Ground Proximity Warning System (GPWS)

Ground Proximity Warning System

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

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