Ground-Proximity Warning Systems (GPWS/EGPWS): Modes, Terrain Logic, and Escape

022-12-09Pablo Asensio Martínez2026-03-272 min

The GPWS (Ground Proximity Warning System) is designed to generate visual and aural warnings when the aircraft's proximity to the terrain poses a safety threat, specifically to prevent CFIT (Controlled Flight Into Terrain).

System Inputs and Operation

The system requires inputs from the Air Data Computer (ADC) (barometric altitude, vertical speed, CAS), Radio Altimeter (height AGL), ILS (glide slope), and aircraft configuration (flaps, landing gear).

  • Operating Range: Automatically active between 50 ft and 2500 ft Radio Altitude.
  • Requirement: Mandatory for turbine-powered aircraft > 5,700 kg or > 9 passengers.

Modes of Operation

The basic GPWS (non-enhanced) has 7 modes based on distinct conditions:

Mode Condition Aural Alert Aural Warning
1 Excessive Descent Rate "SINK RATE" "PULL UP"
2 Excessive Terrain Closure Rate "TERRAIN" "PULL UP"
3 Altitude Loss after Take-off/Go-around "DON'T SINK" -
4 Unsafe Terrain Clearance (Config) "TOO LOW GEAR/FLAPS/TERRAIN" -
5 Below Glide Slope Deviation "GLIDE SLOPE" (Soft) "GLIDE SLOPE" (Loud)
6 Altitude Callouts / Excessive Bank "MINIMUMS", "BANK ANGLE" -
7 Windshear (Reactive) "WINDSHEAR" (x3) Siren + Voice
  • Mode 5 Inhibition: Can be cancelled by the pilot (e.g., for visual approaches or localizer-only approaches) using a specific button ("G/S INHIBIT" or similar).
  • Mode 7 Priority: Windshear alerts take priority over all other GPWS modes.

Enhanced GPWS (EGPWS) / TAWS

Basic GPWS relies on the Radio Altimeter, which looks straight down. It effectively "looks down, not ahead".

  • Limitation: A sheer cliff or rapid terrain rise might not be detected in time by Mode 2.
  • Solution: EGPWS (or TAWS - Terrain Awareness and Warning System) adds a Forward-Looking Terrain Avoidance (FLTA) function.
  • Mechanism: Uses a worldwide Terrain Database matched with the aircraft's position (GPS/FMS) to predict conflicts.

Terrain Display (EGPWS)

Using the navigation display to show terrain threats:

  • Red: Terrain well above aircraft altitude (Warning).
  • Amber/Yellow: Terrain near aircraft altitude (Caution).
  • Green: Terrain safely below aircraft.
  • Magenta: No terrain data available.

Pilot Response (PULL UP Warning)

If a "PULL UP" or "TERRAIN AHEAD, PULL UP" warning occurs:

  1. Autopilot: Disconnect.
  2. Pitch: Apply Maximum Backstick (Full Aft) / Pitch up to the limit (stick shaker/PLI).
    • Note: In Fly-By-Wire aircraft, full backstick commands max available alpha without stalling (alpha protection).
  3. Thrust: Apply Maximum Power (TOGA).
  4. Speedbrakes: Retract.
  5. Bank: Wings Level.
  6. Configuration: Do not change gear or flap configuration until clear of the obstacle (drag considerations).