In an aircraft ground proximity warning system that compares the rate of descent of an aircraft with its altitude above the terrain and in response thereto generates a warning signal when the aircraft's sink rate exceeds a predetermined limit for a particular altitude, a first type of advisory voice warning is generated for combinations of lesser descent rates and greater altitudes and a second type of imperative voice warning is generated for greater sink rates at lower altitudes where the danger of the aircraft impact with the terrain is more significant. In addition, the repetition rate of the advisory warning is increased as a function of increasing descent rate and the amplitude of the advisory warning can similarly be increased as function of increasing descent rate in order to call attention to an increasing undesirable operating condition.

An electronic sensor circuit is provided which generates predetermined frequencies corresponding to the resultant force vector of the gravitational and inertial forces. There is further provided electronic circuit means for generating an audible response which increases in intensity in correspondence with the magnitude of the deviation of the force vector from a defined datum. In the preferred embodiment an electrolytic transducer is used to sense gravitational and inertial forces and a phase detection circuit is provided to monitor the output and control the input generator for selection of a frequency corresponding to the deviation occurring.

A ground proximity warning system monitors the flight path angle or barometric or inertially derived descent rate of the aircraft and modulates the glide slope alerting function in accordance with flight path angle or descent rate when the aircraft is not descending rapidly even though it is below the glide slope in order to reduce nuisance warnings without compromising system sensitivity. Alternatively, the repetition rate of the alert may be varied as a function of the flight path angle or descent rate to provide less frequent alerts when the aircraft is not descending rapidly.

There is disclosed an improved annunciator for verbalizing altitude-related messages during the descent of an aircraft. In the past, announcements such as "nine hundred," "eight hundred," etc. were made when the aircraft crossed a respective "hundreds" level. During circle-to-land maneuvers multiple announcements of the same level sometimes occurred, primarily due to small terrain variations which in effect caused the aircraft altitude to vary above and below a "hundreds" level. In accordance with the invention, the annunciator is enabled following a previous announcement only if the aircraft first passes through a mid-"hundreds" level, e.g., 750 feet. Terrain variations are usually no more than twenty feet or so; thus multiple announcements of the same level are inhibited.

There is disclosed an annunciator for verbalizing altitude-related messages during the descent of an aircraft. Announcements such as "nine hundred," "eight hundred," etc. are made as the aircraft descends through respective "hundreds" levels; such announcements are not made during an ascent. In addition, the word "terrain" is heard when the aircraft descends through the 2,000-foot and 1,000-foot levels, and the word "minimum" is heard when the aircraft drops below a "decision height" selected by the pilot. Lastly, the word "glideslope" is out-putted in response to the detection of a glideslope deviation, the frequency of this announcement as well as its volume being a function of the magnitude of the deviation.