Gas turbine emissions

[0001] This specification is based upon and claims the benefit of priority from UK Patent Application Number 2408231.5 filed on 10 Jun. 2024, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to the emissions of non-volatile particulate matter (nvPM) from gas turbine engines, specifically gas turbine engines for an aircraft. The present disclosure provides various methods of operating a gas turbine engine and gas turbine engines. More specifically the present application relates methods of operating gas turbine engines using a fuel which comprises a sustainable aviation fuel (SAF) and gas turbine engines configured to operate using fuel comprising a SAF.

[0003] There is an expectation in the aviation industry of a trend towards the use of fuels different from the traditional kerosene-based jet fuels generally used at present.

[0004] The inventors have identified that the emissions of a gas turbine engine are sensitive to the fuel being used, in particular the amount of nvPM produced by the engine varies depending on the operating parameters and the type of fuel being used. Thus, there is a need to take account of fuel properties of these different fuels and to adjust methods of operating gas turbine engines accordingly.

[0005] According to a first aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0006] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0007] wherein:
  • [0008] a first idle-MTO nvPM emissions index ratio is defined as:

[0009] $\frac{E{I}_{\mathrm{idle}}}{E{I}_{\mathrm{maxTO}}}$

• • • [0010] where:
    • [0011] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
    • [0012] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions:
  • [0013] the first idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 3; and
  • [0014] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0015] Advantageously, reduced nvPM in the exhaust of a gas turbine engine contributes to a reduction in undesirable emissions of the engine. For example, according to operational conditions, reducing nvPM in such a manner may lead to a reduced degree of soot deposits within the engine within and/or downstream of the combustor, and/or an improvement in local air quality. Furthermore, at certain stages of an aircraft flight (where contrails are otherwise expected to form) reduced nvPM in the exhaust may lead to reduced contrail strength and/or time taken for a contrail to disperse. Still further, it has been recognised that certain parts of the flight cycle at which the nvPM is reduced (or most reduced) can be targeted in order to achieve a desired outcome, for example in terms of environmental impact. Purely by way of example, lower nvPM at cruise conditions may particularly reduce the radiative forcing impact of contrails. Purely by way of further example, lower nvPM at idle conditions may particularly improve local air quality on the ground in the region of engine operation. Purely by way of further example, lower nvPM at MTO conditions may particularly reduce the maximum rate of nvPM production during the flight cycle and/or improve air quality on the ground and/or in the region of engine operation. These considerations may apply to all aspects of the disclosure.

[0016] A number of parameters related to gas turbine engine operation have been determined to have an influence on, or are an important factor in, the configuration and arrangement of the combustor of the engine when certain types of fuel, such as a sustainable aviation fuel, are being combusted. Accordingly, any one or more parameters of the following aspects may be advantageously taken into account when determining, for example, operational settings, combustor arrangement and/or combustor configuration, to influence and/or optimise how that fuel is to be distributed, ignited, and/or combusted within the gas turbine engine. These considerations may apply to all aspects of the disclosure.

[0017] The first idle-MTO nvPM emissions index ratio of the gas turbine engine may be greater than zero.

[0018] The first idle-MTO nvPM emissions index ratio may be less than 2.54 and preferably may be less than 2.33 and more preferably may be less than 2.12.

[0019] The first idle-MTO nvPM emissions index ratio may be less than or equal to 1.5 and preferably may be less than or equal to 1 and more preferably may be less than or equal to 0.5.

[0020] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.377 and preferably may be less than or equal to 0.346 and more preferably may be less than or equal to 0.314.

[0021] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.319 and preferably may be less than or equal to 0.293 and more preferably may be less than or equal to 0.266.

[0022] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.184 and preferably may be greater than or equal to 0.207 and more preferably may be greater than or equal to 0.23.

[0023] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.212 and preferably may be greater than or equal to 0.239 and more preferably may be greater than or equal to 0.265.

[0024] The first idle-MTO nvPM emissions index ratio may be in the range of 0.184 to 0.377 and preferably may be in the range of 0.207 to 0.346 and more preferably may be in the range of 0.230 to 0.314.

[0025] The first idle-MTO nvPM emissions index ratio may be in the range of 0.212 to 0.319 and preferably may be in the range of 0.239 to 0.293 and more preferably may be in the range of 0.265 to 0.266.

[0026] The first idle-MTO nvPM emissions index ratio may be less than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.377, or in any range defined between any two of these values.

[0027] A Second Idle-MTO nvPM Emissions Index Ratio May be Defined as:

[0028] $\frac{E{I}_{\mathrm{idle},\mathrm{SAF}}/E{I}_{\mathrm{maxTO},\mathrm{SAF}}}{E{I}_{\mathrm{idle},\mathrm{FF}}/E{I}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0029] where:
    • [0030] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
  • [0031] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
  • [0032] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
  • [0033] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel; and
  • [0034] the second idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0035] The second idle-MTO nvPM emissions index ratio may be greater than zero.

[0036] The second idle-MTO nvPM emissions index ratio may be less than or equal to 0.8 and preferably may be less than or equal to 0.6 and more preferably may be less than or equal to 0.4.

[0037] The second idle-MTO nvPM emissions index ratio may be less than or equal to 0.178 and preferably may be less than or equal to 0.164 and more preferably may be less than or equal to 0.149.

[0038] The second idle-MTO nvPM emissions index ratio may be greater than or equal to 0.03 and preferably may be greater than or equal to 0.06 and more preferably may be greater than or equal to 0.09.

[0039] The second idle-MTO nvPM emissions index ratio may be greater than or equal to 0.118 and preferably may be greater than or equal to 0.133 and more preferably may be greater than or equal to 0.148.

[0040] The second idle-MTO nvPM emissions index ratio may be in the range 0.118 to 0.178 and preferably may be in the range 0.133 to 0.164 and more preferably may be in the range 0.148 to 0.149.

[0041] The second idle-MTO nvPM emissions index ratio may be less than 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1, or in any range defined between any two of these values.

[0042] The second idle-MTO nvPM emissions index ratio of the gas turbine engine may be 0.118, 0.12, 0.125, 0.13, 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.175, 0.178, or within any range defined between any two of these values.

[0043] According to a second aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0044] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0045] wherein:
  • [0046] a second idle-MTO nvPM emissions index ratio is defined as:

[0047] $\frac{E{I}_{\mathrm{idle},\mathrm{SAF}}/E{I}_{\mathrm{maxTO},\mathrm{SAF}}}{E{I}_{\mathrm{idle},\mathrm{FF}}/E{I}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0048] where:
    • [0049] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [0050] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [0051] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [0052] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [0053] the second idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
    • [0054] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0055] The second idle-MTO nvPM emissions index ratio defined in the second aspect may be as defined above in connection with the first aspect.

[0056] According to a third aspect, there is provided a method of operating the gas turbine engine of the first aspect or the second aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0057] According to a fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0058] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray 15 nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0059] wherein:
  • [0060] a first idle-MTO nvPM emissions index ratio may be defined as:

[0061] $\frac{{\mathrm{EI}}_{\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}}$

• • • [0062] where:
    • [0063] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
    • [0064] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions; and
  • [0065] the first idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 3; and
  • [0066] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0067] The first idle-MTO nvPM emissions index ratio may be as defined above in connection with the first aspect.

[0068] A second idle-MTO nvPM emissions index ratio may be defined as:

[0069] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0070] where:
    • [0071] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [0072] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [0073] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [0074] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel; and wherein
  • [0075] the second idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0076] The second idle-MTO nvPM emissions index ratio may be as defined above in connection with the first aspect.

[0077] According to a fifth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0078] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0079] wherein:
  • [0080] a second idle-MTO nvPM emissions index ratio may be defined as:

[0081] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0082] where:
      • [0083] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions and if a fuel provided to the combustor comprises a sustainable aviation fuel;
      • [0084] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
      • [0085] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
      • [0086] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
  • [0087] the second idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0088] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0089] The second idle-MTO nvPM emissions index ratio may be as defined above in connection with the first aspect.

[0090] According to a sixth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0091] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0092] wherein:
  • [0093] a fuel-flow nvPM emissions index ratio may be defined as:

[0094] $\frac{{\mathrm{EI}}_{\mathrm{idle}}\times W_{f,\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}\times W_{f,\mathrm{maxTO}}}$

• • • [0095] where:
    • [0096] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions;
    • [0097] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
    • [0098] W_f,idle is the rate of fuel flow to the fuel spray nozzles in kg/s at around 7% available thrust for the given operating conditions; and
    • [0099] W_f,maxTO is the rate of fuel flow to the fuel spray nozzles in kg/s at around 100% available thrust for the given operating conditions;
  • [0100] the fuel-flow nvPM emissions index ratio of the gas turbine engine is less than 0.3; and
  • [0101] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0102] The fuel-flow nvPM emissions index ratio may be less than 0.241 and preferably may be less than 0.221 and more preferably may be less than 0.201.

[0103] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.15 and preferably may be less than or equal to 0.1 and more preferably may be less than or equal to 0.05.

[0104] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0357 and preferably may be less than or equal to 0.0327 and more preferably may be less than or equal to 0.0297.

[0105] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0285 and preferably may be less than or equal to 0.0261 and more preferably may be less than or equal to 0.0238.

[0106] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.0138 and preferably may be greater than or equal to 0.0156 and more preferably may be greater than or equal to 0.0173.

[0107] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.0189 and preferably may be greater than or equal to 0.0213 and more preferably may be greater than or equal to 0.0237.

[0108] The fuel-flow nvPM emissions index ratio may be in the range of 0.0138 to 0.0357 and preferably may be in the range of 0.0156 to 0.0327 and even more preferably may be in the range of 0.0173 to 0.0297.

[0109] The fuel-flow nvPM emissions index ratio may be in the range of 0.0189 to 0.0285 and preferably may be in the range of 0.0213 to 0.0261 and even more preferably may be in the range of 0.0237 to 0.0238.

[0110] The fuel-flow nvPM emissions index ratio may be less than 0.003, 0.004, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.055, 0.06, 0.065, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.245, 0.35, 0.357 or any range defined between any two of these values.

[0111] W_f,idle may be in the range of 0.142 to 0.263 kg/s and preferably may be in the range of 0.160 to 0.241 kg/s and more preferably may be in the range of 0.178 to 0.219 kg/s.

[0112] W_f,maxTO may be in the range 1.50 to 3.36 kg/s and preferably may be in the range of 1.69 to 3.08 kg/s and more preferably may be in the range of 1.88 to 2.80 kg/s.

[0113] According to a seventh aspect, there is provided a method of operating the gas turbine engine of the sixth aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0114] According to an eighth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0115] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0116] wherein:
  • [0117] a fuel-flow nvPM emissions index ratio may be defined as:

[0118] $\frac{{\mathrm{EI}}_{\mathrm{idle}}\times W_{f,\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}\times W_{f,\mathrm{maxTO}}}$

• • • [0119] where:
    • [0120] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions;
    • [0121] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
    • [0122] W_f,idle is the rate of fuel flow to the fuel spray nozzles in kg/s at around 7% available thrust for the given operating conditions; and
    • [0123] W_f,maxTO is the rate of fuel flow to the fuel spray nozzles in kg/s at around 100% available thrust for the given operating conditions; and
  • [0124] the fuel-flow nvPM emissions index ratio of the gas turbine engine is less than 0.3; and
  • [0125] wherein the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0126] Any of the fuel-flow nvPM emissions index ratio, W_f,idle and W_f,maxTO may be as defined above in connection with the sixth aspect.

[0127] According to a ninth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0128] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0129] wherein:
  • [0130] a thrust nvPM emissions index ratio may be defined as:

[0131] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}/F_{\mathrm{maxTO}}}{{\mathrm{EI}}_{\mathrm{idle}}/F_{\mathrm{idle}}}$

• • • [0132] where:
    • [0133] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions;
    • [0134] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
    • [0135] F_maxTO is the thrust of the gas turbine engine at around 100% available thrust in kN for the given operating conditions; and
    • [0136] F_idle is the thrust of the gas turbine engine at around 7% available thrust in kN for the given operating conditions;
  • [0137] the thrust nvPM emissions index ratio is greater than 0.02; and
  • [0138] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0139] The thrust nvPM emissions index ratio may be greater than 0.0264 and preferably may be greater than 0.0297 and more preferably may be greater than 0.033.

[0140] The thrust nvPM emissions index ratio may be greater than 0.0312 and preferably may be greater than 0.0351 and more preferably may be greater than 0.039.

[0141] The thrust nvPM emissions index ratio may be greater than or equal to 0.07 and preferably may be greater than or equal to 0.1 and more preferably may be greater than or equal to 0.13.

[0142] The thrust nvPM emissions index ratio may be greater than or equal to 0.178 and preferably may be greater than or equal to 0.2 and more preferably may be greater than or equal to 0.223.

[0143] The thrust nvPM emissions index ratio may be greater than or equal to 0.21 and preferably may be greater than or equal to 0.237 and more preferably may be greater than or equal to 0.263.

[0144] The thrust nvPM emissions index ratio may be less than or equal to 0.365 and preferably may be less than or equal to 0.335 and more preferably may be less than or equal to 0.304.

[0145] The thrust nvPM emissions index ratio may be less than or equal to 0.317 and preferably may be less than or equal to 0.29 and more preferably may be less than or equal to 0.264.

[0146] The thrust nvPM emissions index ratio may be in the range of 0.178 to 0.365 and preferably may be in the range of 0.200 to 0.335 and even more preferably may be in the range 0.223 to 0.304.

[0147] The thrust nvPM emissions index ratio may be in the range of 0.210 to 0.317 and preferably may be in the range of 0.237 to 0.290 and even more preferably may be in the range 0.263 to 0.264.

[0148] The thrust nvPM emissions index ratio may be greater than 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38, 0.4 or any range defined between any two of these values.

[0149] The thrust nvPM emissions index ratio may be 0.178, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.365 or any range defined between any two of these values.

[0150] F_maxTO may be in the range 204 kN to 420 kN and preferably may be in the range 229 kN to 385 kN and more preferably may be in the range 255 kN to 350 kN.

[0151] F_idle may be in the range 14.2 kN to 29.4 kN and preferably may be in the range 16.0 kN to 26.9 KN and more preferably may be in the range 17.8 kN to 24.5 kN.

[0152] According to a tenth aspect, there is provided a method of operating the gas turbine engine of the ninth aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0153] According to an eleventh aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0154] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0155] wherein:
  • [0156] a thrust nvPM emissions index ratio may be defined as:

[0157] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}/F_{\mathrm{maxTO}}}{{\mathrm{EI}}_{\mathrm{idle}}/F_{\mathrm{idle}}}$

• • • [0158] where:
    • [0159] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
    • [0160] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
    • [0161] F_maxTO is the thrust of the gas turbine engine at around 100% available thrust in kN for the given operating conditions,
    • [0162] F_idle is the thrust of the gas turbine engine at around 7% available thrust in kN for the given operating conditions; and
  • [0163] the thrust nvPM emissions index ratio is greater than 0.02; and
  • [0164] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0165] Any of the thrust nvPM emissions index ratio, F_maxTO and F_idle may be as defined in connection with the ninth aspect.

[0166] According to a twelfth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0167] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0168] wherein:
  • [0169] a lean cruise-MTO nvPM emissions index ratio may be defined as:

[0170] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [0171] where:
    • [0172] EI_{cruise (lean)} may be defined as:

[0173] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}+{\mathrm{EI}}_{\mathrm{climb}}}{2}$

• • • [0174] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions;
    • [0175] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions; and
    • [0176] BPR is the bypass ratio of the gas turbine engine;
  • [0177] the lean cruise-MTO nvPM emissions index ratio is less than 0.2; and
  • [0178] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0179] The lean cruise-MTO nvPM emissions index ratio may be less than 0.151, preferably may be less than 0.138 and further preferably may be less than 0.126.

[0180] The lean cruise-MTO nvPM emissions index ratio may be less than 0.136, preferably may be less than 0.125, and further preferably may be less than 0.114.

[0181] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.131, preferably may be less than or equal to 0.12, and further preferably may be less than or equal to 0.109.

[0182] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.118, preferably may be less than or equal to 0.108, and further preferably may be less than or equal to 0.098.

[0183] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0751, preferably may be greater than or equal to 0.0845, and further preferably may be greater than or equal to 0.0938.

[0184] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0869, preferably may be greater than or equal to 0.0977, and further preferably may be greater than or equal to 0.108.

[0185] The lean cruise-MTO nvPM emissions index ratio may be in the range of 0.0751 to 0.131, preferably may be in the range of 0.0845 to 0.120 and further preferably may be in the range of 0.0938 to 0.109.

[0186] The lean cruise-MTO nvPM emissions index ratio may be in the range of 0.0751 to 0.118, preferably may be in the range of 0.0845 to 0.108 and further preferably may be in the range of 0.0938 to 0.0980.

[0187] The lean cruise-MTO nvPM emissions index ratio may be in the range of 0.0869 to 0.131, preferably may be in the range of 0.0977 to 0.120 and further preferably may be in the range of 0.108 to 0.109.

[0188] The lean cruise-MTO nvPM emissions index ratio may be less than 0.098, 0.11, 0.115, 0.12, 0.125, 0.13, 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.175, 0.18, 0.185, 0.19, 0.195 or 0.2, or within any range defined between any two of these values.

[0189] The lean cruise-MTO nvPM emissions index ratio may be 0.0751, 0.08, 0.085, 0.09, 0.095, 0.1, 0.105, 0.11, 0.115, 0.12, 0.125, 0.13, 0.131, or within any range defined between any two of these values.

[0190] A rich cruise-MTO nvPM emissions index ratio may be defined as:

[0191] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • [0192] where:
  • [0193] EI_{cruise (rich)} may be defined as:

[0194] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • [0195] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions;
  • [0196] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated; and
  • [0197] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and wherein the rich cruise-MTO nvPM emissions index ratio may be less than 0.3.

[0198] The rich cruise-MTO nvPM emissions index ratio may be less than 0.29, preferably may be less than 0.266 and further preferably may be less than 0.242.

[0199] The rich cruise-MTO nvPM emissions index ratio may be less than 0.285, preferably may be less than 0.261 and further preferably may be less than 0.237.

[0200] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.2, preferably may be less than or equal to 0.16, and further preferably may be less than or equal to 0.12.

[0201] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.105, preferably may be less than or equal to 0.0963, and further preferably may be less than or equal to 0.0875.

[0202] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.102, preferably may be less than or equal to 0.0926, and further preferably may be less than or equal to 0.0842.

[0203] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0509, preferably may be greater than or equal to 0.0573, and further preferably may be greater than or equal to 0.0637.

[0204] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0699, preferably may be greater than or equal to 0.0787, and further preferably may be greater than or equal to 0.0874.

[0205] The rich cruise-MTO nvPM emissions index ratio may be in the range of 0.0509 to 0.105, preferably may be in the range of 0.0573 to 0.0963 and further preferably may be in the range of 0.0637 to 0.0875.

[0206] The rich cruise-MTO nvPM emissions index ratio may be in the range of 0.0509 to 0.102, preferably may be in the range of 0.0573 to 0.0926 and further preferably may be in the range of 0.0637 to 0.0842.

[0207] The rich cruise-MTO nvPM emissions index ratio may be in the range of 0.0699 to 0.105, preferably may be in the range of 0.0787 to 0.0963 and further preferably may be in the range of 0.0874 to 0.0875.

[0208] The rich cruise-MTO nvPM emissions index ratio may be less than 0.3, 0.28, 0.26, 0.24, 0.22, 0.2, 0.18, 0.16, 0.14, 0.12, 0.1, 0.0842, or within any range defined between any two of these values.

[0209] The rich cruise-MTO nvPM emissions index ratio may be 0.0509, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, 0.105, or within any range defined between any two of these values.

[0210] The BPR may be in the range of 6.38 to 11.3 and preferably may be in the range of 7.18 to 10.4 and further preferably may be in the range of 7.98 to 9.40.

[0211] The BPR may be in the range of 6.38 to 9.59 and preferably may be in the range of 7.18 to 8.79 and further preferably may be in the range of 7.98 to 7.99.

[0212] The BPR may be in the range of 6.85 to 11.3 and preferably may be in the range of 7.70 to 10.4 and further preferably may be in the range of 8.56 to 9.40.

[0213] According to a thirteenth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0214] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0215] wherein:
  • [0216] a rich cruise-MTO nvPM emissions index ratio may be defined as:

[0217] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • [0218] where:
  • [0219] EI_{cruise (rich)} may be defined as:

[0220] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • [0221] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions;
  • [0222] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated; and
  • [0223] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and the rich cruise-MTO nvPM emissions index ratio is less than 0.3; and
  • [0224] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0225] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the twelfth aspect.

[0226] According to a fourteenth aspect, there is provided a method of operating the gas turbine engine of the twelfth aspect or the thirteenth aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0227] According to a fifteenth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0228] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0229] wherein:
  • [0230] a lean cruise-MTO nvPM emissions index ratio may be defined as:

[0231] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [0232] where:
    • [0233] EI_{cruise (lean)} may be defined as:

[0234] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}+{\mathrm{EI}}_{\mathrm{climb}}}{2}$

• • • [0235] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions;
    • [0236] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions; and
    • [0237] BPR is the bypass ratio of the gas turbine engine; and the lean cruise-MTO nvPM emissions index ratio is less than 0.2; and
  • [0238] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0239] The lean cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the twelfth aspect.

[0240] A rich cruise-MTO nvPM emissions index ratio may be defined as:

[0241] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\max \mathrm{TO}}}{\mathrm{BPR}}$

• • [0242] where:
  • [0243] EI_{cruise (rich)} may be defined as:

[0244] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • [0245] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions;
  • [0246] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated;
  • [0247] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and wherein the rich cruise-MTO nvPM emissions index ratio may be less than 0.3.

[0248] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the twelfth aspect.

[0249] According to a sixteenth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0250] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0251] wherein:
  • [0252] a rich cruise-MTO nvPM emissions index ratio may be defined as:

[0253] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\max \mathrm{TO}}}{\mathrm{BPR}}$

• • [0254] where:
  • [0255] EI_{cruise (rich)} may be defined as:

[0256] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • [0257] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust given operating conditions;
  • [0258] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated;
  • [0259] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and
  • [0260] the rich cruise-MTO nvPM emissions index ratio is less than 0.3; and
  • [0261] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0262] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the twelfth aspect.

[0263] According to a seventeenth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0264] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0265] wherein:
  • [0266] a MTO nvPM emissions index ratio may be defined as:

[0267] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0268] where:
    • [0269] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
    • [0270] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0271] the MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0272] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0273] The MTO nvPM emissions index ratio may be greater than zero.

[0274] The MTO nvPM emissions index ratio may be less than or equal to 0.93, and preferably may be less than or equal to 0.86, and more preferably may be less than or equal to 0.79.

[0275] The MTO nvPM emissions index ratio may be less than or equal to 0.776, and preferably may be less than or equal to 0.711, and more preferably may be less than or equal to 0.646.

[0276] The MTO nvPM emissions index ratio may be greater than or equal to 0.15, and preferably may be greater than or equal to 0.3, and more preferably may be greater than or equal to 0.45.

[0277] The MTO nvPM emissions index ratio may be greater than or equal to 0.516, and preferably may be greater than or equal to 0.581, and more preferably may be greater than or equal to 0.645.

[0278] The MTO nvPM emissions index ratio may be in the range of 0.516 to 0.776, and preferably may be in the range of 0.581 to 0.711, and more preferably may be in the range of 0.645 to 0.646.

[0279] The MTO nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0280] The MTO nvPM emissions index ratio may be 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, or within any range defined between any two of these values.

[0281] A climb nvPM emissions index ratio may be defined as:

[0282] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • [0283] where:
  • [0284] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0285] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0286] and wherein the climb nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0287] The climb nvPM emissions index ratio may be greater than zero.

[0288] The climb nvPM emissions index ratio may be less than or equal to 0.9, and preferably may be less than or equal to 0.75, and more preferably may be less than or equal to 0.6.

[0289] The climb nvPM emissions index ratio may be less than or equal to 0.57, and preferably may be less than or equal to 0.523, and more preferably may be less than or equal to 0.475.

[0290] The climb nvPM emissions index ratio may be greater than or equal to 0.1, and preferably may be greater than or equal to 0.2, and more preferably may be greater than or equal to 0.3.

[0291] The climb nvPM emissions index ratio may be greater than or equal to 0.379, and preferably may be greater than or equal to 0.427, and more preferably may be greater than or equal to 0.474.

[0292] The climb nvPM emissions index ratio may be in the range of 0.379 to 0.570, and preferably may be in the range of 0.427 to 0.523, and more preferably may be in the range of 0.474 to 0.475.

[0293] The climb nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0294] The climb nvPM emissions index ratio may be 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, or within any range defined between any two of these values.

[0295] An approach nvPM emissions index ratio may be defined as:

[0296] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • [0297] where:
  • [0298] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0299] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein the approach nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0300] The approach nvPM emissions index ratio may be greater than zero.

[0301] The approach nvPM emissions index ratio may be less than or equal to 0.8, and preferably may be less than or equal to 0.5, and more preferably may be less than or equal to 0.2.

[0302] The approach nvPM emissions index ratio may be less than or equal to 0.185, and preferably may be less than or equal to 0.169, and more preferably may be less than or equal to 0.154.

[0303] The approach nvPM emissions index ratio may be greater than or equal to 0.03, and preferably may be greater than or equal to 0.06, and more preferably may be greater than or equal to 0.09.

[0304] The approach nvPM emissions index ratio may be greater than or equal to 0.122, and preferably may be greater than or equal to 0.138, and more preferably may be greater than or equal to 0.153.

[0305] The approach nvPM emissions index ratio may be in the range of 0.122 to 0.185, and preferably may be in the range of 0.138 to 0.169, and more preferably may be in the range of 0.153 to 0.154.

[0306] The approach nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0307] The approach nvPM emissions index ratio may be 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, or within any range defined between any two of these values.

An Idle nvPM Emissions Index Ratio May be Defined as:

[0308] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}$

• • [0309] where:
  • [0310] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0311] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein the idle nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0312] The idle nvPM emissions index ratio may be greater than zero.

[0313] The idle nvPM emissions index ratio may be less than or equal to 0.8, and preferably may be less than or equal to 0.5, and more preferably may be less than or equal to 0.2.

[0314] The idle nvPM emissions index ratio may be less than or equal to 0.115, and preferably may be less than or equal to 0.106, and more preferably may be less than or equal to 0.0959.

[0315] The idle nvPM emissions index ratio may be greater than or equal to 0.02, and preferably may be greater than or equal to 0.04, and more preferably may be greater than or equal to 0.06.

[0316] The idle nvPM emissions index ratio may be greater than or equal to 0.0766, and preferably may be greater than or equal to 0.0862, and more preferably may be greater than or equal to 0.0958.

[0317] The idle nvPM emissions index ratio may be in the range of 0.0766 to 0.115, and preferably may be the range of 0.0862 to 0.106, and more preferably may be the range of 0.0958 to 0.0959.

[0318] The idle nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0319] The idle nvPM emissions index ratio of the gas turbine engine may be 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, or within any range defined between any two of these values.

[0320] According to an eighteenth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0321] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0322] wherein:
  • [0323] a climb nvPM emissions index ratio may be defined as:

[0324] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • [0325] where:
  • [0326] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0327] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0328] and the climb nvPM emissions index ratio of the gas turbine engine may be less than 1; and
  • [0329] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0330] The climb nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0331] According to a nineteenth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0332] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0333] wherein:
  • [0334] an approach nvPM emissions index ratio may be defined as:

[0335] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • [0336] where:
  • [0337] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0338] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0339] the approach nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0340] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0341] The approach nvPM emissions index ratio may be as defined above in connection with the seventeenth aspect.

[0342] According to a twentieth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0343] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0344] wherein:
  • [0345] an idle nvPM emissions index ratio may be defined as:

[0346] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}$

• • [0347] where:
  • [0348] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0349] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0350] the idle nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0351] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0352] The idle nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0353] According to a twenty-first aspect, there is provided a method of operating the gas turbine engine of any of the seventeenth, eighteenth, nineteenth or twentieth aspects, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0354] According to a twenty-second aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0355] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0356] a MTO nvPM emissions index ratio may be defined as:

[0357] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0358] where:
    • [0359] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
    • [0360] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0361] the MTO nvPM emissions index ratio of the gas turbine engine (10) is less than 1; and
  • [0362] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0363] The MTO nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0364] A climb nvPM emissions index ratio may be defined as:

[0365] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • [0366] where:
  • [0367] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0368] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein the climb nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0369] The climb nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0370] An approach nvPM emissions index ratio may be defined as:

[0371] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • [0372] where:
  • [0373] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0374] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein the approach nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0375] The approach nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0376] An idle nvPM emissions index ratio may be defined as:

[0377] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}$

• • [0378] where:
  • [0379] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
  • [0380] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein the idle nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0381] The idle nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0382] According to a twenty-third aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0383] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0384] a climb nvPM emissions index ratio may be defined as:

[0385] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • • [0386] where:
      • [0387] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [0388] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0389] the climb nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0390] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0391] The climb nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0392] According to a twenty-fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0393] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0394] an approach nvPM emissions index ratio may be defined as:

[0395] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • • [0396] where:
      • [0397] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [0398] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0399] the approach nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0400] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0401] The approach nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0402] According to a twenty-fifth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0403] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0404] an idle nvPM emissions index ratio may be defined as:

[0405] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}$

• • • [0406] where:
      • [0407] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions, and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [0408] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0409] the idle nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0410] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0411] The idle nvPM emissions index ratio may be as defined in connection with the seventeenth aspect.

[0412] According to a twenty-sixth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0413] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and
  • [0414] wherein:
  • [0415] an MTO nvPM emissions index ratio-modified fuel flow may be defined as:

[0416] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}\times W_{f,\mathrm{maxTO}}$

• • • [0417] where:
    • [0418] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
    • [0419] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0420] W_f,maxTO is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 100% available thrust for the given operating conditions;
  • [0421] the MTO nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 4; and
  • [0422] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0423] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be greater than zero.

[0424] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be less than 3.36, more preferably may be less than 3.08 and yet even more preferably may be less than 2.8.

[0425] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.974, preferably may be greater than or equal to 1.09 and further preferably may be greater than or equal to 1.21.

[0426] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 2.17, more preferably may be less than or equal to 1.99 and further preferably may be less than or equal to 1.81.

[0427] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.974 to 2.17, preferably may be in the range 1.09 to 1.99 and further preferably may be in the range 1.21 to 1.81.

[0428] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be less than 2.95, more preferably may be less than 2.7 and yet even more preferably may be less than 2.46.

[0429] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 1.26, preferably may be greater than or equal to 1.42 and further preferably may be greater than or equal to 1.58.

[0430] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 1.91, more preferably may be less than or equal to 1.75 and further preferably may be less than or equal to 1.59.

[0431] The MTO nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 1.26 to 1.91, preferably may be in the range 1.42 to 1.75 and further preferably may be in the range 1.58 to 1.59.

[0432] The MTO nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.974, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.17, or within any range defined between any two of these values.

[0433] W_f,maxTO may be in the range of 1.50 to 3.36 kg/s, and preferably may be in the range of 1.69 to 3.08 kg/s, and more preferably may be in the range of 1.88 to 2.80 kg/s.

[0434] W_f,maxTO may be in the range of 1.96 to 2.95 kg/s, and preferably may be in the range of 2.20 to 2.70 kg/s, and more preferably may be in the range of 2.45 to 2.46 kg/s.

[0435] A climb nvPM emissions index ratio-modified fuel flow may be defined as:

[0436] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}\times W_{f,\mathrm{climb}}$

• • [0437] where:
    • [0438] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0439] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0440] W_f,climb is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF and EI_climb,FF are calculated; and wherein
  • [0441] the climb nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than 3.

[0442] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be greater than zero.

[0443] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be less than 2.73, more preferably may be less than 2.5 and yet even more preferably may be less than 2.27.

[0444] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.589, preferably may be greater than or equal to 0.663 and further preferably may be greater than or equal to 0.737.

[0445] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 1.3, more preferably may be less than or equal to 1.19 and further preferably may be less than or equal to 1.08.

[0446] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.589 to 1.30, preferably may be in the range 0.663 to 1.19 and further preferably may be in the range 0.737 to 1.08.

[0447] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be less than 2.42, more preferably may be less than 2.21 and yet even more preferably may be less than 2.01.

[0448] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.763, preferably may be greater than or equal to 0.858 and further preferably may be greater than or equal to 0.953.

[0449] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 1.15, more preferably may be less than or equal to 1.05 and further preferably may be less than or equal to 0.954.

[0450] The climb nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.763 to 1.15, preferably may be in the range 0.858 to 1.05 and further preferably may be in the range 0.953 to 0.954.

[0451] The climb nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than or equal to 0.589, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, or any range defined between any two of these values.

[0452] W_f,climb may be in the range of 1.24 to 2.73 kg/s, and preferably may be in the range of 1.39 to 2.50 kg/s, and more preferably may be in the range of 1.55 to 2.27 kg/s.

[0453] W_f,climb may be in the range of 1.60 to 2.42 kg/s, and preferably may be in the range of 1.80 to 2.21 kg/s, and more preferably may be in the range of 2.00 to 2.01 kg/s.

[0454] An approach nvPM emissions index ratio-modified fuel flow may be defined as:

[0455] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}\times W_{f,\mathrm{approach}}$

• • [0456] where:
    • [0457] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0458] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0459] W_f,approach is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 30% available thrust for the same
  • [0460] operating conditions at which EI_approach,SAF and EI_approach,FF are calculated; and wherein the approach nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 0.9.

[0461] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be greater than zero.

[0462] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be less than 0.841, more preferably may be less than 0.771 and yet even more preferably may be less than 0.701.

[0463] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.0636, preferably may be greater than or equal to 0.0716 and further preferably may be greater than or equal to 0.0795.

[0464] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 0.13, more preferably may be less than or equal to 0.119 and further preferably may be less than or equal to 0.108.

[0465] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.0636 to 0.130, preferably may be in the range 0.0716 to 0.119 and further preferably may be in the range 0.0795 to 0.108.

[0466] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be less than 0.771, more preferably may be less than 0.707 and yet even more preferably may be less than 0.642.

[0467] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.0788, preferably may be greater than or equal to 0.0887 and further preferably may be greater than or equal to 0.0986.

[0468] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 0.119, more preferably may be less than or equal to 0.109 and further preferably may be less than or equal to 0.0987.

[0469] The approach nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.0788 to 0.119, preferably may be in the range 0.0887 to 0.109 and further preferably may be in the range 0.0986 to 0.0987.

[0470] The approach nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than or equal to 0.0636, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, 0.105, 0.12, 0.125, 0.13, or any range defined between any two of these values.

[0471] W_f,approach may be in the range of 0.414 to 0.841 kg/s, and preferably may be in the range of 0.466 to 0.771 kg/s, and more preferably may be in the range of 0.517 to 0.701 kg/s.

[0472] W_f,approach may be in the range of 0.513 to 0.771 kg/s, and preferably may be in the range of 0.577 to 0.707 kg/s, and more preferably may be in the range of 0.641 to 0.642 kg/s.

[0473] An idle nvPM emissions index ratio-modified fuel flow may be defined as:

[0474] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}\times W_{f,\mathrm{idle}}$

• • [0475] where:
    • [0476] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0477] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0478] W_f,idle is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF and EI_idle,FF are calculated; and wherein the idle nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than 0.3.

[0479] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be greater than zero.

[0480] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than 0.263, more preferably may be less than 0.241 and yet even more preferably may be less than 0.219.

[0481] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.0136, preferably may be greater than or equal to 0.0153 and further preferably may be greater than or equal to 0.017.

[0482] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 0.0252, more preferably may be less than or equal to 0.0231 and further preferably may be less than or equal to 0.021.

[0483] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.0136 to 0.0252, preferably may be in the range 0.0153 to 0.0231 and further preferably may be in the range 0.0170 to 0.0210.

[0484] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than 0.263, more preferably may be less than 0.241 and yet even more preferably may be less than 0.219.

[0485] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.0167, preferably may be greater than or equal to 0.0188 and further preferably may be greater than or equal to 0.0209.

[0486] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 0.0252, more preferably may be less than or equal to 0.0231 and further preferably may be less than or equal to 0.021.

[0487] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.0167 to 0.0252, preferably may be in the range 0.0188 to 0.0231 and further preferably may be in the range 0.0209 to 0.0210.

[0488] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than 0.253, more preferably may be less than 0.232 and yet even more preferably may be less than 0.211.

[0489] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be greater than or equal to 0.0136, preferably may be greater than or equal to 0.0153 and further preferably may be greater than or equal to 0.017.

[0490] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be less than or equal to 0.0243, more preferably may be less than or equal to 0.0223 and further preferably may be less than or equal to 0.0202.

[0491] The idle nvPM emissions index ratio-modified fuel flow in kg/s may be in the range 0.0136 to 0.0243, preferably may be in the range 0.0153 to 0.0223 and further preferably may be in the range 0.0170 to 0.0202.

[0492] The idle nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than or equal to 0.0136, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, 0.023, 0.024, 0.025, 0.0252, or any range defined between any two of these values.

[0493] W_f,idle may be in the range of 0.142 to 0.263 kg/s, and preferably may be in the range of 0.160 to 0.241 kg/s, and more preferably may be in the range of 0.178 to 0.219 kg/s.

[0494] W_f,idle may be in the range of 0.142 to 0.253 kg/s, and preferably may be in the range of 0.160 to 0.232 kg/s, and more preferably may be in the range of 0.178 to 0.211 kg/s.

[0495] W_f,idle may be in the range of 0.175 to 0.263 kg/s, and preferably may be in the range of 0.196 to 0.241 kg/s, and more preferably may be in the range of 0.218 to 0.219 kg/s.

[0496] According to a twenty-seventh aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0497] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0498] a climb nvPM emissions index ratio-modified fuel flow may be defined as:

[0499] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}\times W_{f,\mathrm{climb}}$

• • • [0500] where:
      • [0501] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0502] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0503] W_f,climb is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF and EI_climb,FF are calculated;
  • [0504] the climb nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 3; and
  • [0505] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0506] The climb nvPM emissions index ratio-modified fuel flow and/or W_f,climb may be as defined above in connection with the twenty-sixth aspect.

[0507] According to a twenty-eighth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0508] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0509] an approach nvPM emissions index ratio-modified fuel flow may be defined as:

[0510] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}\times W_{f,\mathrm{approach}}$

• • • [0511] where:
      • [0512] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0513] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0514] W_f,approach is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF and EI_approach,FF are calculated; and wherein
  • [0515] the approach nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 0.9; and
  • [0516] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0517] The approach nvPM emissions index ratio-modified fuel flow and/or W_f,approach may be as defined above in connection with the twenty-sixth aspect.

[0518] According to a twenty-ninth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0519] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0520] an idle nvPM emissions index ratio-modified fuel flow may be defined as:

[0521] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}\times W_{f,\mathrm{idle}}$

• • • [0522] where:
      • [0523] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0524] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0525] W_f,idle is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF and EI_idle,FF are calculated;
  • [0526] the idle nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 0.3; and
  • [0527] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[0528] The idle nvPM emissions index ratio-modified fuel flow and/or W_f,idle may be as defined above in connection with the twenty-sixth aspect.

[0529] According to a thirtieth aspect, there is provided a method of operating the gas turbine engine of any one or more of the twenty-sixth, twenty-seventh, twenty-eighth or twenty-ninth aspects, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0530] According to a thirty-first aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0531] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6;
  • [0532] wherein:
  • [0533] an MTO nvPM emissions index ratio-modified fuel flow may be defined as:

[0534] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}\times W_{f,\mathrm{maxTO}}$

• • • [0535] where:
    • [0536] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
    • [0537] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0538] W_f,maxTO is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 100% available thrust for the given operating conditions;
  • [0539] the MTO nvPM emissions index ratio-modified fuel flow in kg/s is less than 4; and
  • [0540] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0541] The MTO nvPM emissions index ratio-modified fuel flow and/or W_f,maxTO may be as defined above in connection with the twenty-sixth aspect.

[0542] A climb nvPM emissions index ratio-modified fuel flow may be defined as:

[0543] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}\times W_{f,\mathrm{climb}}$

• • [0544] where:
  • [0545] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
  • [0546] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0547] W_f,climb is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF and EI_climb,FF are calculated; and wherein
  • [0548] the climb nvPM emissions index ratio-modified fuel flow of the gas turbine engine (10) in kg/s may be less than 3.

[0549] The climb nvPM emissions index ratio-modified fuel flow and/or W_f,climb may be as defined above in connection with the twenty-sixth aspect.

[0550] An approach nvPM emissions index ratio-modified fuel flow may be defined as:

[0551] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}\times W_{f,\mathrm{approach}}$

• • [0552] where:
  • [0553] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
  • [0554] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0555] W_f,approach is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF and EI_approach,FF are calculated; and wherein
  • [0556] the approach nvPM emissions index ratio-modified fuel flow of the gas turbine engine (10) kg/s may be less than 0.9.

[0557] The approach nvPM emissions index ratio-modified fuel flow and/or W_f,approach may be as defined above in connection with the twenty-sixth aspect.

[0558] An idle nvPM emissions index ratio-modified fuel flow may be defined as:

[0559] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}\times W_{f,\mathrm{idle}}$

• • [0560] where:
  • [0561] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
  • [0562] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0563] W_f,idle is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF and EI_idle,FF are calculated; and wherein
  • [0564] the idle nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s may be less than 0.3.

[0565] The idle nvPM emissions index ratio-modified fuel flow and/or W_f,idle may be as defined above in connection with the twenty-sixth aspect.

[0566] According to a thirty-second aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0567] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0568] a climb nvPM emissions index ratio-modified fuel flow may be defined as:

[0569] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}\times W_{f,\mathrm{climb}}$

• • • [0570] where:
      • [0571] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0572] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0573] W_f,climb is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF and EI_climb,FF are calculated;
  • [0574] the climb nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 3; and
  • [0575] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0576] The climb nvPM emissions index ratio-modified fuel flow and/or W_f,climb may be as defined above in connection with the twenty-sixth aspect.

[0577] According to a thirty-third aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0578] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0579] an approach nvPM emissions index ratio-modified fuel flow may be defined as:

[0580] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}\times W_{f,\mathrm{approach}}$

• • • [0581] where:
      • [0582] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0583] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0584] W_f,approach is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 30% available thrust for the same operating conditions at which EI_approach,SAF and EI_approach,FF are calculated;
  • [0585] the approach nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 0.9; and
  • [0586] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0587] The approach nvPM emissions index ratio-modified fuel flow and/or W_f,approach may be as defined above in connection with the twenty-sixth aspect.

[0588] According to a thirty-fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0589] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; and wherein:
  • [0590] an idle nvPM emissions index ratio-modified fuel flow may be defined as:

[0591] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}\times W_{f,\mathrm{idle}}$

• • • [0592] where:
      • [0593] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0594] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0595] W_f,idle is the mass flow rate of fuel provided to the plurality of fuel spray nozzles in kg/s when the gas turbine engine is operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF and EI_idle,FF are calculated;
  • [0596] the idle nvPM emissions index ratio-modified fuel flow of the gas turbine engine in kg/s is less than 0.3; and
  • [0597] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0598] The idle nvPM emissions index ratio-modified fuel flow and/or W_f,idle may be as defined above in connection with the twenty-sixth aspect.

[0599] According to a thirty-fifth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0600] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6;
  • [0601] wherein:
  • [0602] a lean cruise nvPM emissions index ratio may be defined as:

[0603] $\frac{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • • [0604] where:
    • [0605] EI_{cruise(lean),SAF} may be defined as:

[0606] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0607] EI_{cruise(lean),FF} may be defined as:

[0608] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0609] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0610] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0611] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0612] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0613] the lean cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0614] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0615] The lean cruise nvPM emissions index ratio may be greater than zero.

[0616] The lean cruise nvPM emissions index ratio may be less than or equal to 0.9 and preferably may be less than or equal to 0.8 and further preferably may be less than or equal to 0.7.

[0617] The lean cruise nvPM emissions index ratio may be less than or equal to 0.677, preferably may be less than or equal to 0.621 and further preferably may be less than or equal to 0.564.

[0618] The lean cruise nvPM emissions index ratio may be greater than or equal to 0.446, preferably may be greater than or equal to 0.501 and further preferably may be greater than or equal to 0.557.

[0619] The lean cruise nvPM emissions index ratio may be in the range 0.446 to 0.677, preferably may be in the range 0.501 to 0.621 and further preferably may be in the range 0.557 to 0.564.

[0620] The lean cruise nvPM emissions index ratio may be less than or equal to 0.673, preferably may be less than or equal to 0.617 and further preferably may be less than or equal to 0.561.

[0621] The lean cruise nvPM emissions index ratio may be greater than or equal to 0.448, preferably may be greater than or equal to 0.504 and further preferably may be greater than or equal to 0.56.

[0622] The lean cruise nvPM emissions index ratio may be in the range 0.448 to 0.673, preferably may be in the range 0.504 to 0.617 and further preferably may be in the range 0.560 to 0.561.

[0623] The lean cruise nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or in any range defined between any two of these values.

[0624] The lean cruise nvPM emissions index ratio of the gas turbine engine may be 0.446, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.677, or within any range defined between any two of these values.

[0625] An idle-MTO nvPM emissions index ratio may be defined as:

[0626] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0627] where:
    • [0628] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0629] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0630] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0631] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0632] wherein
  • [0633] the idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0634] The idle-MTO nvPM emissions index ratio may be greater than zero.

[0635] The idle-MTO nvPM emissions index ratio may be less than or equal to 0.8 and preferably may be less than or equal to 0.6, even preferably may be less than or equal to 0.4 and yet even further preferably may be less than or equal to 0.2.

[0636] The idle-MTO nvPM emissions index ratio may be less than or equal to 0.178, preferably may be less than or equal to 0.164 and further preferably may be less than or equal to 0.149.

[0637] The idle-MTO nvPM emissions index ratio may be greater than or equal to 0.118, preferably may be greater than or equal to 0.133 and further preferably may be greater than or equal to 0.148.

[0638] The idle-MTO nvPM emissions index ratio may be in the range 0.118 to 0.178, preferably may be in the range 0.133 to 0.164 and further preferably may be in the range 0.148 to 0.149.

[0639] The idle-MTO nvPM emissions index ratio may be less than 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1, or in any range defined between any two of these values.

[0640] The idle-MTO nvPM emissions index ratio of the gas turbine engine may be 0.118, 0.12, 0.125, 0.13, 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.175, 0.178, or within any range defined between any two of these values.

[0641] A lean cruise/MTO nvPM emissions index ratio may be defined as:

[0642] $\frac{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0643] where:
    • [0644] EI_{cruise(lean),SAF} may be defined as:

[0645] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0646] EI_{cruise(lean),FF} may be defined as:

[0647] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0648] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0649] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0650] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0651] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0652] wherein
  • [0653] the lean cruise/MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0654] The lean cruise/MTO nvPM emissions index ratio may be greater than zero.

[0655] The lean cruise/MTO nvPM emissions index ratio may be less than or equal to 0.99 and preferably may be less than or equal to 0.98.

[0656] The lean cruise/MTO nvPM emissions index ratio may be less than or equal to 0.97, preferably may be less than or equal to 0.961 and further preferably may be less than or equal to 0.873.

[0657] The lean cruise/MTO nvPM emissions index ratio may be greater than or equal to 0.69, preferably may be greater than or equal to 0.776 and further preferably may be greater than or equal to 0.863.

[0658] The lean cruise/MTO nvPM emissions index ratio may be in the range 0.690 to 0.970, preferably may be in the range 0.776 to 0.961 and further preferably may be in the range 0.863 to 0.873.

[0659] The lean cruise/MTO nvPM emissions index ratio may be less than or equal to 0.97, preferably may be less than or equal to 0.955 and further preferably may be less than or equal to 0.868.

[0660] The lean cruise/MTO nvPM emissions index ratio may be greater than or equal to 0.693, preferably may be greater than or equal to 0.78 and further preferably may be greater than or equal to 0.867.

[0661] The lean cruise/MTO nvPM emissions index ratio may be in the range 0.693 to 0.970, preferably may be in the range 0.780 to 0.955 and further preferably may be in the range 0.867 to 0.868.

[0662] The lean cruise/MTO nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0663] The lean cruise/MTO nvPM emissions index ratio of the gas turbine engine 10 may be 0.69, 0.7, 0.725, 0.75, 0.775, 0.8, 0.825, 0.85, 0.875, 0.9, 0.925, 0.95, 0.97, 0.99 or within any range defined between any two of these values.

[0664] An idle/lean cruise nvPM emissions index ratio may be defined as:

[0665] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • [0666] where:
    • [0667] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0668] EI_{cruise(lean),SAF} may be defined as:

[0669] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0670] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF IS calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0671] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0672] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0673] EI_{cruise(lean),FF} may be defined as:

[0674] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0675] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0676] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0677] wherein
    the idle/lean cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0678] The idle/lean cruise nvPM emissions index ratio may be greater than zero.

[0679] The idle/lean cruise nvPM emissions index ratio may be less than or equal to 0.8, preferably may be less than or equal to 0.6, even preferably may be less than or equal to 0.4, and even further preferably may be less than or equal to 0.3.

[0680] The idle/lean cruise nvPM emissions index ratio may be less than or equal to 0.207, preferably may be less than or equal to 0.189 and further preferably may be less than or equal to 0.172.

[0681] The idle/lean cruise nvPM emissions index ratio may be greater than or equal to 0.135, preferably may be greater than or equal to 0.152 and further preferably may be greater than or equal to 0.169.

[0682] The idle/lean cruise nvPM emissions index ratio may be in the range 0.135 to 0.207, preferably may be in the range 0.152 to 0.189 and further preferably may be in the range 0.169 to 0.172.

[0683] The idle/lean cruise nvPM emissions index ratio may be less than or equal to 0.206, preferably may be less than or equal to 0.189 and further preferably may be less than or equal to 0.171.

[0684] The idle/lean cruise nvPM emissions index ratio may be greater than or equal to 0.136, preferably may be greater than or equal to 0.153 and further preferably may be greater than or equal to 0.17.

[0685] The idle/lean cruise nvPM emissions index ratio may be in the range 0.136 to 0.206, preferably may be in the range 0.153 to 0.189 and further preferably may be in the range 0.170 to 0.171.

[0686] The idle/lean cruise nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0687] The idle/lean cruise nvPM emissions index ratio may be 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.175, 0.18, 0.185, 0.19, 0.195, 0.2, 0.205, 0.207 or within any range defined between any two of these values.

[0688] A rich cruise nvPM emissions index ratio may be defined as:

[0689] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • [0690] where:
    • [0691] EI_{cruise(rich),SAF} may be defined as:

[0692] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0693] EI_{cruise(rich),FF} may be defined as:

[0694] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [0695] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0696] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0697] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0698] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein
    • [0699] the rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0700] The rich cruise nvPM emissions index ratio may be greater than zero.

[0701] The rich cruise nvPM emissions index ratio may be less than or equal to 0.8, preferably may be less than or equal to 0.6, even preferably may be less than or equal to 0.4.

[0702] The rich cruise nvPM emissions index ratio may be less than or equal to 0.325, preferably may be less than or equal to 0.298 and further preferably may be less than or equal to 0.271.

[0703] The rich cruise nvPM emissions index ratio may be greater than or equal to 0.18, preferably may be greater than or equal to 0.202 and further preferably may be greater than or equal to 0.225.

[0704] The rich cruise nvPM emissions index ratio may be in the range 0.180 to 0.325, preferably may be in the range 0.202 to 0.298 and further preferably may be in the range 0.225 to 0.271.

[0705] The rich cruise nvPM emissions index ratio may be less than or equal to 0.287, preferably may be less than or equal to 0.263 and further preferably may be less than or equal to 0.239.

[0706] The rich cruise nvPM emissions index ratio may be greater than or equal to 0.19, preferably may be greater than or equal to 0.214 and further preferably may be greater than or equal to 0.238.

[0707] The rich cruise nvPM emissions index ratio may be in the range 0.190 to 0.287, preferably may be in the range 0.214 to 0.263 and further preferably may be in the range 0.238 to 0.239.

[0708] The rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0709] The rich cruise nvPM emissions index ratio of the gas turbine engine may be 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.325 or within any range defined between any two of these values.

[0710] A rich cruise/MTO nvPM emissions index ratio may be defined as:

[0711] $\frac{{\mathrm{EI}}_{cruise\left(ri\mathrm{ch}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{crui\mathrm{se}\left(\mathrm{rich}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0712] where:
    • [0713] EI_{cruise(rich),SAF} may be defined as:

[0714] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0715] and EI_{cruise(rich),FF} may be defined as:

[0716] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • [0717] and where:
    • [0718] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0719] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0720] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0721] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0722] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0723] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0724] wherein
    the rich cruise/MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0725] The rich cruise/MTO nvPM emissions index ratio may be greater than zero.

[0726] The rich cruise/MTO nvPM emissions index ratio may be less than or equal to 0.8, preferably may be less than or equal to 0.7 and even preferably may be less than or equal to 0.6.

[0727] The rich cruise/MTO nvPM emissions index ratio may be less than or equal to 0.503, preferably may be less than or equal to 0.461 and further preferably may be less than or equal to 0.419.

[0728] The rich cruise/MTO nvPM emissions index ratio may be greater than or equal to 0.279, preferably may be greater than or equal to 0.313 and further preferably may be greater than or equal to 0.348.

[0729] The rich cruise/MTO nvPM emissions index ratio may be in the range 0.279 to 0.503, preferably may be in the range 0.313 to 0.461 and further preferably may be in the range 0.348 to 0.419.

[0730] The rich cruise/MTO nvPM emissions index ratio may be less than or equal to 0.444, preferably may be less than or equal to 0.407 and further preferably may be less than or equal to 0.37.

[0731] The rich cruise/MTO nvPM emissions index ratio may be greater than or equal to 0.295, preferably may be greater than or equal to 0.332 and further preferably may be greater than or equal to 0.369.

[0732] The rich cruise/MTO nvPM emissions index ratio may be in the range 0.295 to 0.444, preferably may be in the range 0.332 to 0.407 and further preferably may be in the range 0.369 to 0.370.

[0733] The rich cruise/MTO nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0734] The rich cruise/MTO nvPM emissions index ratio of the gas turbine engine may be 0.279, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.503, or within any range defined between any two of these values.

[0735] An idle/rich cruise nvPM emissions index ratio may be defined as:

[0736] $\frac{{\mathrm{EI}}_{i\mathrm{dle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • [0737] where:
    • [0738] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
    • [0739] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0740] EI_{cruise(rich),SAF} may be defined as:

[0741] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0742] EI_{cruise(rich),FF} may be defined as:

[0743] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [0744] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0745] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0746] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0747] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein
  • [0748] the idle/rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0749] The idle/rich cruise nvPM emissions index ratio may be greater than zero.

[0750] The idle/rich cruise nvPM emissions index ratio may be less than or equal to 0.9 and preferably may be less than or equal to 0.8 and further preferably may be less than or equal to 0.7.

[0751] The idle/rich cruise nvPM emissions index ratio may be less than or equal to 0.511, preferably may be less than or equal to 0.468 and further preferably may be less than or equal to 0.426.

[0752] The idle/rich cruise nvPM emissions index ratio may be greater than or equal to 0.283, preferably may be greater than or equal to 0.319 and further preferably may be greater than or equal to 0.354.

[0753] The idle/rich cruise nvPM emissions index ratio may be in the range 0.283 to 0.511, preferably may be in the range 0.319 to 0.468 and further preferably may be in the range 0.354 to 0.426.

[0754] The idle/rich cruise nvPM emissions index ratio may be less than or equal to 0.482, preferably may be less than or equal to 0.442 and further preferably may be less than or equal to 0.402.

[0755] The idle/rich cruise nvPM emissions index ratio may be greater than or equal to 0.321, preferably may be greater than or equal to 0.361 and further preferably may be greater than or equal to 0.401.

[0756] The idle/rich cruise nvPM emissions index ratio may be in the range 0.321 to 0.482, preferably may be in the range 0.361 to 0.442 and further preferably may be in the range 0.401 to 0.402.

[0757] The idle/rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[0758] The idle/rich cruise nvPM emissions index ratio of the gas turbine engine may be 0.283, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.511, or within any range defined between any two of these values.

[0759] According to a thirty-sixth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0760] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0761] an idle-MTO nvPM emissions index ratio may be defined as:

[0762] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0763] where:
      • [0764] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0765] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0766] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0767] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which
    • [0768] EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0769] the idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0770] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0771] The idle-MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0772] According to a thirty-seventh aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0773] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6;
    wherein:
  • [0774] a lean cruise/MTO nvPM emissions index ratio may be defined as:

[0775] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0776] where:
      • [0777] EI_{cruise(lean),SAF} may be defined as:

[0778] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • • [0779] EI_{cruise(lean),FF} may be defined as:

[0780] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • • [0781] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0782] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0783] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0784] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which
    • [0785] EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0786] the lean cruise/MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0787] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0788] The lean cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0789] According to a thirty-eighth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0790] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0791] an idle/lean cruise nvPM emissions index ratio may be defined as:

[0792] $\frac{{\mathrm{EI}}_{i\mathrm{dle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{i\mathrm{dle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • • [0793] where:
      • [0794] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0795] EI_{cruise(lean),SAF} may be defined as:

[0796] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • • [0797] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0798] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0799] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0800] EI_{cruise(lean),FF} may be defined as:

[0801] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • • [0802] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0803] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0804] the idle/lean cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0805] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0806] The idle/lean cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0807] According to a thirty-ninth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0808] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0809] a rich cruise nvPM emissions index ratio may be defined as:

[0810] $\frac{{\mathrm{EI}}_{c\mathrm{ruise}\left(ri\mathrm{ch}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{c\mathrm{ruise}\left(ri\mathrm{ch}\right),\mathrm{FF}}}$

• • • [0811] where:
      • [0812] EI_{cruise(rich),SAF} may be defined as:

[0813] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [0814] EI_{cruise(rich),FF} may be defined as:

[0815] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • • [0816] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0817] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0818] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0819] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0820] the rich cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0821] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0822] The rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0823] According to a fortieth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0824] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0825] a rich cruise/MTO nvPM emissions index ratio may be defined as:

[0826] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0827] where:
      • [0828] EI_{cruise(rich),SAF} may be defined as:

[0829] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [0830] and EI_{cruise(rich),FF} may be defined as:

[0831] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [0832] and where:
      • [0833] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0834] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0835] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0836] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0837] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0838] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0839] the rich cruise/MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0840] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0841] The rich cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0842] According to a forty-first aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [0843] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0844] an idle/rich cruise nvPM emissions index ratio may be defined as:

[0845] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • • [0846] where:
      • [0847] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
      • [0848] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0849] EI_{cruise(rich),SAF} may be defined as:

[0850] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [0851] EI_{cruise(rich),FF} may be defined as:

[0852] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • • [0853] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0854] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0855] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [0856] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0857] the idle/rich cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
    the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0858] The idle/rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0859] According to a forty-second aspect, there is provided a method of operating the gas turbine engine of any one or more of the thirty-fifth, thirty-sixth, thirty-seventh, thirty-eighth, thirty-ninth, fortieth or forty-first aspects, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[0860] According to a forty-third aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0861] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6;
  • [0862] wherein:
  • [0863] a lean cruise nvPM emissions index ratio may be defined as:

[0864] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • • [0865] where:
    • [0866] EI_{cruise(lean),SAF} may be defined as:

[0867] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0868] EI_{cruise(lean),FF} may be defined as:

[0869] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0870] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0871] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0872] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0873] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0874] the lean cruise nvPM emissions index ratio is less than 1; and
  • [0875] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0876] The lean cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0877] An idle-MTO nvPM emissions index ratio may be defined as:

[0878] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0879] where:
    • [0880] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0881] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0882] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0883] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and the idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0884] The idle-MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0885] A lean cruise/MTO nvPM emissions index ratio may be defined as:

[0886] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0887] where:
    • [0888] EI_{cruise(lean),SAF} may be defined as:

[0889] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0890] EI_{cruise(lean),FF} may be defined as:

[0891] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0892] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0893] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0894] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0895] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0896] wherein
    the lean cruise/MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0897] The lean cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0898] An idle/lean cruise nvPM emissions index ratio may be defined as:

[0899] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • [0900] where:
    • [0901] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0902] EI_{cruise(lean),SAF} may be defined as:

[0903] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • [0904] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0905] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0906] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0907] EI_{cruise(lean),FF} may be defined as:

[0908] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • [0909] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0910] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [0911] wherein
    the idle/lean cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0912] The idle/lean cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0913] A rich cruise nvPM emissions index ratio may be defined as:

[0914] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • [0915] where:
    • [0916] EI_{cruise(rich),SAF} may be defined as:

[0917] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0918] EI_{cruise(rich),FF} may be defined as:

[0919] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [0920] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0921] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0922] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
    • [0923] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and wherein
      the rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0924] The rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0925] A rich cruise/MTO nvPM emissions index ratio may be defined as:

[0926] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [0927] where:
    • [0928] EI_{cruise(rich),SAF} may be defined as:

[0929] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0930] and EI_{cruise(rich),FF} may be defined as:

[0931] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • [0932] and where:
    • [0933] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0934] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0935] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0936] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0937] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0938] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      the rich cruise/MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0939] The rich cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0940] An idle/rich cruise nvPM emissions index ratio may be defined as:

[0941] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • [0942] where:
    • [0943] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for other different operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
    • [0944] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0945] EI_{cruise(rich),SAF} may be defined as:

[0946] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • [0947] EI_{cruise(rich),FF} may be defined as:

[0948] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [0949] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
    • [0950] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
    • [0951] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
    • [0952] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      the idle/rich cruise nvPM emissions index ratio of the gas turbine engine may be less than 1.

[0953] The idle/rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0954] According to a forty-fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0955] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0956] an idle-MTO nvPM emissions index ratio may be defined as:

[0957] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0958] where:
      • [0959] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0960] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0961] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0962] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0963] the idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0964] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0965] The idle-MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0966] According to a forty-fifth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0967] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0968] a lean cruise/MTO nvPM emissions index ratio may be defined as:

[0969] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{lean}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [0970] where:
      • [0971] EI_{cruise(lean),SAF} may be defined as:

[0972] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • • [0973] EI_{cruise(lean),FF} may be defined as:

[0974] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • • [0975] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0976] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0977] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0978] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_maxTO,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0979] the lean cruise/MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0980] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0981] The lean cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[0982] According to a forty-sixth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [0983] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [0984] an idle/lean cruise nvPM emissions index ratio may be defined as:

[0985] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right),\mathrm{FF}}}$

• • • [0986] where:
      • [0987] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0988] EI_{cruise(lean),SAF} may be defined as:

[0989] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{2}$

• • • • [0990] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0991] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [0992] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [0993] EI_{cruise(lean),FF} may be defined as:

[0994] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}{2}$

• • • • [0995] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [0996] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [0997] the idle/lean cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [0998] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[0999] The idle/lean cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[1000] According to a forty-seventh aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1001] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [1002] a rich cruise nvPM emissions index ratio may be defined as:

[1003] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • • [1004] where:
      • [1005] EI_{cruise(rich),SAF} may be defined as:

[1006] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [1007] EI_{cruise(rich),FF} may be defined as:

[1008] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • • [1009] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [1010] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [1011] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
      • [1012] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1013] the rich cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1014] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1015] The rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[1016] According to a forty-eighth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1017] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [1018] a rich cruise/MTO nvPM emissions index ratio may be defined as:

[1019] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [1020] where:
      • [1021] EI_{cruise(rich),SAF} may be defined as:

[1022] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [1023] and EI_{cruise(rich),FF} may be defined as:

[1024] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • [1025] and where:
      • [1026] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [1027] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [1028] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [1029] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [1030] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [1031] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1032] the rich cruise/MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1033] the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1034] The rich cruise/MTO nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[1035] According to a forty-ninth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1036] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber, wherein the plurality of fuel spray nozzles comprises a first subset of fuel spray nozzles and a second subset of fuel spray nozzles, wherein the combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6; wherein:
  • [1037] an idle/rich cruise nvPM emissions index ratio may be defined as:

[1038] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right),\mathrm{FF}}}$

• • • [1039] where:
      • [1040] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for given operating conditions and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
      • [1041] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [1042] EI_{cruise(rich),SAF} may be defined as:

[1043] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{2}$

• • • • [1044] EI_{cruise(rich),FF} may be defined as:

[1045] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}+{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}{2}$

• • • • [1046] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel;
      • [1047] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
      • [1048] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel; and
      • [1049] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_idle,SAF is calculated if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1050] the idle/rich cruise nvPM emissions index ratio of the gas turbine engine is less than 1; and
    the method comprises providing fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1051] The idle/rich cruise nvPM emissions index ratio may be as defined above in connection with the thirty-fifth aspect.

[1052] The following statements may apply to any of the first to forty-ninth aspects:

[1053] The ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles may be in the range of 1:4 to 1:5.

[1054] The ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles may be in the range of 1:4.25 to 1:4.75.

[1055] The first subset of fuel spray nozzles may include between 1 and 10 fuel spray nozzles.

[1056] The first subset of fuel spray nozzles may include between 3 and 5 fuel spray nozzles.

[1057] The second subset of fuel spray nozzles may include between 10 and 25 fuel spray nozzles.

[1058] The second subset of fuel spray nozzles may include between 14 and 22 fuel spray nozzles.

[1059] The second subset of fuel spray nozzles may include between 16 and 20 fuel spray nozzles.

[1060] The combustor may comprise one or more ignitors.

[1061] Each of the first subset of fuel spray nozzles may be located nearer a respective one or more of the ignitors than the second subset,

[1062] One or more of the ignitors may be arranged diametrically opposite another one or more of the ignitors.

[1063] The fuel provided to the plurality of fuel spray nozzles may comprise a % SAF in the range of 50% to 100%.

[1064] The fuel provided to the plurality of fuel spray nozzles may comprise a % SAF in the range of 70% to 100%.

[1065] The fuel provided to the plurality of fuel spray nozzles may comprise a % SAF in the range of 90% to 100%.

[1066] According to a fiftieth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1067] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber;
    wherein:
  • [1068] a first idle-MTO nvPM emissions index ratio may be defined as:

[1069] $\frac{{\mathrm{EI}}_{\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}}$

• • • [1070] where:
    • [1071] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
    • [1072] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
  • [1073] the first idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 2; and
  • [1074] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1075] The first idle-MTO nvPM emissions index ratio of the gas turbine engine may be greater than zero.

[1076] The first idle-MTO nvPM emissions index ratio may be less than 1.26 and preferably may be less than 1.16 and more preferably may be less than 1.05.

[1077] The first idle-MTO nvPM emissions index ratio may be less than 1.15 and preferably may be less than 1.05 and more preferably may be less than 0.951.

[1078] The first idle-MTO nvPM emissions index ratio may be less than 0.96 and preferably may be less than 0.88 and more preferably may be less than 0.8.

[1079] The first idle-MTO nvPM emissions index ratio may be less than 0.791 and preferably may be less than 0.725 and more preferably may be less than 0.659.

[1080] The first idle-MTO nvPM emissions index ratio may be less than 0.415 and preferably may be less than 0.381 and more preferably may be less than 0.346.

[1081] The first idle-MTO nvPM emissions index ratio may be less than 1.5 and preferably may be less than 1 and more preferably may be less than 0.5.

[1082] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.187 and preferably may be less than or equal to 0.171 and more preferably may be less than or equal to 0.156.

[1083] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.143 and preferably may be less than or equal to 0.131 and more preferably may be less than or equal to 0.119.

[1084] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.170 and preferably may be less than or equal to 0.156 and more preferably may be less than or equal to 0.142.

[1085] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.0615 and preferably may be less than or equal to 0.0564 and more preferably may be less than or equal to 0.0513.

[1086] The first idle-MTO nvPM emissions index ratio may be less than or equal to 0.118 and preferably may be less than or equal to 0.108 and more preferably may be less than or equal to 0.0978.

[1087] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.0409 and preferably may be greater than or equal to 0.0461 and more preferably may be greater than or equal to 0.0512.

[1088] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.0956 and preferably may be greater than or equal to 0.107 and more preferably may be greater than or equal to 0.119.

[1089] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.0509 and preferably may be greater than or equal to 0.0573 and more preferably may be greater than or equal to 0.0636.

[1090] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.124 and preferably may be greater than or equal to 0.139 and more preferably may be greater than or equal to 0.155.

[1091] The first idle-MTO nvPM emissions index ratio may be greater than or equal to 0.0524 and preferably may be greater than or equal to 0.059 and more preferably may be greater than or equal to 0.0656.

[1092] The first idle-MTO nvPM emissions index ratio may be in the range 0.0409 to 0.187 and preferably may be in the range 0.0461 to 0.171 and more preferably may be in the range 0.0512 to 0.156.

[1093] The first idle-MTO nvPM emissions index ratio may be in the range 0.0956 to 0.187 and preferably may be in the range 0.107 to 0.171 and more preferably may be in the range 0.119 to 0.156.

[1094] The first idle-MTO nvPM emissions index ratio may be in the range 0.0509 to 0.143 and preferably may be in the range 0.0573 to 0.131 and more preferably may be in the range 0.0636 to 0.119.

[1095] The first idle-MTO nvPM emissions index ratio may be in the range 0.0956 to 0.17 and preferably may be in the range 0.107 to 0.156 and more preferably may be in the range 0.119 to 0.142.

[1096] The first idle-MTO nvPM emissions index ratio may be in the range 0.124 to 0.187 and preferably may be in the range 0.139 to 0.171 and more preferably may be in the range 0.155 to 0.156.

[1097] The first idle-MTO nvPM emissions index ratio may be in the range 0.0409 to 0.0615 and preferably may be in the range 0.0461 to 0.0564 and more preferably may be in the range 0.0512 to 0.0513.

[1098] The first idle-MTO nvPM emissions index ratio may be in the range 0.0524 to 0.118 and preferably may be in the range 0.059 to 0.108 and more preferably may be in the range 0.0656 to 0.0978.

[1099] The first idle-MTO nvPM emissions index ratio may be less than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, or in any range defined between any two of these values.

[1100] A second idle-MTO nvPM emissions index ratio may be defined as:

[1101] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{MaxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [1102] where:
    • [1103] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1104] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1105] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [1106] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
  • [1107] and wherein the second idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[1108] The second idle-MTO nvPM emissions index ratio may be greater than zero.

[1109] The second idle-MTO nvPM emissions index ratio may be less than or equal to 0.8 and preferably may be less than or equal to 0.6 and more preferably may be less than or equal to 0.4.

[1110] The second idle-MTO nvPM emissions index ratio may be less than or equal to 0.178 and preferably may be less than or equal to 0.164 and more preferably may be less than or equal to 0.149.

[1111] The second idle-MTO nvPM emissions index ratio may be greater than or equal to 0.03 and preferably may be greater than or equal to 0.06 and more preferably may be greater than or equal to 0.09.

[1112] The second idle-MTO nvPM emissions index ratio may be greater than or equal to 0.118 and preferably may be greater than or equal to 0.133 and more preferably may be greater than or equal to 0.148.

[1113] The second idle-MTO nvPM emissions index ratio may be in the range 0.118 to 0.178 and preferably may be in the range 0.133 to 0.164 and more preferably may be in the range 0.148 to 0.149.

[1114] The second idle-MTO nvPM emissions index ratio may be less than 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1, or in any range defined between any two of these values.

[1115] According to a fifty-first aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1116] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1117] a second idle-MTO nvPM emissions index ratio may be defined as:

[1118] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{MaxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [1119] where:
    • [1120] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1121] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1122] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [1123] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
  • [1124] the second idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1125] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1126] The second idle-MTO nvPM emissions index ratio defined in the fifty-first aspect may be as defined above in connection with the fiftieth aspect.

[1127] According to a fifty-second aspect, there is provided a method of operating the gas turbine engine of the fiftieth aspect or the fifty-first aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1128] According to a fifty-third aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1129] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1130] a first idle-MTO nvPM emissions index ratio may be defined as:

[1131] $\frac{{\mathrm{EI}}_{\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}}$

• • • [1132] where:
      • [1133] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
      • [1134] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
  • [1135] the first idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 2; and
  • [1136] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1137] The first idle-MTO nvPM emissions index ratio may be as defined above in connection with the fiftieth aspect.

[1138] A second idle-MTO nvPM emissions index ratio may be defined as:

[1139] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • [1140] where:
    • [1141] around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1142] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
    • [1143] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
    • [1144] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel; and
  • [1145] wherein the second idle-MTO nvPM emissions index ratio of the gas turbine engine may be less than 1.

[1146] The second idle-MTO nvPM emissions index ratio may be as defined above in connection with the fiftieth aspect.

[1147] According to a fifty-fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1148] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1149] a second idle-MTO nvPM emissions index ratio may be defined as:

[1150] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}/{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [1151] where:
      • [1152] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions and if a fuel provided to the combustor comprises a sustainable aviation fuel;
      • [1153] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor comprises a sustainable aviation fuel;
      • [1154] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
      • [1155] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the same given operating conditions at which EI_idle,SAF is calculated, and if a fuel provided to the combustor is a fossil-based hydrocarbon fuel;
  • [1156] the second idle-MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1157] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1158] The second idle-MTO nvPM emissions index ratio may be as defined above in connection with the fiftieth aspect.

[1159] According to a fifty-fifth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1160] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1161] a fuel-flow nvPM emissions index ratio may be defined as:

[1162] $\frac{{\mathrm{EI}}_{\mathrm{idle}}\times W_{f,\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}\times W_{f,\mathrm{maxTO}}}$

• • • [1163] where:
      • [1164] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions;
      • [1165] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
      • [1166] W_f,idle is the rate of fuel flow to the fuel spray nozzles in kg/s at around 7% available thrust for the given operating conditions; and
      • [1167] W_f,maxTO is the rate of fuel flow to the fuel spray nozzles in kg/s at around 100% available thrust for the given operating conditions;
  • [1168] the fuel-flow nvPM emissions index ratio of the gas turbine engine is less than 0.2; and
  • [1169] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1170] The fuel-flow nvPM emissions index ratio may be less than 0.13 and preferably may be less than 0.119 and more preferably may be less than 0.108.

[1171] The fuel-flow nvPM emissions index ratio may be less than 0.091 and preferably may be less than 0.0834 and more preferably may be less than 0.0758.

[1172] The fuel-flow nvPM emissions index ratio may be less than 0.123 and preferably may be less than 0.113 and more preferably may be less than 0.103.

[1173] The fuel-flow nvPM emissions index ratio may be less than 0.0819 and preferably may be less than 0.075 and more preferably may be less than 0.0682.

[1174] The fuel-flow nvPM emissions index ratio may be less than 0.0357 and preferably may be less than 0.0327 and more preferably may be less than 0.0298.

[1175] The fuel-flow nvPM emissions index ratio may be less than 0.17 and preferably may be less than 0.15 and more preferably may be less than 0.12.

[1176] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0193 and preferably may be less than or equal to 0.0177 and more preferably may be less than or equal to 0.0161.

[1177] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0135 and preferably may be less than or equal to 0.0124 and more preferably may be less than or equal to 0.0113.

[1178] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0182 and preferably may be less than or equal to 0.0167 and more preferably may be less than or equal to 0.0152.

[1179] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.00529 and preferably may be less than or equal to 0.00485 and more preferably may be less than or equal to 0.00441.

[1180] The fuel-flow nvPM emissions index ratio may be less than or equal to 0.0122 and preferably may be less than or equal to 0.0112 and more preferably may be less than or equal to 0.0102.

[1181] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.00352 and preferably may be greater than or equal to 0.00396 and more preferably may be greater than or equal to 0.0044.

[1182] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.0107 and preferably may be greater than or equal to 0.012 and more preferably may be greater than or equal to 0.0133.

[1183] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.00561 and preferably may be greater than or equal to 0.00631 and more preferably may be greater than or equal to 0.00701.

[1184] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.0128 and preferably may be greater than or equal to 0.0144 and more preferably may be greater than or equal to 0.016.

[1185] The fuel-flow nvPM emissions index ratio may be greater than or equal to 0.00564 and preferably may be greater than or equal to 0.00635 and more preferably may be greater than or equal to 0.00705.

[1186] The fuel-flow nvPM emissions index ratio may be in the range 0.00352 to 0.0193 and preferably may be in the range 0.00396 to 0.0177 and more preferably may be in the range 0.00440 to 0.0161.

[1187] The fuel-flow nvPM emissions index ratio may be in the range 0.0107 to 0.0193 and preferably may be in the range 0.0120 to 0.0177 and more preferably may be in the range 0.0133 to 0.0161.

[1188] The fuel-flow nvPM emissions index ratio may be in the range 0.00561 to 0.0135 and preferably may be in the range 0.00631 to 0.0124 and more preferably may be in the range 0.00701 to 0.0113.

[1189] The fuel-flow nvPM emissions index ratio may be in the range 0.0107 to 0.0182 and preferably may be in the range 0.0120 to 0.0167 and more preferably may be in the range 0.0133 to 0.0152.

[1190] The fuel-flow nvPM emissions index ratio may be in the range 0.0128 to 0.0193 and preferably may be in the range 0.0144 to 0.0177 and more preferably may be in the range 0.0160 to 0.0161.

[1191] The fuel-flow nvPM emissions index ratio may be in the range 0.00352 to 0.00529 and preferably may be in the range 0.00396 to 0.00485 and more preferably may be in the range 0.00440 to 0.00441.

[1192] The fuel-flow nvPM emissions index ratio may be in the range 0.00564 to 0.0122 and preferably may be in the range 0.00635 to 0.0112 and more preferably may be in the range 0.00705 to 0.0102.

[1193] The fuel-flow nvPM emissions index ratio may be less than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2 or may be in any range defined between any two of these values.

[1194] W_f,idle in kg/s may be in the range 0.185 to 0.362 and preferably may be in the range 0.209 to 0.331 and more preferably may be in the range 0.232 to 0.301.

[1195] W_f,idle in kg/s may be in the range 0.215 to 0.362 and preferably may be in the range 0.242 to 0.331 and more preferably may be in the range 0.269 to 0.301.

[1196] W_f,idle in kg/s may be in the range 0.215 to 0.349 and preferably may be in the range 0.242 to 0.320 and more preferably may be in the range 0.269 to 0.291.

[1197] W_f,idle in kg/s may be in the range 0.185 to 0.324 and preferably may be in the range 0.209 to 0.297 and more preferably may be in the range 0.232 to 0.270.

[1198] W_f,idle in kg/s may be in the range 0.215 to 0.336 and preferably may be in the range 0.242 to 0.308 and more preferably may be in the range 0.269 to 0.280.

[1199] W_f,idle in kg/s may be in the range 0.232 to 0.349 and preferably may be in the range 0.261 to 0.320 and more preferably may be in the range 0.290 to 0.291.

[1200] W_f,idle in kg/s may be in the range 0.24 to 0.362 and preferably may be in the range 0.27 to 0.331 and more preferably may be in the range 0.300 to 0.301.

[1201] W_f,idle in kg/s may be in the range 0.196 to 0.311 and preferably may be in the range 0.220 to 0.285 and more preferably may be in the range 0.245 to 0.259.

[1202] W_f,maxTO in kg/s may be in the range 1.68 to 4.20 and preferably may be in the range 1.89 to 3.85 and more preferably may be in the range 2.10 to 3.50.

[1203] W_f,maxTO in kg/s may be in the range 1.92 to 4.20 and preferably may be in the range 2.16 to 3.85 and more preferably may be in the range 2.41 to 3.50.

[1204] W_f,maxTO in kg/s may be in the range 1.92 to 3.39 and preferably may be in the range 2.16 to 3.11 and more preferably may be in the range 2.41 to 2.82.

[1205] W_f,maxTO in kg/s may be in the range 1.68 to 3.45 and preferably may be in the range 1.89 to 3.16 and more preferably may be in the range 2.10 to 2.88.

[1206] W_f,maxTO in kg/s may be in the range 1.92 to 3.13 and preferably may be in the range 2.16 to 2.87 and more preferably may be in the range 2.41 to 2.61.

[1207] W_f,maxTO in kg/s may be in the range 2.25 to 3.39 and preferably may be in the range 2.53 to 3.11 and more preferably may be in the range 2.81 to 2.82.

[1208] W_f,maxTO in kg/s may be in the range 2.79 to 4.20 and preferably may be in the range 3.14 to 3.85 and more preferably may be in the range 3.49 to 3.50.

[1209] W_f,maxTO in kg/s may be in the range 1.83 to 3.01 and preferably may be in the range 2.05 to 2.76 and more preferably may be in the range 2.28 to 2.51.

[1210] According to a fifty-sixth aspect, there is provided a method of operating the gas turbine engine of the fifty-fifth aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1211] According to a fifty-seventh aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1212] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1213] a fuel-flow nvPM emissions index ratio may be defined as:

[1214] $\frac{{\mathrm{EI}}_{\mathrm{idle}}\times W_{f,\mathrm{idle}}}{{\mathrm{EI}}_{\mathrm{maxTO}}\times W_{f,\mathrm{maxTO}}}$

• • • [1215] where:
      • [1216] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
      • [1217] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
      • [1218] W_f,idle is the rate of fuel flow to the fuel spray nozzles in kg/s at around 7% available thrust for the given operating conditions; and
      • [1219] W_f,maxTO is the rate of fuel flow to the fuel spray nozzles in kg/s at around 100% available thrust for the given operating conditions;
  • [1220] the fuel-flow nvPM emissions index ratio of the gas turbine engine is less than 0.2; and
  • [1221] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1222] Any of the fuel-flow nvPM emissions index ratio, W_f,idle and W_f,maxTO may be as defined above in connection with the fifty-fifth aspect.

[1223] According to a fifty-eighth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1224] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1225] a thrust nvPM emissions index ratio may be defined as:

[1226] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}/F_{\mathrm{maxTO}}}{{\mathrm{EI}}_{\mathrm{idle}}/F_{\mathrm{idle}}}$

• • • [1227] where:
      • [1228] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions;
      • [1229] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
      • [1230] F_maxTO is the thrust of the gas turbine engine at around 100% available thrust in kN for the given operating conditions; and
      • [1231] F_idle is the thrust of the gas turbine engine at around 7% available thrust in kN for the given operating conditions;
  • [1232] the thrust nvPM emissions index ratio is greater than 0.05; and
  • [1233] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1234] The thrust nvPM emissions index ratio may be greater than 0.0535 and preferably may be greater than 0.0602 and more preferably may be greater than 0.0669.

[1235] The thrust nvPM emissions index ratio may be greater than 0.07 and preferably may be greater than 0.0788 and more preferably may be greater than 0.0875.

[1236] The thrust nvPM emissions index ratio may be greater than 0.0588 and preferably may be greater than 0.0662 and more preferably may be greater than 0.0736.

[1237] The thrust nvPM emissions index ratio may be greater than 0.162 and preferably may be greater than 0.182 and more preferably may be greater than 0.202.

[1238] The thrust nvPM emissions index ratio may be greater than 0.0849 and preferably may be greater than 0.0956 and more preferably may be greater than 0.106.

[1239] The thrust nvPM emissions index ratio may be greater than or equal to 0.1 and preferably may be greater than or equal to 0.2 and more preferably may be greater than or equal to 0.3.

[1240] The thrust nvPM emissions index ratio may be greater than or equal to 0.36 and preferably may be greater than or equal to 0.405 and more preferably may be greater than or equal to 0.451.

[1241] The thrust nvPM emissions index ratio may be greater than or equal to 0.472 and preferably may be greater than or equal to 0.531 and more preferably may be greater than or equal to 0.59.

[1242] The thrust nvPM emissions index ratio may be greater than or equal to 0.397 and preferably may be greater than or equal to 0.446 and more preferably may be greater than or equal to 0.496.

[1243] The thrust nvPM emissions index ratio may be greater than or equal to 1.09 and preferably may be greater than or equal to 1.22 and more preferably may be greater than or equal to 1.36.

[1244] The thrust nvPM emissions index ratio may be greater than or equal to 0.572 and preferably may be greater than or equal to 0.644 and more preferably may be greater than or equal to 0.716.

[1245] The thrust nvPM emissions index ratio may be less than or equal to 1.64 and preferably may be less than or equal to 1.51 and more preferably may be less than or equal to 1.37.

[1246] The thrust nvPM emissions index ratio may be less than or equal to 0.703 and preferably may be less than or equal to 0.645 and more preferably may be less than or equal to 0.586.

[1247] The thrust nvPM emissions index ratio may be less than or equal to 1.32 and preferably may be less than or equal to 1.21 and more preferably may be less than or equal to 1.1.

[1248] The thrust nvPM emissions index ratio may be less than or equal to 0.542 and preferably may be less than or equal to 0.497 and more preferably may be less than or equal to 0.452.

[1249] The thrust nvPM emissions index ratio may be less than or equal to 1.64 and preferably may be less than or equal to 1.51 and more preferably may be less than or equal to 1.37.

[1250] The thrust nvPM emissions index ratio may be less than or equal to 1.29 and preferably may be less than or equal to 1.18 and more preferably may be less than or equal to 1.07.

[1251] The thrust nvPM emissions index ratio may be greater than 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, 2 or any range defined between any two of these values.

[1252] The thrust nvPM emissions index ratio may be in the range 0.36 to 1.64 and preferably may be in the range 0.405 to 1.51 and more preferably may be in the range 0.451 to 1.37.

[1253] The thrust nvPM emissions index ratio may be in the range 0.36 to 0.703 and preferably may be in the range 0.405 to 0.645 and more preferably may be in the range 0.451 to 0.586.

[1254] The thrust nvPM emissions index ratio may be in the range 0.472 to 1.32 and preferably may be in the range 0.531 to 1.21 and more preferably may be in the range 0.59 to 1.1.

[1255] The thrust nvPM emissions index ratio may be in the range 0.397 to 0.703 and preferably may be in the range 0.446 to 0.645 and more preferably may be in the range 0.496 to 0.586.

[1256] The thrust nvPM emissions index ratio may be in the range 0.360 to 0.542 and preferably may be in the range 0.405 to 0.497 and more preferably may be in the range 0.451 to 0.452.

[1257] The thrust nvPM emissions index ratio may be in the range 1.09 to 1.64 and preferably may be in the range 1.22 to 1.51 and more preferably may be in the range 1.36 to 1.37.

[1258] The thrust nvPM emissions index ratio may be in the range 0.572 to 1.29 and preferably may be in the range 0.644 to 1.18 and more preferably may be in the range 0.716 to 1.07.

[1259] F_maxTO in kN may be in the range 229 to 525 and preferably may be in the range 258 to 481 and more preferably may be in the range 287 to 437.

[1260] F_maxTO in kN may be in the range 267 to 525 and preferably may be in the range 300 to 481 and more preferably may be in the range 334 to 437.

[1261] F_maxTO in kN may be in the range 267 to 455 and preferably may be in the range 300 to 417 and more preferably may be in the range 334 to 380.

[1262] F_maxTO in kN may be in the range 229 to 437 and preferably may be in the range 258 to 401 and more preferably may be in the range 287 to 364.

[1263] F_maxTO in kN may be in the range 267 to 427 and preferably may be in the range 300 to 391 and more preferably may be in the range 334 to 356.

[1264] F_maxTO in kN may be in the range 303 to 455 and preferably may be in the range 341 to 417 and more preferably may be in the range 379 to 380.

[1265] F_maxTO in kN may be in the range 349 to 525 and preferably may be in the range 393 to 481 and more preferably may be in the range 436 to 437.

[1266] F_maxTO in kN may be in the range 246 to 394 and preferably may be in the range 277 to 361 and more preferably may be in the range 308 to 328.

[1267] F_idle in kN may be in the range 16.0 to 36.7 and preferably may be in the range 18.0 to 33.7 and more preferably may be in the range 20.0 to 30.6.

[1268] F_idle in kN may be in the range 18.7 to 36.7 and preferably may be in the range 21.0 to 33.7 and more preferably may be in the range 23.3 to 30.6.

[1269] F_idle in kN may be in the range 18.7 to 31.9 and preferably may be in the range 21.0 to 29.2 and more preferably may be in the range 23.3 to 26.6.

[1270] F_idle in kN may be in the range 16.0 to 30.6 and preferably may be in the range 18.0 to 28.1 and more preferably may be in the range 20.0 to 25.5.

[1271] F_idle in kN may be in the range 18.7 to 29.9 and preferably may be in the range 21.0 to 27.4 and more preferably may be in the range 23.3 to 24.9.

[1272] F_idle in kN may be in the range 21.2 to 31.9 and preferably may be in the range 23.8 to 29.2 and more preferably may be in the range 26.5 to 26.6.

[1273] F_idle in kN may be in the range 24.4 to 36.7 and preferably may be in the range 27.5 to 33.7 and more preferably may be in the range 30.5 to 30.6.

[1274] F_idle in kN may be in the range 17.2 to 27.6 and preferably may be in the range 19.4 to 25.3 and more preferably may be in the range 21.6 to 23.0.

[1275] According to a fifty-ninth aspect, there is provided a method of operating the gas turbine engine of the fifty-eighth aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1276] According to a sixtieth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1277] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1278] a thrust nvPM emissions index ratio may be defined as:

[1279] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}/F_{\mathrm{maxTO}}}{{\mathrm{EI}}_{\mathrm{idle}}/F_{\mathrm{idle}}}$

• • • [1280] where:
      • [1281] EI_idle is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 7% available thrust for given operating conditions; and
      • [1282] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions;
      • [1283] F_maxTO is the thrust of the gas turbine engine at around 100% available thrust in kN for the given operating conditions,
      • [1284] F_idle is the thrust of the gas turbine engine at around 7% available thrust in kN for the given operating conditions; and
  • [1285] the thrust nvPM emissions index ratio is greater than 0.05; and
  • [1286] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1287] Any of the thrust nvPM emissions index ratio, F_maxTO and F_idle may be as defined in connection with the fifty-eighth aspect.

[1288] According to a sixty-first aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1289] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1290] a lean cruise-MTO nvPM emissions index ratio may be defined as:

[1291] $\frac{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [1292] where:
      • [1293] EI_{cruise (lean)} may be defined as:

[1294] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}+{\mathrm{EI}}_{\mathrm{climb}}}{2}$

• • • • [1295] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions;
      • [1296] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions; and
      • [1297] BPR is the bypass ratio of the gas turbine engine;
  • [1298] the lean cruise-MTO nvPM emissions index ratio is less than 0.2; and
  • [1299] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1300] The lean cruise-MTO nvPM emissions index ratio may be less than 0.194 and preferably may be less than 0.178 and more preferably may be less than 0.162.

[1301] The lean cruise-MTO nvPM emissions index ratio may be less than 0.19 and preferably may be less than 0.174 and more preferably may be less than 0.158.

[1302] The lean cruise-MTO nvPM emissions index ratio may be less than 0.181 and preferably may be less than 0.166 and more preferably may be less than 0.151.

[1303] The lean cruise-MTO nvPM emissions index ratio may be less than 0.172 and preferably may be less than 0.158 and more preferably may be less than 0.144.

[1304] The lean cruise-MTO nvPM emissions index ratio may be less than 0.14 and preferably may be less than 0.128 and more preferably may be less than 0.117.

[1305] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.18 and preferably may be less than or equal to 0.16 and more preferably may be less than or equal to 0.14.

[1306] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.16 and preferably may be less than or equal to 0.147 and more preferably may be less than or equal to 0.134.

[1307] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.151 and preferably may be less than or equal to 0.138 and more preferably may be less than or equal to 0.126.

[1308] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.145 and preferably may be less than or equal to 0.133 and more preferably may be less than or equal to 0.121.

[1309] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.123 and preferably may be less than or equal to 0.112 and more preferably may be less than or equal to 0.102.

[1310] The lean cruise-MTO nvPM emissions index ratio may be less than or equal to 0.157 and preferably may be less than or equal to 0.144 and more preferably may be less than or equal to 0.131.

[1311] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0814 and preferably may be greater than or equal to 0.0915 and more preferably may be greater than or equal to 0.101.

[1312] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.096 and preferably may be greater than or equal to 0.108 and more preferably may be greater than or equal to 0.12.

[1313] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0958 and preferably may be greater than or equal to 0.107 and more preferably may be greater than or equal to 0.119.

[1314] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0979 and preferably may be greater than or equal to 0.11 and more preferably may be greater than or equal to 0.122.

[1315] The lean cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0961 and preferably may be greater than or equal to 0.108 and more preferably may be greater than or equal to 0.12.

[1316] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0814 to 0.160 and preferably may be in the range 0.0915 to 0.147 and more preferably may be in the range 0.101 to 0.134.

[1317] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0814 to 0.151 and preferably may be in the range 0.0915 to 0.138 and more preferably may be in the range 0.101 to 0.126.

[1318] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0960 to 0.151 and preferably may be in the range 0.108 to 0.138 and more preferably may be in the range 0.120 to 0.126.

[1319] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0958 to 0.160 and preferably may be in the range 0.107 to 0.147 and more preferably may be in the range 0.119 to 0.134.

[1320] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0979 to 0.151 and preferably may be in the range 0.110 to 0.138 and more preferably may be in the range 0.122 to 0.126.

[1321] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0960 to 0.145 and preferably may be in the range 0.108 to 0.133 and more preferably may be in the range 0.120 to 0.121.

[1322] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0814 to 0.123 and preferably may be in the range 0.0915 to 0.112 and more preferably may be in the range 0.101 to 0.102.

[1323] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0961 to 0.160 and preferably may be in the range 0.108 to 0.147 and more preferably may be in the range 0.120 to 0.134.

[1324] The lean cruise-MTO nvPM emissions index ratio may be in the range 0.0958 to 0.157 and preferably may be in the range 0.107 to 0.144 and more preferably may be in the range 0.119 to 0.131.

[1325] The lean cruise-MTO nvPM emissions index ratio may less than 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.2, or within any range defined between any two of these values.

[1326] BPR may be in the range 6.44 to 11.2 and preferably may be in the range 7.25 to 10.3 and more preferably may be in the range 8.05 to 9.29.

[1327] BPR may be in the range 7.20 to 11.2 and preferably may be in the range 8.10 to 10.3 and more preferably may be in the range 9.01 to 9.28.

[1328] BPR may be in the range 7.10 to 11.2 and preferably may be in the range 7.99 to 10.3 and more preferably may be in the range 8.88 to 9.29.

[1329] BPR may be in the range 7.32 to 11.2 and preferably may be in the range 8.23 to 10.3 and more preferably may be in the range 9.15 to 9.28.

[1330] BPR may be in the range 7.20 to 10.9 and preferably may be in the range 8.10 to 9.92 and more preferably may be in the range 9.00 to 9.02.

[1331] BPR may be in the range 6.44 to 9.68 and preferably may be in the range 7.25 to 8.87 and more preferably may be in the range 8.05 to 8.06.

[1332] BPR may be in the range 7.15 to 11.1 and preferably may be in the range 8.04 to 10.2 and more preferably may be in the range 8.93 to 9.19.

[1333] A rich cruise-MTO nvPM emissions index ratio may be defined as:

[1334] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • [1335] where:
    • [1336] EI_{cruise (rich)} may be defined as:

[1337] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • • [1338] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for different given operating conditions;
    • [1339] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated; and
    • [1340] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and
  • [1341] wherein the rich cruise-MTO nvPM emissions index ratio is less than 0.4.
    The rich cruise-MTO nvPM emissions index ratio may be less than 0.38 and preferably may be less than 0.36, and more preferably may be less than 0.34.

[1342] The rich cruise-MTO nvPM emissions index ratio may be less than 0.323 and preferably may be less than 0.296 and more preferably may be less than 0.269.

[1343] The rich cruise-MTO nvPM emissions index ratio may be less than 0.307 and preferably may be less than 0.282 and more preferably may be less than 0.256.

[1344] The rich cruise-MTO nvPM emissions index ratio may be less than 0.298 and preferably may be less than 0.273 and more preferably may be less than 0.248.

[1345] The rich cruise-MTO nvPM emissions index ratio may be less than 0.277 and preferably may be less than 0.254 and more preferably may be less than 0.231.

[1346] The rich cruise-MTO nvPM emissions index ratio may be less than 0.211 and preferably may be less than 0.193 and more preferably may be less than 0.176.

[1347] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.13 and preferably may be less than or equal to 0.119 and more preferably may be less than or equal to 0.108.

[1348] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.129 and preferably may be less than or equal to 0.119 and more preferably may be less than or equal to 0.108.

[1349] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.128 and preferably may be less than or equal to 0.118 and more preferably may be less than or equal to 0.107.

[1350] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.0823 and preferably may be less than or equal to 0.0754 and more preferably may be less than or equal to 0.0686.

[1351] The rich cruise-MTO nvPM emissions index ratio may be less than or equal to 0.129 and preferably may be less than or equal to 0.118 and more preferably may be less than or equal to 0.108.

[1352] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0548 and preferably may be greater than or equal to 0.0616 and more preferably may be greater than or equal to 0.0685.

[1353] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0773 and preferably may be greater than or equal to 0.087 and more preferably may be greater than or equal to 0.0966.

[1354] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0633 and preferably may be greater than or equal to 0.0712 and more preferably may be greater than or equal to 0.0791.

[1355] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.086 and preferably may be greater than or equal to 0.0968 and more preferably may be greater than or equal to 0.107.

[1356] The rich cruise-MTO nvPM emissions index ratio may be greater than or equal to 0.0676 and preferably may be greater than or equal to 0.0761 and more preferably may be greater than or equal to 0.0845.

[1357] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0548 to 0.130 and preferably may be in the range 0.0616 to 0.119 and more preferably may be in the range 0.0685 to 0.108.

[1358] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0773 to 0.130 and preferably may be in the range 0.0870 to 0.119 and more preferably may be in the range 0.0966 to 0.108.

[1359] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0633 to 0.129 and preferably may be in the range 0.0712 to 0.119 and more preferably may be in the range 0.0791 to 0.108.

[1360] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0773 to 0.128 and preferably may be in the range 0.0870 to 0.118 and more preferably may be in the range 0.0966 to 0.107.

[1361] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0860 to 0.130 and preferably may be in the range 0.0968 to 0.119 and more preferably may be in the range 0.107 to 0.108.

[1362] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0548 to 0.0823 and preferably may be in the range 0.0616 to 0.0754 and more preferably may be in the range 0.0685 to 0.0686.

[1363] The rich cruise-MTO nvPM emissions index ratio may be in the range 0.0676 to 0.129 and preferably may be in the range 0.0761 to 0.119 and more preferably may be in the range 0.0845 to 0.108.

[1364] The rich cruise-MTO nvPM emissions index ratio may be less than 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38, 0.4 or within any range defined between any two of these values.

[1365] According to a sixty-second aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1366] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1367] a rich cruise-MTO nvPM emissions index ratio may be defined as:

[1368] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [1369] where:
      • [1370] EI_{cruise (rich)} may be defined as:

[1371] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • • • [1372] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions;
      • [1373] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated;
      • [1374] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated;
      • [1375] BPR is the bypass ratio of the gas turbine engine;
  • [1376] the rich cruise-MTO nvPM emissions index ratio is less than 0.4; and
  • [1377] the gas turbine engine is configured to provide fuel comprising a sustainable aviation fuel (SAF) to the plurality of fuel spray nozzles.

[1378] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the sixty-first aspect.

[1379] According to a sixty-third aspect, there is provided a method of operating the gas turbine engine of the sixty-first or sixty-second aspect, the method comprising providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1380] According to a sixty-fourth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1381] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1382] a lean cruise-MTO nvPM emissions index ratio may be defined as:

[1383] $\frac{{\mathrm{EI}}_{\mathrm{crui}\mathrm{se}\left(\mathrm{lean}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [1384] where:
      • [1385] EI_{cruise (lean)} may be defined as:

[1386] $\frac{{\mathrm{EI}}_{\mathrm{maxTO}}+{\mathrm{EI}}_{\mathrm{climb}}}{2}$

• • • • [1387] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions;
      • [1388] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions; and
      • [1389] BPR is the bypass ratio of the gas turbine engine;
  • [1390] the lean cruise-MTO nvPM emissions index ratio is less than 0.2; and
  • [1391] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1392] The lean cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the sixty-first aspect.

[1393] A rich cruise-MTO nvPM emissions index ratio may be defined as:

[1394] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • [1395] where:
    • [1396] EI_{cruise (rich)} may be defined as:

[1397] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • • [1398] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for different given operating conditions;
    • [1399] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated;
    • [1400] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated; and
  • [1401] wherein the rich cruise-MTO nvPM emissions index ratio may be less than 0.4.

[1402] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the sixty-first aspect.

[1403] According to a sixty-fifth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

• • [1404] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1405] a rich cruise-MTO nvPM emissions index ratio may be defined as:

[1406] $\frac{{\mathrm{EI}}_{\mathrm{cruise}\left(\mathrm{rich}\right)}/{\mathrm{EI}}_{\mathrm{maxTO}}}{\mathrm{BPR}}$

• • • [1407] where:
      • [1408] EI_{cruise (rich)} may be defined as:

[1409] $\frac{{\mathrm{EI}}_{\mathrm{climb}}+{\mathrm{EI}}_{\mathrm{approach}}}{2}$

• • • • [1410] EI_climb is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions;
      • [1411] EI_approach is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same operating conditions at which EI_climb is calculated;
      • [1412] EI_maxTO is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the same operating conditions at which EI_climb is calculated;
      • [1413] BPR is the bypass ratio of the gas turbine engine;
  • [1414] the rich cruise-MTO nvPM emissions index ratio is less than 0.4; and
  • [1415] the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.

[1416] The rich cruise-MTO nvPM emissions index ratio and/or the BPR may be as defined above in connection with the sixty-first aspect.

[1417] According to a sixty-sixth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1418] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1419] a MTO nvPM emissions index ratio may be defined as:

[1420] $\frac{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{maxTO},\mathrm{FF}}}$

• • • [1421] where:
      • [1422] EI_maxTO,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [1423] EI_maxTO,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1424] the MTO nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1425] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[1426] The MTO nvPM emissions index ratio may be greater than zero.

[1427] The MTO nvPM emissions index ratio may be less than or equal to 0.93, and preferably may be less than or equal to 0.86, and more preferably may be less or equal to than 0.79.

[1428] The MTO nvPM emissions index ratio may be less than or equal to 0.776, and preferably may be less than or equal to 0.711, and more preferably may be less than or equal to 0.646.

[1429] The MTO nvPM emissions index ratio may be greater than or equal to 0.15, and preferably may be greater than or equal to 0.3, and more preferably may be greater than or equal to 0.45.

[1430] The MTO nvPM emissions index ratio may be greater than or equal to 0.516, and preferably may be greater than or equal to 0.581, and more preferably may be greater than or equal to 0.645.

[1431] The MTO nvPM emissions index ratio may be in the range of 0.516 to 0.776, and preferably may be in the range of 0.581 to 0.711, and more preferably may be in the range of 0.645 to 0.646.

[1432] The MTO nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[1433] The MTO nvPM emissions index ratio may be 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, or within any range defined between any two of these values.

[1434] A climb nvPM emissions index ratio may be defined as:

[1435] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • [1436] where:
    • [1437] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
    • [1438] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1439] and wherein the climb nvPM emissions index ratio of the gas turbine engine may be less than 1.

[1440] The climb nvPM emissions index ratio may be greater than zero.

[1441] The climb nvPM emissions index ratio may be less than or equal to 0.9, and preferably may be less than or equal to 0.75, and more preferably may be less than or equal to 0.6.

[1442] The climb nvPM emissions index ratio may be less than or equal to 0.57, and preferably may be less than or equal to 0.523, and more preferably may be less than or equal to 0.475.

[1443] The climb nvPM emissions index ratio may be greater than or equal to 0.1, and preferably may be greater than or equal to 0.2, and more preferably may be greater than or equal to 0.3.

[1444] The climb nvPM emissions index ratio may be greater than or equal to 0.379, and preferably may be greater than or equal to 0.427, and more preferably may be greater than or equal to 0.474.

[1445] The climb nvPM emissions index ratio may be in the range of 0.379 to 0.570, and preferably may be in the range of 0.427 to 0.523, and more preferably may be in the range of 0.474 to 0.475.

[1446] The climb nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[1447] The climb nvPM emissions index ratio may be 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.6, or within any range defined between any two of these values.

[1448] An approach nvPM emissions index ratio may be defined as:

[1449] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • [1450] where:
    • [1451] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the given operating conditions, or for different given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
    • [1452] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [1453] wherein the approach nvPM emissions index ratio of the gas turbine engine may be less than 1.

[1454] The approach nvPM emissions index ratio may be greater than zero.

[1455] The approach nvPM emissions index ratio may be less than or equal to 0.8, and preferably may be less than or equal to 0.5, and more preferably may be less than or equal to 0.2.

[1456] The approach nvPM emissions index ratio may be less than or equal to 0.185, and preferably may be less than or equal to 0.169, and more preferably may be less than or equal to 0.154.

[1457] The approach nvPM emissions index ratio may be greater than or equal to 0.03, and preferably may be greater than or equal to 0.06, and more preferably may be greater than or equal to 0.09.

[1458] The approach nvPM emissions index ratio may be greater than or equal to 0.122, and preferably may be greater than or equal to 0.138, and more preferably may be greater than or equal to 0.153.

[1459] The approach nvPM emissions index ratio may be in the range of 0.122 to 0.185, and preferably may be in the range of 0.138 to 0.169, and more preferably may be in the range of 0.153 to 0.154.

[1460] The approach nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[1461] The approach nvPM emissions index ratio may be 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, or within any range defined between any two of these values.

[1462] An idle nvPM emissions index ratio may be defined as:

[1463] $\frac{{\mathrm{EI}}_{\mathrm{idle},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{idle},\mathrm{FF}}}$

• • [1464] where:
    • [1465] EI_idle,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the given operating conditions, or for different given operating conditions, and if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
    • [1466] EI_idle,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 7% available thrust for the same given operating conditions at which EI_idle,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel; and
  • [1467] wherein the idle nvPM emissions index ratio of the gas turbine engine may be less than 1.

[1468] The idle nvPM emissions index ratio may be greater than zero.

[1469] The idle nvPM emissions index ratio may be less than or equal to 0.8, and preferably may be less than or equal to 0.5, and more preferably may be less than or equal to 0.2.

[1470] The idle nvPM emissions index ratio may be less than or equal to 0.115, and preferably may be less than or equal to 0.106, and more preferably may be less than or equal to 0.0959.

[1471] The idle nvPM emissions index ratio may be greater than or equal to 0.02, and preferably may be greater than or equal to 0.04, and more preferably may be greater than or equal to 0.06.

[1472] The idle nvPM emissions index ratio may be greater than or equal to 0.0766, and preferably may be greater than or equal to 0.0862, and more preferably may be greater than or equal to 0.0958.

[1473] The idle nvPM emissions index ratio may be in the range of 0.0766 to 0.115, and preferably may be in the range of 0.0862 to 0.106, and more preferably may be in the range of 0.0958 to 0.0959.

[1474] The idle nvPM emissions index ratio may be less than 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, or 0.05, or any range defined between any two of these values.

[1475] The idle nvPM emissions index ratio may be 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2 or within any range defined between any two of these values.

[1476] According to a sixty-seventh aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1477] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1478] a climb nvPM emissions index ratio may be defined as:

[1479] $\frac{{\mathrm{EI}}_{\mathrm{climb},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{climb},\mathrm{FF}}}$

• • • [1480] where:
      • [1481] EI_climb,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for given operating conditions, if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [1482] EI_climb,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 85% available thrust for the same given operating conditions at which EI_climb,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1483] the climb nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1484] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.

[1485] The climb nvPM emissions index ratio may be as defined in connection with the sixty-sixth aspect.

[1486] According to a sixty-eighth aspect, there is provided a gas turbine engine for an aircraft, comprising any one or more of the following features:

• • [1487] a combustor, comprising a combustion chamber and a plurality of fuel spray nozzles configured to inject fuel into the combustion chamber; and wherein:
  • [1488] an approach nvPM emissions index ratio may be defined as:

[1489] $\frac{{\mathrm{EI}}_{\mathrm{approach},\mathrm{SAF}}}{{\mathrm{EI}}_{\mathrm{approach},\mathrm{FF}}}$

• • • [1490] where:
      • [1491] EI_approach,SAF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF); and
      • [1492] EI_approach,FF is the nvPM emissions index in mg/kg of the gas turbine engine when operating at around 30% available thrust for the same given operating conditions at which EI_approach,SAF is calculated and if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
  • [1493] the approach nvPM emissions index ratio of the gas turbine engine is less than 1; and
  • [1494] the gas turbine engine is configured to provide fuel comprising a SAF to the plurality of fuel spray nozzles.