Warning Systems - GPWS

Amendment: Note regarding Look-Ahead Terrain Alerting positional inputs.

Ground Proximity Alerts

The Ground Proximity Warning System (GPWS) provides alerts for potentially hazardous flight conditions involving imminent impact with the ground.

Ground proximity time critical warnings are accompanied by master WARNING light illumination and voice aural alerts. Ground proximity cautions are accompanied by ground proximity light illumination and a voice aural alert.

If illuminated, pushing a master warning/caution reset switch resets the master WARNING lights but does not deactivate the ground proximity warning.

Aural cautions are accompanied by ground proximity light illumination. The master CAUTION lights do not illuminate for ground proximity cautions.

Note: Terrain ahead of the airplane may exceed available climb performance. A ground proximity caution or warning does not guarantee terrain clearance.

Note: The GPWS does not provide an alert for flight toward vertically sheer terrain or slow descents into terrain while in the landing configuration.

GPWS versus EGPWS The GPWS requires inputs from the following systems for proper operation:

In addition, the terrain awareness alerting and terrain display functions [Look-Ahead Terrain Alerting] require inputs from the:

Note: Loss of an input does not necessarily inhibit operation of the entire GPWS.


capability Radio Altitude based alerts
(with aural callouts)
Look-Ahead Alerts
(note 1)
Terrain Display
(note 1)
Peaks Display Mode
(note 2)
Basic GPWS YES - - -
Basic EGPWS YES YES YES -
Enhanced EGPWS YES YES YES YES

Notes:

  1. Requires a terrain database plus barometric altitude and position inputs.
  2. The ability to display terrain more than 2000 feet below current altitude.
Radio Altitude Based Alerts

The GPWS provides alerts based on radio altitude and combinations of barometric altitude, airspeed, glide slope deviation, and airplane configuration and is operative between 30 ft and 2450 ft RA. The alerts are for:

  1. Excessive descent rate
  2. Excessive terrain closure rate
  3. Altitude loss after takeoff or go-around
  4. Unsafe terrain clearance when not in the landing configuration
  5. Excessive deviation below an ILS glide slope
  6. Altitude Callouts
  7. Windshear Detection

These alerts are "radio altitude based alerts".

Radio altitude based alerts are prioritised based on the level of hazard and the required flight crew reaction time.

As installed
Radio altitude based alerts are inhibited by an actual windshear warning (airplane in windshear).


Mode 1 - Excessive Descent Rate
Aural Alert SINK RATE sound clip GND PROX (G/S INHIB) Excessive Descent Rate
Visual Alert GND PROX light
Description Excessive descent rate.

Aural Alert PULL UP sound clip PULL UP warning Excessive Descent Rate
Visual Alert

Master WARNING lights.
As installed - Red PULL UP message on both ADIs.

Description

Follows SINK RATE alert if descent rate lights becomes severe.


Mode 2a - Excessive Terrain Closure Rate with Flaps not in Landing Position
Aural Alert TERRAIN sound clip GND PROX (G/S INHIB) Mode 2a - Excessive Terrain Closure Rate with Flaps not in Landing Position
Visual Alert GND PROX light
Description Excessive terrain closure rate.



Aural Alert PULL UP sound clip PULL UP warning Mode 2a - Excessive Terrain Closure Rate with Flaps not in Landing Position
Visual Alert

Master WARNING lights.
As installed - Red PULL UP message on both ADIs.

Description

Follows radio altitude based TERRAIN alert if excessive terrain closure rate continues and landing gear and/or flaps are not in landing configuration.


Mode 2b - Excessive Terrain Closure Rate with Flaps in Landing Position
Aural Alert TERRAIN sound clip GND PROX (G/S INHIB) Mode 2b - Excessive Terrain Closure Rate with Flaps in Landing Position
Visual Alert GND PROX light
Description Excessive terrain closure rate.

The height of the floor of the Mode 2B envelope will vary between 200 and 600 feet radio altitude based on barometric descent rate. This provides additional protection against nuisance alerts during approaches where the runway threshold is in close proximity to rising terrain, as in the case of an airport on a plateau.


Mode 3 - Altitude Loss after Takeoff or Go-Around
Aural Alert DON'T SINK sound clip GND PROX (G/S INHIB) Mode 3 - Atitude Loss after Takeoff or Go-Around
Visual Alert GND PROX light
Description Excessive altitude loss after takeoff or go-around.



Aural Alert TOO LOW, TERRAIN sound clip GND PROX (G/S INHIB) Mode 3 - Atitude Loss after Takeoff or Go-Around
Visual Alert GND PROX light
Description

Follows DON'T SINK if another descent is initiated after initial alert, before climbing to the altitude where the initial descent began.


Mode 4a - Unsafe Terrain Clearance with Landing Gear Not Down AND Flaps Not in Landing Position
Aural Alert TOO LOW, GEAR sound clip GND PROX (G/S INHIB) Mode 4a - Unsafe Terrain Clearance with Landing Gear Not Down AND Flaps Not in Landing Position
Visual Alert GND PROX light
Description

Ground Proximity / Configuration Gear Override SwitchUnsafe terrain clearance at low airspeed with landing gear not down.

Pushing the ground proximity GEAR override switch to OVRD inhibits the alert.


Aural Alert TOO LOW, TERRAIN sound clip GND PROX (G/S INHIB) Mode 4a - Unsafe Terrain Clearance with Landing Gear Not Down AND Flaps Not in Landing Position
Visual Alert GND PROX light
Description

Unsafe terrain clearance at high airspeed with either landing gear not down or flaps not in landing position.


Mode 4b - Unsafe Terrain Clearance with Landing Gear Not Down OR Flaps Not in Landing Position
Aural Alert TOO LOW, FLAPS sound clip GND PROX (G/S INHIB) Mode 4b - Unsafe Terrain Clearance with Landing Gear Not Down OR Flaps Not in Landing Position
Visual Alert GND PROX light
Description Ground Proximity Flap Override Switch

Unsafe terrain clearance at low airspeed with flaps not in landing configuration.

Pushing the ground proximity FLAP override switch to OVRD inhibits the alert.


Aural Alert TOO LOW, TERRAIN sound clip GND PROX (G/S INHIB) Mode 4b - Unsafe Terrain Clearance with Landing Gear Not Down OR Flaps Not in Landing Position
Visual Alert GND PROX light
Description

Unsafe terrain clearance at high airspeed with either landing gear not down or flaps not in landing position.


Mode 5 - Below Glide Slope Deviation

The mode is armed when a valid signal is being received by the left glide slope receiver, the radio altitude is 1000 feet or less and the landing gear is down.

Aural Alert GLIDESLOPE sound clip GND PROX (G/S INHIB) Mode 5 - Below Glide Slope Deviation
Visual Alert GND PROX light
Description

Deviation of more than 1.3 dots below glide slope. Volume and repetition rate increase as deviation increases. sound clip

Pushing the ground proximity G/S INHB switch cancels or inhibits the alert below 1,000 feet radio altitude.

The mode rearms when climbing above 1000 feet radio altitude or descending below 30 feet radio altitude.


TCX Although some manufacturers of GPWS equipment may show in their literature TOO LOW TERRAIN to be an alert, Company procedure [dictates] that the response is as for a warning.

Look-Ahead Terrain Alerting

As installed
The GPWS monitors terrain proximity using an internal worldwide terrain database. Proximate terrain data shows on the HSI. If there is a potential terrain conflict, alerts are provided based on estimated time to impact. These alerts are "look-ahead terrain alerts".

The GPWS terrain database contains detailed terrain data near major airports, and data in lesser detail for areas between airports. Terrain within 2,000 feet of airplane barometric altitude shows on the navigation display. The terrain data is not designed to be an independent navigation aid.

Note: The GPWS terrain database, look-ahead terrain alerting, and terrain display do not account for man made obstructions. Where such information is available in the data base, the warnings OBSTACLE PULL UP and CAUTION OBSTACLE can be generated.

Although the terrain display is generated using a built in sweep, the display is not a radar display; the terrain display is computer-generated from a database contained in the GPWS computer and correlated to GPS position. Terrain Alerting and Display is not available in areas without terrain data.

Terrain and Weather Radar cannot show together on a display. If one pilot selects terrain and the other pilot selects weather radar, each display updates on alternating sweeps. All other displays (TCAS, LNAV routing, etc.) can show with terrain data.

Look-ahead terrain alerts are based on the airplane's position, barometric altitude, vertical flight path, and ground speed.

Look-ahead terrain alerts are prioritised based on the level of hazard and the required flight crew reaction time. Look-ahead terrain alerts are inhibited by an actual windshear warning (airplane in windshear).

Ground Proximity Terrain Override Switch
  • OVRD - Inhibits look-ahead terrain alerts and terrain display.
Look-Ahead Terrain Alerts
Aural Alert Visual Alert Description Display
TERRAIN, TERRAIN, PULL UP

Red PULL UP message on both ADIs.

Master WARNING lights.

Red TERRAIN message on HSI (all modes).

Solid red terrain on HSI

20 to 30 seconds from projected impact with terrain shown solid red on the HSI (in MAP, CTR MAP, VOR, or ILS modes only).

Pushing the ground proximity terrain override switch to OVRD inhibits the alert.

EGPWS Warning
CAUTION TERRAIN

Amber TERRAIN message on HSI (all modes)

Solid amber terrain on HSI.

GND PROX light

40 to 60 seconds from projected impact with terrain shown solid amber on the HSI (in MAP, CTR MAP, VOR, or APP modes only).

Pushing the ground proximity terrain override switch to OVRD inhibits the alert.

EGPWS Caution
TOO LOW, TERRAIN GND PROX light

Descent below unsafe radio altitude while too far from any airport in the terrain database.

Pushing the ground proximity terrain override switch to OVRD inhibits the alert.


GPWS Terrain Display
Colour Terrain Elevation. Display
Solid
Red
  Look-ahead terrain warning active. EHSI - Terrain Display
Solid
Amber
  Look-ahead terrain caution active.
Dotted
Red
  Terrain more than 2000 feet above airplane's current altitude.
Dotted
Amber
  Terrain 500 feet below (250 feet with gear down) to 2000 feet above the airplane's current altitude.
Dotted
Green
  Terrain from 2000 feet below to 500 feet (250 feet with gear down) below the airplane's current altitude.
Dotted
Magenta
  No terrain data available.

Note: In areas without terrain data, look ahead terrain alerting and terrain display functions not available. Radio altitude based terrain alerts function normally.

Note: Terrain more than 2,000 feet below airplane altitude or within 400 feet of nearest airport runway elevation does not show. (see Peaks Display Mode)

Automatically shows when:

  • a look-ahead terrain alert occurs, and
  • neither pilot has the terrain display selected, and
  • in expanded MAP, CTR MAP, VOR or APP modes.

Updates with a display sweep, similar to weather radar display.


EGPWS - Peaks Display Mode

As installed
An enhancement to the current "standard" EGPWS terrain display, the Peaks Mode terrain display, allows terrain below the aircraft to be viewed on the HSI during the cruise portion of flight. At altitudes safely above all terrain for the display range chosen, the terrain is displayed independent of aircraft altitude emphasising the highest and lowest elevations to provide increased situational awareness. This increased awareness can be particularly valuable to the flight crew in the event of an unplanned descent or off route deviation and for the purpose of previewing terrain prior to descent.

Previously, the standard GPWS/EGPWS terrain display used colours and shading patterns corresponding to the vertical displacement between terrain elevation and the current altitude of the aircraft. Terrain more than 2000 feet below the aircraft is not displayed and the terrain display is typically blank during the enroute portion of the flight. The Peaks Mode display is thus a "merged" display applicable to all phases of flight.

Typical radar display showing aircraft more than 2000 feet below highest point Typical radar display showing aircraft more than 2000 feet above highest point
Aircraft more than 2000 feet below
highest point.
Aircraft more than 2000 feet above
highest point.

With the new Peaks Mode display, two elevation numbers indicating the highest and lowest terrain currently being displayed are overlaid on the display. The elevation numbers indicate terrain in hundreds of feet above sea level (MSL). The terrain elevation numbers are displayed with the "highest" terrain number on top, and the lowest terrain number beneath it. The "highest" terrain number is shown in the same colour as the highest terrain colour pattern on the display, and the "lowest" terrain number is shown in the colour of the lowest terrain colour pattern on the display. A single elevation number is displayed when the screen is all black or blue as a result of flying over water or relatively flat terrain where there is no appreciable difference in terrain elevations. The elevation numbers on the display are an additional indication that the terrain display is selected.

The background colour dot patterns and terrain elevation threshold values are shown in the following table:

(Bold Italics indicate new functionality in Peaks Mode)

Colour Terrain Elevation.
Solid
Red
  Terrain Threat Area - Warning.
Solid
Yellow
  Terrain Threat Area - Caution.
50%
Red
Dots
  Terrain that is more than 2000 feet above aircraft altitude.
50% Yellow Dots   Terrain that is between 1000 and 2000 feet above aircraft altitude.
25% Yellow Dots   Terrain that is 500 (250 with gear down) feet below to 1000 feet above aircraft altitude.
Solid Green   Shown only when no Red or Yellow terrain areas are within range on the display. Highest terrain not within 500 (250 with gear down) feet of aircraft altitude.
50%
Green
Dots
  Terrain that is 500 (250 with gear down) feet below to 1000 feet below aircraft altitude. OR Terrain that is the middle elevation band when there are no Red or Yellow terrain areas within range on the display.
16%
Green
Dots
  Terrain that is 1000 to 2000 feet below aircraft altitude. OR Terrain that is the lower elevation band when there are no Red or Yellow terrain areas within range on the display.
Black   No significant terrain
16%
Cyan
  Terrain Elevation equal to 0 feet MSL (Optional, requires compatible display).
Magenta Dots   Unknown terrain.

EGPWS Elevation The Peaks Mode display adds a new solid green level to indicate the highest, non-threatening terrain. The existing lower density green display patterns now indicate mid and upper terrain in the display area as well as terrain that is within 2000 feet below the aircraft. The red and Yellow dot patterns are unchanged and continue to indicate terrain that is near or above the current altitude of the aircraft. Solid Yellow and Red colours are unchanged and continue to indicate alert and warning areas relative to the aircraft flight path. Terrain identified as water (0 feet MSL) may optionally be displayed as cyan colour dot patterns if the aircraft display hardware supports the colour cyan. The Peaks Mode display is prioritised such that higher-level colours and densities override lower colour and densities for maximum situational awareness.

GPWS Callouts

As installed
GPWS provides voice alerts for terrain awareness and potentially unsafe flight conditions.

As installed
GPWS provides a voice callout at selected radio altitudes to advise the flight crew of the approximate height above ground level. Voice callouts are provided at:

GPWS also provides a voice callout reaching decision height or minimum descent altitude set on the captain's radio altimeter:

(SB changes)
GPWS provides a voice callout at selected bank angles to advise the flight crew when exceeding bank angle thresholds. A one cycle aural warning BANK ANGLE, BANK ANGLE sound clip sounds when roll angle exceeds 35 degrees, 40 degrees and 45 degrees. Once the warning sounds it will not sound again until roll angle decreases below 30 degrees and again exceeds 35 degrees.

As installed
An aural alert is enabled as the airplane descends from decision height (DH) plus 75 feet to DH plus 50 feet. It is a pulsed tone that activates as the airplane passes through DH plus 50 feet and continues to DH. Also, the repetition rate of the tone increases as the airplane approaches DH. The tone is automatically turned off when the airplane descends below DH or climbs above DH plus 50 feet. The alert can be enabled again by climbing above DH plus 75 feet and then descending through DH plus 50 feet.

As installed
A voice callout of FIFTY ABOVE occurs when the airplane reaches fifty feet prior to the decision height altitude set on the captain's radio altimeter. The voice callout DECIDE occurs at the decision height.

As installed
Some airplanes have a 500 foot callout that sounds only when the airplane is not within ±2 dots of a valid ILS glide slope or valid ILS glide slope is not available. If the Mode 5 aural warning GLIDE SLOPE is active, the 500 foot callout will not sound unless the warning is cancelled with the Glide Slope Inhibit Switch.

Windshear Next