ground-proximity warning system

A system that warns the crew if the aircraft’s current flight path would result in impact with the ground. The system automatically and continuously monitors the aircraft’s flight path with respect to the terrain at all altitudes between 50 and 2450 ft. The GPWS uses inputs from systems providing radio altitude, air speed/Mach number, landing gear and flap position, and decision height (DH) setting. The system provides both visual and aural warnings. The various hazardous conditions that can be encountered in flight are divided into five modes. These are:
A typical GPWS control panel.
i. Mode 1. Excessive descent rate. This mode has an inner and outer warning boundary. Initial penetration activates the warning lights and two “SINKRATE” aural messages. If the condition is not corrected and the inner boundary is penetrated, the aural message changes to “WHOOP WHOOP PULL UP,” which repeats continuously until the unsafe condition is corrected.
ii. Mode 2. Excessive terrain closure rate. Mode 2 has two submodes, 2A and 2B. Mode 2A is active when flaps are not in the landing configuration, and 2B is active when flaps are in the landing configuration. When the boundary is penetrated, the warning lights come on and two “TERRAIN” messages sound. If the closure condition remains, the aural annunciation changes to “WHOOP WHOOP PULL UP.”
iii. Mode 3. Loss of altitude after takeoff (or go-around) when not in the landing configuration. When penetrating the boundary, the warning lights come on and two “DONT SINK” messages sound.
iv. Mode 4. Insufficient terrain clearance when not in the landing configuration. This mode also has two submodes, 4A and 4B. When mode 4A is activated and the gear is not down, the warning lights come on and a “TOO LOW, TERRAIN” message sounds. Below a certain height and speed, the message is replaced by “TOO LOW, GEAR.” If the gear is down but the flaps are not in the landing configuration, mode 4B is activated and the “TOO LOW, FLAPS” message sounds.
v. Mode 5. Descent below ILS glide slope. When mode 5 is activated, amber caution lights come on and the message “GLIDESLOPE, GLIDESLOPE” is spoken with a low voice (soft warning). If the terrain clearance decreases and/or the glide-slope deviation increases, the “GLIDESLOPE” message will become louder and continuous (hard warning). The soft warning may be canceled.
vi. Mode 6. Descent below selected minimum decision height (DH). Mode 6 is activated when descending through the selected decision height with the gear down at where “MINIMUMS, MINIMUMS” will be announced.
Advanced versions of the equipment have additional facilities of radio altitude callouts and aural warnings at excessive high bank angles. The other feature in these versions is that spurious and nuisance warnings are minimized. The system has a major drawback in that it cannot look ahead at terrain. Consequently, it cannot always give pilots sufficient time to predict and plan avoidance maneuvers.
Enhanced GPWS (EGPWS), besides providing traditional GPWS alerting functions, displays the surrounding terrain (up to 320 NM) on an EFIS (electronic flight instrument system) screen or weather radar CRT (cathode-ray tube) and provides alerts about a minute’s flight time or more away from terrain.

Aviation dictionary. 2014.

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