Excessive Descent Rate Warning System for Aircraft

The invention relates to the field of aircraft ground proximity warning systems and more particularly to systems providing warnings of excessive aircraft descent rates.

In a prior art system, as represented by the U.S. patent application of Bateman, Ser. No. 480,727, entitled "Aircraft Ground Proximity Warning Instrument, " filed June 19, 1974, and assigned to the assignee of this application, there is a linear relationship between the aircraft altitude above ground and the descent rate that will generate a warning signal. For example, at an altitude of 2500 feet, a descent rate of 3500 feet per minute is tolerated without generating a warning, whereas at 50 feet, a descent rate of 1300 feet per minute or less is tolerated. In other words, the greater the altitude, the greater the rate of descent that is tolerated by the warning system.

However, there are certain circumstances where normal operating procedures will call for the aircraft to descend at a rate sufficient to trigger the warning system that is based on this particular relationship between aircraft altitude and rate of descent. For example, under visual operating conditions, aircraft are occasionally cleared for landing approach well above the normal 3.degree. descent angle thereby tending to require that the aircraft descend at a relatively rapid rate. As a result, an aircraft attempting to get down to a normal, stabilized approach angle will occasionally trigger the warning system and, in effect, generate a nuisance warning. It is considered important to reduce the number of this type of nuisance warnings due to the fact that air crews will tend to discount the seriousness of the warnings thereby reducing the overall effectiveness and credibility of the ground proximity warning systems.

It has been determined that preventing the triggering of an alarm for a calculated time to ground impact of greater than a predetermined duration, for example 30 seconds, will tend to eliminate the type of nuisance warnings described and, at the same time, have a minimal effect on warning times when the aircraft is in actual danger. By using a "time line" of 30 seconds to limit warnings, the general credibility of the system is enhanced. In addition, a 30 second time line will not only prevent nuisance warnings at higher altitudes but will not affect the warning system at lower altitudes where the aircraft may be in actual danger of impacting the ground.

It is therefore an object of the invention to provide an excessive descent rate warning system wherein the warning signal is inhibited by a function representing a predetermined time to impact.

It is an additional object of the invention to provide an excessive descent rate warning circuit that alters the relationship between descent rate and altitude above a predefined descent rate in order to limit the generation of a warning signal to a predetermined function of the time to ground impact.

The excessive sink rate warning circuit functions by comparing the aircraft rate of altitude, change as measured by a barometric altimeter, and the aircraft actual altitude above the ground, as measured by a radio altimeter. As the aircraft altitude above ground is reduced, the rate of descent required to trigger a warning is similarly reduced on a linear basis. However, above certain altitudes the combination of distance to the ground and the aircraft rate of descent that triggers a warning, as in the prior art systems, would provide an excessive warning time, i.e., time to potential ground impact.

In order to eliminate these excessive warning times at greater altitudes, a characteristic of the relationship between rate of descent and altitude is changed so as to provide a maximum warning time. This is accomplished by changing the slope of the rate of descent vs. altitude relationship above a selected altitude.

FIG. 1 is a graphic representation of the relationship between descent rate and altitude above ground that will cause an excessive sink rate warning signal to be generated.

FIG. 2 is a functional block diagram of the circuitry for implementing the warning characteristic shown in FIG. 1.

The graphic representation of the warning envelope illustrated in FIG. 1, is provided to show the preferred warning characteristics of the excessive sink rate warning system. The shaded area 10 of the graph, represents the combinations of descent rate and altitude above ground that will generate a warning signal. The first portion 12 of the curve illustrates the boundary relationship between altitude and rate of descent for lower altitudes. The warning is inhibited for altitudes of 50 feet or below in order to prevent warnings from being sounded as the aircraft is landing. From altitudes of 50 feet along the portion 12 of the curve to the point 14, the relationship between altitude and descent rate is linear. At point 14 the slope of the warning boundary curve changes to correspond to a constant time line 16 representing those combinations of descent rates and altitudes that will cause the aircraft to impact the ground at a constant time. The time line shown at 16 represents 30 seconds to impact, e.g., with a descent rate of 2500 fpm a warning is given at an altitude of 1250 feet. At altitudes above point 14 on the warning curve it is considered that a 30 second warning would give the crew of an aircraft ample time to maneuver in order to avoid impacting the ground.

The preferred embodiment of circuitry for generating the warning curves shown in FIG. 1 is provided in FIG. 2 of the drawings. The aircraft descent rate in terms of barometric altitude is represented by the signal h.sub.B, which can be generated with a barometric altimeter and rate circuit in the manner disclosed in the Application of Batemen, Ser. No. 480,727. The h.sub.B signal, which is negative when the aircraft is descending, is an input to one of the positive terminals of the summing junction 20. The other input to the summing junction 20 is a signal of positive polarity that represents an increase in altitude of 1300 feet per minute. The sum of these two signals is then applied to the limiter 22. The function of the limiter 22 is to limit the magnitude of the biased h.sub.B signal from the summing junction 20 to a maximum absolute value. In the preferred embodiment, this will be a signal equal to a minus 1057 fpm. The scaling amplifier 24 serves to multiply the output of the limiter 22 by a factor of 1.11. The biased h.sub.B signal, so scaled, is transmitted through the summing junction 26 to the summing junction 28 where it is combined with the h.sub.R signal representing the aircraft altitude above ground. The h.sub.R signal is preferably obtained from a radio altimeter in order to given an accurate measure of the aircraft altitude above the ground.

When the magnitude of the scaled h.sub.B signal from the amplifier 24 exceeds the magnitude of the positive h.sub.R signal, the output of the summing junction 28 will be negative, which in turn, causes the detector 30 to generate a positive output or logic signal to the AND gate 32. Therefore, when the AND gate 32 is enabled, a negative output from the summing junction 28 will generate a warning signal on the line 34 to activate an alarm.

The h.sub.B signal is also utilized as input to the summing junction 36. Here the h.sub.b signal is biased by a positive signal representing a rate of altitude change of 2357 fpm. The rectifier 38 eliminates any positive voltage or signal from being applied to the second scaling amplifier 40. As a result, the scaling amplifier 40 will only have a signal applied to it when the aircraft rate of descent is greater than 2357 fpm. Point 14 on the warning curve of FIG. 1 is on the 30 second ground impact line and has coordinates of 2357 fpm and 1178 feet. The biased h.sub.B signal from the rectifier 38 is then scaled by a factor of 0.5 which corresponds to the slope of the 30 second time to impact line 16, of FIG. 1. The scaled h.sub.B signal from the second scaling amplifier 40 is combined with the limited h.sub.B signal from the first scaling amplifier 24 in the summing junction 26. The combined h.sub.B signals are added to the h.sub.R signal in the summing junction 28 to produce the warning curve of FIG. 1. When the aircraft rate of descent is greater than 2357 fpm, corresponding to point 14 on the curve, the upper portion 18 of the warning curve will correspond to the 30 second time line 16.

With a descent rate h.sub.B less than 1300 fpm, the output of scaling amplifier 24 is 0. The output increases linearly for greater descent rates to a maximum with a descent rate h.sub.B of 2357 fpm, point 14 on the curve. This corresponds with a limiter input of 1057 fpm. The output of scaling amplifier 24 establishes the segment 12 of the warning boundary. The output of scaling amplifier 40 is 0 for descent rates less than 2357 fpm. It increases linearly with greater descent rates and is added to the output of amplifier 24 establishing segment 18 of the warning boundary.

The AND gate 32 has a second input which enables the gate whenever the aircraft is within 2450 feet of the ground. In addition to being the appropriate maximum range of most radio altimeters, it is considered unnecessary to provide excessive sink rate warnings at altitudes of greater than 2500 feet.

The invention has been described in terms of specific rates of descent and altitudes which are preferred. It will be understood that the invention may utilize other values.