Ground Proximity Warning System (GPWS)

Wg Cdr Rajagopal

Ground Proximity Warning System

Index

Introduction to GPWS

  • Ground proximity warning system prevents controlled flight into terrain
  • Warns of proximity to the terrain which are potential threats to safety
  • Operates in height band 50 to 2450 feet, which is same as radio altimeter
  • Automatic selection of operation mode based on flight profile
  • Radio altimeter, Mach meter, ADC and Glidepath provide inputs
  • Undercarriage, Flaps and Navigation systems also provide inputs

Stages of GPWS Warnings

  • GPWS warnings fall under two categories
    • Alert is a caution which means advisory instructions.
    • Warnings are commands which means obey the instructions.

Mode 1 – Excessive Barometric Descent Rate

  • Mode 1 has two boundaries and is independent of aircraft configuration
  • This mode activates below 2500 feet on radio altimeter.
  • Penetration of the first boundary generates an voice alert of “SINK RATE”
  • Audio warning repeats every 1.5 seconds.
  • Penetrating the second boundary causes the repeated warnings
  • Audio of “WHOOP, WHOOP PULL UP”
  • Visual light indication of ‘PULL UP’

Mode 2 – Excessive Terrain Closure Rate

  • Mode 2 monitors five parameters to set its boundaries
  • Mach, radio altitude, rate of change, barometric altitude and configuration
  • Mode 2 operates below 1500 feet at 0.35 mach
  • Increase in mach number to 0.45 with gear up increases height to 2450 feet
  • Flap override switch overrides the higher portion of envelope
  • First boundary triggers an audio alert of “Terrain, Terrain” every 1.5 seconds.
  • Second boundary generates audio warning of “WHOOP, WHOOP PULL UP”
  • In landing configuration, only the repeating TERRAIN voice alert is heard.

Mode 3 – Descent after Take-off or Go-around

  • Mode 3 alerts for descent is made during initial climb or go-around
  • The audio alert of “DON’T SINK” is repeated each 1.5 seconds
  • Operating height is between 50 and 700 feet of radio altitude.
  • Alerts when aircraft descends more than 10 percent of barometric altitude.
  • Mode 3 does trigger below 200 feet of radio altitude.

Mode 4A – Unsafe Terrain Clearance – Landing Gear Not down

  • Mode 4A operates if aircraft descends below set height without landing gear.
  • This mode operates below 500 feet of radio altitude
  • Landing gear mode armed after aircraft climbs above 700 feet radio altitude.
  • Upper limit increases from 500 feet to 1000 feet radio altitude.
  • Below Mach 0.35, the aural alert “TOO LOW GEAR” is heard every 1.5 seconds
  • Above Mach 0.35, voice alert “TOO LOW TERRAIN” is heard every 1.5 seconds

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

  • Mode 4B alerts if landing gear is down without flaps below 300 feet.
  • Below Mach 0.28, voice alert of “TOO LOW FLAPS” is heard every 1.5 seconds
  • Above Mach 0.28, audio of “TOO LOW TERRAIN” is heard every 1.5 seconds
  • Upper boundary increases to 1000 feet radio altitude.
  • “TOO LOW GEAR” alert takes priority over the “TOO LOW FLAPS”.
  • Mode 4B is inhibited by ‘FLAP OVRD’ switch

Mode 5 – Below Glide Slope Deviation Alert

  • Mode 5 alerts descent of 1.3 dots below glide slope, below 1000 feet on ILS
  • Operates when glide slope receiver is active and landing gear is down
  • Hard alert sounds at full volume whereas Soft alert is at reduced volume
  • Repetition rate increases with glide slope deviation and reducing in altitude
  • Mode 5 is cancelled by pressing ‘BELOW G/S’ light
  • Above 1000 feet radio altitude mode 5 will get re armed.
  • Mode 1 to 4 audio alerts have priority over mode 5
  • PULL UP and BELOW G/S lights could light up simultaneously

Mode 6A – Descent Below Selected Minimum Radio Altitude

  • Mode 6A alerts on descent below the minimum decision altitude
  • MDA is set using a cursor on captain’s radio altimeter.
  • Mode 6A is active between 50 and 1000 feet of radio altitude.
  • Audio alert consists of “MINIMUMS, MINIMUMS” sounded only once.
  • The mode is re-armed when the radio altitude increases above the MDA.

Mode 6B – Altitude Call-outs and Bank Angle Alert

  • Mode 6B calls out at altitudes at pre selected intervals and at minimums
  • Normally, calls out at 200 ft and 100 ft to decision height
  • Alternately absolute height call-outs from the radio altimeter.
  • Mode 6B also calls out “BANK ANGLE” to alert for excessive roll angles.
  • Bank angle limits reduce with proximity to the ground

Mode 7 – Wind Shear Alert

  • Mode 7 provides visual and audio wind shear warnings based on calculations
  • Pilot can initiate quicker wind shear go-around actions.
  • Mode 7 warnings take priority over all other modes.
  • GPWS calculates the initial conditions for entering an area of wind shear.

Mode 7 – Wind Shear Alert Calculation

  • The calculations are based on five inputs
    • Ground speed
    • Air speed
    • Barometric height
    • Rate of descent
    • Radio altitude

Enhanced Ground Proximity Warning System

  • EGPWS provides warnings from rising terrain ahead using TAWS
  • Terrain Awareness and Warning System is a world-wide terrain data base
  • Terrain ahead is shown in shades of green, yellow and red on a display
  • Display could be independent or of weather radar or navigation screen
  • Audio warning may be of Caution Terrain Ahead or Whoop Whoop Pull Up
  • Voice message in this mode depends on the type of danger

Action on GPWS Warning

  • Pilots must take positive and immediate response to GPWS warnings
  • Wings level and initiate a maximum gradient climb until minimum safe altitude
  • Find the cause of the warning and verify the aircraft’s position.

Pilot Override on GPWS Warning

  • Action can be deferred only under the following circumstances
    • Positively identification of cause
    • Warning has ceased
    • During day
    • Clear visual conditions
  • Obviously not in any dangerous situation with regard to three factors
  • Terrain clearance, Aircraft configuration or the Manoeuvre

Runway Awareness and Advisory System

  • Runway Awareness and Advisory System increases situational awareness
  • Reduce the likely-hood of runway incursions.
  • Provides advisory during taxi, take off approach and landing
  • RAAS uses Terrain Clearance Floor for its operation
  • Terrain clearance floor is a height map in the vicinity of airfields
  • Avoids premature descent on non-precision approaches

Share this Page