Ground Proximity Warning System (GPWS)

Wing Commander Rajagopal

Warning of Controlled Flight into Terrain by Ground Proximity Warning System (GPWS)

Introduction to GPWS

  • The Ground Proximity Warning System (GPWS) helps prevent Controlled Flight Into Terrain (CFIT).
  • It warns the flight crew when the aircraft is in unsafe proximity to terrain.
  • The system typically operates between 50 ft and 2450 ft radio altitude, similar to the radio altimeter range.
  • GPWS automatically selects operating modes based on flight conditions.
  • It uses inputs from the radio altimeter, Mach meter, Air Data Computer (ADC), and glide slope receiver.
  • Additional inputs include landing gear, flap position, and navigation systems.

Stages of GPWS Warnings

  • GPWS warnings are categorised into two levels:
    • Alerts – Cautionary warnings requiring pilot awareness and preparation.
    • Warnings – Mandatory commands requiring immediate pilot action.

Operating Modes of GPWS

Mode 1 – Excessive Descent Rate (Sink Rate)

  • Mode 1 operates independently of aircraft configuration.
  • Active below approximately 2500 ft radio altitude.
  • First threshold triggers the alert: “SINK RATE” (repeated every 1.5 seconds).
  • Second threshold triggers urgent warning: “WHOOP WHOOP PULL UP”.
  • A red PULL UP visual warning is also displayed.

Mode 2 – Excessive Terrain Closure Rate

  • Mode 2 monitors aircraft closure rate to terrain using multiple parameters:
    • Mach number
    • Radio altitude
    • Rate of descent
    • Barometric altitude
    • Aircraft configuration
  • Typically active below 1500 ft at Mach 0.35.
  • At higher speeds or gear-up configuration, activation height increases up to 2450 ft.
  • First alert: “TERRAIN, TERRAIN” (repeated every 1.5 seconds).
  • Second warning: “WHOOP WHOOP PULL UP”.
  • In landing configuration, only the “TERRAIN” alert may sound.

Mode 3 – Descent After Take-off or Go-Around

  • Active during initial climb or go-around phases.
  • Triggers when the aircraft loses more than ~10% of barometric altitude gain.
  • Operates between 50 ft and 700 ft radio altitude.
  • Audio alert: “DON’T SINK” repeated every 1.5 seconds.

Mode 4A – Unsafe Terrain Clearance (Gear Not Down)

  • Active below approximately 500 ft radio altitude.
  • Warns when landing gear is not extended during descent.
  • Upper limit increases up to 1000 ft under certain conditions.
  • Below Mach 0.35: “TOO LOW GEAR”.
  • Above Mach 0.35: “TOO LOW TERRAIN”.

Mode 4B – Unsafe Terrain Clearance (Flaps Not in Landing Configuration)

  • Active when gear is down but flaps are not properly configured below ~300 ft.
  • Below Mach 0.28: “TOO LOW FLAPS”.
  • Above Mach 0.28: “TOO LOW TERRAIN”.
  • Gear warning has higher priority than flap warning.
  • This mode can be inhibited using the flap override switch.

Mode 5 – Glide slope Deviation Alert

  • Activates when aircraft deviates significantly below Glide slope (about 1.3 dots).
  • Operates below 1000 ft during ILS approach.
  • Requires active Glide slope receiver and landing configuration.
  • Audio warning: “GLIDESLOPE” (hard or soft depending on deviation).
  • Can be cancelled using the BELOW G/S switch.

Mode 6A – Minimums Alert

  • Warns when aircraft descends below selected minimum decision altitude.
  • Active between 50 ft and 1000 ft radio altitude.
  • Audio call-out: “MINIMUMS, MINIMUMS” (single alert).
  • Re-armed when aircraft climbs above selected minimum altitude.

Mode 6B – Altitude Callouts and Bank Angle Alert

  • Provides automatic altitude callouts at predefined heights (e.g., 200 ft, 100 ft).
  • Also provides “BANK ANGLE” warnings during excessive roll.
  • Bank angle limits reduce as aircraft gets closer to the ground.

Mode 7 – Wind Shear Alert

  • Provides visual and aural warnings for wind shear conditions.
  • Has highest priority over all other GPWS modes.
  • Enables immediate go-around decision by pilot.
  • Uses multiple inputs:
    • Ground speed
    • Airspeed
    • Barometric altitude
    • Rate of descent
    • Radio altitude

Advancements in GPWS like EGPWS and RAWS

Enhanced Ground Proximity Warning System (EGPWS)

  • EGPWS incorporates a terrain awareness database (TAWS).
  • Provides predictive terrain warnings ahead of the aircraft.
  • Displays terrain in colour-coded format:
    • Green – safe terrain
    • Yellow – caution
    • Red – immediate danger
  • Can be integrated with weather radar or navigation displays.
  • Typical alerts include “TERRAIN AHEAD” and “PULL UP”.

Action on GPWS Warning

  • Pilots must respond immediately and positively to GPWS warnings.
  • Standard response: wings level and maximum climb rate.
  • Climb to a safe altitude as quickly as possible.
  • Verify aircraft position and identify cause of warning.

Pilot Override Conditions

  • GPWS warnings should only be disregarded if:
    • Cause of warning is positively identified.
    • Warning has ceased.
    • Daytime conditions with clear visibility exist.
    • No immediate terrain, configuration, or manoeuvrer hazard is present.

Runway Awareness and Advisory System (RAAS)

  • RAAS improves situational awareness during taxi, take-off, approach, and landing.
  • Reduces risk of runway incursions.
  • Uses a Terrain Clearance Floor database around airports.
  • Helps prevent premature descent during non-precision approaches.

Pilot Actions on GPWS Warning