Following a series of controlled-flight-into-terrain (CFIT) crashes in the 1960s, where aircraft had crashed simply because their pilots did not realize how close to the ground they were flying, Donald Bateman {b. 1932) designed a Ground Proximity Warning System.
The system works by collating data from the radar altimeter (which measures the height above the ground), the barometric altimeter, the sensor that detects the glide path, and sensors attached to the flaps and landing gear. The data are then used to predict trends in flight pattern. The pilot is warned if the system registers an excessive descent rate, excessive terrain closure rate, altitude loss after take-off, unsafe terrain clearance, or below glide path deviation.
Each of the five danger modes has its own audio and visual warnings to alert the pilot. Within each mode, there are also different warnings depending on the severity of the deviation. For example, a sink rate of 4,000 feet (1,200 m) per minute will trigger the warning "Sink Rate" at 1,500 feet (460 m), but "Pull Up" at 500 feet (150m).
The introduction of Bateman's system in 1967 reduced the fatal CFIT crash rate for large passenger aircraft to fall from 3.5 per year to two per year and was made mandatory on such planes by the U.S. Federal Aviation Administration in 1973. The systems were referred to as "Bitching Betty" or "Bitching Bob" by U.S. pilots, depending on whether the voice was female or male. An improved version of the system, Enhanced Ground Proximity Warning System, is linked to a global terrain map, enabling a plane to pinpoint exactly where it is and to predict sudden changes such as steep slopes, giving more timely warnings to pilots and enabling them to fly in poor conditions.
