Learn what a ground proximity warning system is, how its warning modes detect terrain danger, and how GPWS differs from EGPWS and TAWS.
A ground proximity warning system (GPWS) is an aircraft safety system that monitors height above terrain, descent rate, flight path and landing configuration. In real-world aviation, when those inputs match a hazardous pattern, it gives spoken and visual cautions such as “SINK RATE” or urgent warnings such as “PULL UP”, so pilots can correct immediately.
How does GPWS detect dangerous ground proximity?
GPWS compares live aircraft data with predefined warning envelopes rather than searching ahead with terrain radar. Its principal inputs normally include:
- Radio altitude, which measures the aircraft’s height above the surface directly below it.
- Air data, including barometric altitude, airspeed and vertical speed.
- Landing configuration, particularly landing-gear and flap position.
- Glideslope deviation during an instrument approach.
The computer applies mode-specific limits that change with radio altitude, speed and configuration. Crossing a limit triggers a caution or warning; the precise thresholds and wording depend on the aircraft and installed equipment.
Radio altitude is not the same as the altitude shown on a pressure altimeter. Our guide to understanding barometric altitude and the main flight instruments explains that distinction: an aircraft can hold a constant indicated altitude while rising terrain rapidly reduces its height above the ground.
What warnings and modes does GPWS use?
The five core modes in a traditional GPWS cover different ways an aircraft can approach the ground unsafely.
| Mode | Condition monitored | Typical alert |
|---|---|---|
| 1 | Excessive descent rate | “SINK RATE” or “PULL UP” |
| 2 | Excessive closure rate with terrain | “TERRAIN” followed by “PULL UP” |
| 3 | Altitude loss after take-off or go-around | “DON’T SINK” |
| 4 | Insufficient terrain clearance with unsafe gear, flap or speed configuration | “TOO LOW—GEAR”, “TOO LOW—FLAPS” or “TOO LOW—TERRAIN” |
| 5 | Excessive deviation below an ILS glideslope | “GLIDESLOPE” |
Many installations add Mode 6 advisory functions such as bank-angle warnings, minimums and radio-altitude callouts. Some enhanced systems also provide reactive windshear alerts, often described as Mode 7; predictive windshear detection may instead come from the weather-radar system.
A mistake we see constantly in simulators is treating countdown callouts such as “fifty, forty, thirty” as proof that full GPWS is installed. Those are radio-altitude advisories. A basic gauge may play them without modelling terrain-closure logic, configuration modes or forward-looking warnings.
Does GPWS see terrain ahead?
Classic GPWS does not normally see terrain ahead; it reacts mainly to what the radio altimeter and other aircraft sensors report. That leaves a serious limitation when an aircraft is flying nearly level towards steeply rising ground, because the terrain directly underneath may remain safely distant until late in the encounter.
Enhanced GPWS (EGPWS) addresses this limitation by combining aircraft position with a digital terrain, obstacle and runway database. Its forward-looking logic can generate alerts such as “TERRAIN AHEAD” before the older reactive modes would trigger, and it may provide a coloured terrain display on the navigation display.
Terrain awareness and warning system (TAWS) is the broader generic term for equipment meeting defined terrain-warning capabilities; EGPWS is a widely used implementation name. Pilots often use GPWS loosely for both generations, but a traditional GPWS does not automatically include a terrain database or predictive display.
What should a pilot do after a GPWS warning?
A hard GPWS warning requires the aircraft’s published terrain-escape manoeuvre without delay. The aircraft flight manual, checklist and operator procedure govern the exact response, because pitch targets, automation use and thrust handling are not identical across aircraft.
- A caution such as “TERRAIN AHEAD” requires prompt correction and assessment under the applicable procedure.
- A warning such as “PULL UP” normally calls for an immediate escape manoeuvre, generally involving maximum available thrust, the prescribed pitch response, reduced bank and no configuration change until clear of the threat.
- A windshear warning uses the aircraft’s windshear-escape procedure, which must not be replaced with a routine missed approach.
Visual contact with the ground is not, by itself, a reason to ignore a warning. The crew’s first priority is terrain clearance; communication with air traffic control comes after controlling the flight path. Our explanation of a routine A320 go-around and missed-approach sequence also shows why that manoeuvre is different from responding to a terrain or windshear warning.
Why can GPWS produce a false alert or no alert?
A missing or unwanted GPWS alert usually traces to sensor data, aircraft configuration, an inhibited mode or the limitations of the installed system. Common cases include:
- An invalid radio-altimeter, air-data, glideslope, gear or flap input.
- A steep but intentional approach entering a valid warning envelope.
- Incorrect position or outdated terrain data in an enhanced system.
- An approved airport-specific procedure requiring modified alert logic.
- A classic GPWS encountering steep terrain ahead before the surface directly below enters its warning envelope.
- A simulator aircraft that models altitude callouts but not the full warning logic.
Real aircraft may have authorised inhibit controls for particular operations, but these are not general-purpose silence switches. An unexpected alert should be treated as genuine unless the applicable procedure and positive evidence establish otherwise; a fault indication must be handled through the aircraft checklist and maintenance process.
How accurately is GPWS modelled in flight simulators?
GPWS fidelity varies from simple sound triggers to detailed systems using radio altitude, configuration, terrain data and aircraft-specific warning envelopes. High-fidelity airliners may reproduce most operational behaviour, while default or older aircraft often provide only altitude callouts and a few reactive modes.
For FSX, our guide to adding GPWS and radio-altitude callouts to the default 737 explains what a panel gauge can and cannot reproduce. The FS2004 GPWS gauge and its modelled warning modes provides another practical example of how sink-rate, terrain, windshear and altitude alerts can be added to an older simulator.
If a simulated GPWS remains silent, check avionics power, audio settings, radio-altimeter operation and gear/flap data before assuming the terrain logic is faulty. Use the aircraft’s system-test function where one is modelled; deliberately flying towards terrain is neither a reliable test nor a sensible training method.