CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending patent application Serial No. 494,593, filed on May 13, 1983. BACKGROUND OF THE INVENTION

Field of the Invention This invention relates generally to ground proximity warning systems, and more particularly to a system that provides a warning to the pilot of a tac- tical aircraft, such as a fighter/attack aircraft, flying the approach leg of the flight if the descent rate of the aircraft is excessive for the ^' altitude at which the aircraft is flying.

Description of the Prior Art Systems for providing the pilot of an air¬ craft with a warning in the event that the aircraft is descending too rapidly on approach are known. Typical of such systems are the systems disclosed in United States Patent Nos. 3,946,35a, 3,947,808, 3,958,219 and 4,215,334, all assigned to the same assignee as the assignee of the present invention. While all of the above systems serve the basic function of alerting a pilot if the descent rate, for example, barometric rate or Z-velocity, of the air- craft exceeds a predetermined safe rate as determined by the relationship between the radio altitude and the barometric altitude rate, the above-described systems are designed to operate with transport aircraft, particularly with large, turbine powered aircraft of

the type flown by commercial airlines. Since ^• sucn transport aircraft typically have flight profiles that are considerably different from those of a tactical aircraft, for example, a fighter/attack aircraft making a landing onto an aircraft carrier, systems designed for transport aircraft would not be effective on tactical aircraft since they would have a tendency to generate false warnings, thereby losing their effectivness as pilots ignored the warnings. SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a ground proximity warning system that overcomes many of the disadvantages of the prior art warning systems. It is another object of the present inven¬ tion to provide a warning system to the pilot of a tactical aircraft on approach if the sink rate of the aircraft is excessive for the altitude at which the aircraft is flying. It is yet another object of the invention to provide a ground proximity warning system for alert¬ ing the pilot of a tactical aircraft if his descent rate on approach is dangerously high, with the warning criteria having been selected to match the operating procedure used with such tactical aircraft.

Briefly, in accordance with a preferred embodiment of the invention, there is provided a sys¬ tem that monitors the descent rate of the aircraft when the landing gear is down, the indicated airspeed is less than the maximum approach speed of the air¬ craft, for example, approximately 200 knots, and the weapons are not armed, and issues an aural, warning to the pilot if the barometric descent rate or Z-velocity exceeds a predetermined safe limit for the radio alti- tude at which the aircraft is flying.

DESCRIPTION OF THE DRAWING These and other objects and advantages of the invention will become readily apparent upon con¬ sideration of the following detailed description and 5 attached drawing wherein:

FIG. 1 is a logical block diagram of the warning system according to the invention; and

FIG. 2 is a graph of the barometric descent rate required to generate a warning as a function of

10. radio altitude.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, with particular attention to FIG. 1, there is illustrated an embodi¬ ment of the ground proximity warning system according

15 to the invention generally designated by the reference numeral 10. The system 10 according to the invention is illustrated in FIG. 1 in logical block diagram form as a series of gates, comparators and the like for purposes of illustration; however, it should be

20 understood that the actual implementation of the logic can be other than as shown in FIG. 1, with various digital and analog implementations being possible. The signals used by the system as described include radio altitude, barometric altitude rate, airspeed

25 ana signals indicating the position of the landing gear and whether the weapons are armed, along with various validity signals. Depending on the type of aircraft in which the warning system is installed, the signals shown in FIG. 1 can be obtained from indi-

3Q vidual instruments such as a barometric altimeter 12, a barometric rate circuit 14, a radio altimeter 16, an airspeed signal source 18 such as an air data com¬ puter or airspeed indicator and various discrete ele¬ ments indicating the position of the gear, whether

35 the weapons are armed and whether there is weight on

the wheels. Alternatively, the signals can be obtain¬ ed from a digital data bus in certain newer aircraft. In addition, instead of obtaining the altitude rate signal from the barometric altitude rate circuit, the Z-velocity signal from the inertial navigation system may be used to provide a representation of the descent rate of the aircraft.

Because this system is designed to be opera¬ tive only during the approach to a landing phase of a flight profile, its operation is inhibited during other phases of flight in order to avoid the possiDle generation of false warnings during those phases. The inhibiting function is accomplished by a pair of AND gates 20 and 22 that monitor various flight condi- tions and the configuration of the aircraft to deter¬ mine whether the aircraft is on an approach to a land¬ ing. Two of the parameters that are monitored by the gate 20 are whether the landing gear is down and whether there is any weight on the wheels. In addition, a signal indicating that the weapons are not armed is applied to the gate 20, because under normal landing conditions, the weapons would not be armed. If they were, this would indicate that the aircraft was operating in a phase other than an approach to a landing. In addition, the airspeed is monitored by a comparator 24 which pro¬ vides a signal to the AND gate 20 when the airspeed is below the maximum approach speed of tne aircraft, thus indicating a probable landing condition. The maximum approach speed of the aircraft varies as a function of the aircraft, and is on the order of approximate¬ ly 200 knots for aircraft such as the Fairchild A-10 and the Lockheed τ-33. Also, signals indicating that the radio altitude signal and the barometric

altimeter signals are not inhibited are applied to the AND the AND gate 22 in order to prevent the generation of warnings in the event of a faulty barometric or radio altimeter. In addition, in order further to reduce the probability of false- arnings, the system is inhibited below 50 feet of radio altitude and above 1,000 feet of radio altitude by a comparator circuit 26 that is responsive to signals from the radio altimeter 16 and provides enabling signals to the AND gate 22 only when the aircraft is between 50 feet and 1,000 of radio altitude.

If all of the above conditions are met, the AND gate 22 is enabled and placed under the control of an OR gate 28 controlled by a pair of comparators 30 and 32. The comparators 30 and 32 are responsive • to the radio altitude and barometric rate or Z velocity signals, and apply a signal to the OR gate 28 when the sink rate is excessive for a given radio altitude. Upon receipt of a signal from the OR gate 30 or the OR gate 32, the gate 28 causes the gate 22 to apply a warning initiation signal to a warning circuit such as a digital voice warning generator 34. This causes the warning generator 34 to generate a digitally synthesized voice signal, such as, the words "SINK RATE", and apply them, either directly or indirectly, to a transducer 36, which may be a loud¬ speaker, or an earphone or the like.

In the illustrated embodiment, two co para- tors 30 and 32 are utilized to determine the descent rate versus radio altitude relationship required to generate a "SINK RATE" warning, but a single comparator can also be used. However, because of the particular shape of the warning curve defined by the solid line in FIG. 2 {the dashed line will be discussed in a

subsequent section of the specification) , it has been . found convenient to utilize two separate comparators,, The comparator 30 is used to initiate warnings for low radio altitudes and descent rates below 3,000 feet per minute, while the comparator 32 is used to initiate warnings for descent rates in excess of 3,000 feet per minute. Thus, the comparator 32 monitors the signal from the barometric rate circuit 14, and compares it with a reference signal representative of a 3,000 feet per minute descent rate that is applied to the other terminal of the comparator 32. Whenever the signal from the rate circuit 14 exceeds the refer¬ ence signal representative of a 3,000 feet per minute descent rate, the comparator 32 applies a signal to the OR gate 28. This signal causes the OR gate 28 to enable the AND gate 22, and causes a warning to be generated. For descent rates below 3,000 feet per minute, the comparator 30 controls, and causes the warning to be generated as a function of Doth the barometric descent rate and the radio altitude in accordance with the two-step curve below a radio alti¬ tude of 600 feet radio altitude and a 3,000 feet per minute descent rate shown in FIG. 2. Typically, the "SINK RATE" warning will be generated when the descent rate exceeds 3,000 feet per minute at 600 feet of radio altitude, when it exceeds 2,500 feet per minute at 200 feet of radio altitude, and when it exceeds 1,250 feet per minute at a radio altitude of 50 feet. The above-described relationship between the barometric altitude rate and the radio altitude required to generate a warning is illustrated in greater detail in FIG. 2. The shaded area beneath the solid line of the graph of FIG. 2 illustrates the various relationships between descent rate and radio altitude values that cause the warning, "SINK RATE", to be

generated. This warning envelope has been determined by the flight characteristics of fighter/attack bombers such as, the Fairchild A10, and from the manner in which such aircraft are operated during the landing phase of the flight. Because such aircraft often fly at relatively low levels and may encounter relatively high descent rates during such low level flight, the top of the envelope of FIG. 2 is limited at 1,000 feet to avoid false warnings during maneuvering above 1,000 feet. This results in a considerably different envelope than the envelope utilized for transport aircraft which permits warnings up to an altitude of approximately 2,450 feet. Moreover, the descent rates permitted at lower altitudes are substantially greater for the tactical aircraft than for transport aircraft. For example, in the warning envelope illustrated in FIG. 2, a descent rate of up to 2,500 feet per minute is permitted at an altitude of 200 feet, while a warn¬ ing system used in a transport aircraft would generate a warning at approximately one-half of that rate of descent. However, below 200 feet, the permitted des¬ cent rate is rapidly reduced to approximately 1,500 feet per minute at an altitude of 50 feet above ground to avoid excessively high descent rate touchdowns. An alternative envelope is illustrated Dy the dashed line of FIG. 2. In the envelope illustrat¬ ed by the dashed line, the low altitude nose is extend¬ ed such that at 50 feet of altitude, a warning is given if the descent rate exceeds approximately 800 feet per minute. At 300 feet of radio altitude, the descent rate required to generate a warning is approxi¬ mately 2600 feet per minute, as illustrated by the breakpoint between the dashed line and the solid line of FIG. 2. This results in greater warning times at low altitudes. In addition, the vertical portion of

the curve at 3000 feet of radio altitude is eliminated, ^' and the second line segment of radio altitude is elimi¬ nated, and the second line segment of the graph is extended to 1000 feet to permit a descent rate of approximately 3500 feet per minute at 1000 feet of radio altitude. This allows the aircraft to anuever more in the 600 to 1000 feet altitude range without generating nuisance warnings. In addition, by elimi¬ nating the breakpoint at 3000 feet per minute of de- scent rate, the comparator 32 and the OR gate 28 (FIG*. 1) may be eliminated. If this is done, the output of the comparator 30 is applied directly to the AND gate 22. A certain amount of tolerance in the curves of FIG. 2 is permitted. For example, the descent rate necessary to generate a warning may vary approximately plus or minus 200 feet per minute. about the illustrat¬ ed curves without substantially degrading the perfor¬ mance of the system. Also, the 1000 foot ceiling may vary approximately plus or minus 50 feet, and the 50 foot floor may vary plus or minus 25 feet.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically des¬ cribed above.

What is claimed and desired to be secured by Letters Patent of the United States is: