Warning Systems - GPWS

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

Introduction

GPWS provides time-critical alerts for potentially hazardous flight conditions involving imminent impact with the ground. GPWS is enabled whenever power is applied to the airplane. Override or inhibit switches allow the flight crew to inhibit certain GPWS alerts.

GPWS also provides voice Callouts (Mode 6) to assist the flight crew with situational awareness and to advise the flight crew of the aircraft’s approximate height above the ground.

GPWS also provides time-critical alerts for potential hazardous flight conditions involving windshear.

In addition to aircraft configuration, GPWS requires inputs from the following for proper operation:

 

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

GPWS immediate alerts are radio altitude based, available between 30 ft and 2450 ft and provided for the following:

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

Enhanced GPWS

GPWS versus EGPWS

In addition to standard alerts, enhanced GPWS provides look-ahead terrain awareness, including alerting and display functions.

In addition to standard alerts, a non-Boeing installed (STC) enhanced GPWS provides look-ahead terrain awareness, including alerting and display functions.

These functions compare the airplane’s geographic position and altitude against an internal terrain database to predict and display potential conflicts between the airplane flight path and terrain.

In addition to standard GPWS inputs, enhanced GPWS requires inputs from the following for proper operation:

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.

GPWS Alert Prioritization

GPWS and Windshear Warning System alerts are prioritized based on the level of hazard and the required flight crew response. The following are listed in order of decreasing priority:

Condition Alert Level Description
Windshear - Immediate WARNING

Mode 7 - Actual windshear conditions (downdraft)

A windshear immediate-alert warning inhibits all other GPWS and windshear alerts.

Ground proximity -
Immediate
WARNING Mode 1 - Severe decent rate
Ground proximity -
Immediate
WARNING Mode 2 - Severe terrain closure rate

Terrain - Awareness

WARNING Look-Ahead - Terrain along flight path (Near)
Ground proximity -
Immediate
Caution Mode 2 - Excessive terrain closure rate

Terrain - Awareness

Caution Look-Ahead - Terrain along flight path (far)
Ground proximity -
Immediate
Caution Mode 4 - Unsafe terrain clearance
Altitude - Awareness   Mode 6 - Altitude Callouts
Ground proximity -
Immediate
Caution Mode 4 - Unsafe terrain clearance - Gear
Mode 4 - Unsafe terrain clearance - Flaps
Mode 1 - Excessive decent rate
Mode 3 - Decent (sink rate) after takeoff
Mode 5 - Below glideslope - ILS deviation

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

GPWS Immediate-Alert Modes

GPWS immediate-alert (time critical) warnings (Modes 1-2) are accompanied by:

GPWS immediate-alert warnings are accompanied by master WARNING light illumination.

If illuminated, pushing a Master WARNING/CAUTION Reset switch resets the Master WARNING lights but does not inhibit the GPWS warning.

GPWS immediate-alert (time critical) cautions (Modes 1-5) are accompanied by:

The master CAUTION lights do not illuminate for ground proximity cautions.

GPWS Immediate Alert Warnings

Aural Alert Visual Alert Description
PULL UP sound clip PULL UP warning

Master WARNING lights

Severe Descent Rate

Mode 1 - Severe Descent Rate

Follows SINK RATE alert if descent rate becomes severe. (Mode 1)


PULL UP sound clip PULL UP warning

Master WARNING lights

Mode 2a - No Flaps

Mode 2b - With Flaps

Follows TERRAIN alert if excessive terrain closure rate continues and landing gear and/or flaps are not in landing configuration. (Mode 2)

GPWS Immediate Alert Cautions

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

Mode 2a - No Flaps

Excessive terrain closure rate. (Mode 2)

TERRAIN sound clip GND PROX (G/S INHIB) Mode 2b - Excessive Terrain Closure Rate with Flaps in Landing Position

Mode 2b - With Flaps

Excessive terrain closure rate. (Mode 2)

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.

DON'T SINK sound clip GND PROX (G/S INHIB) Mode 3 - Atitude Loss after Takeoff or Go-Around

Mode 3

Excessive altitude loss after takeoff or go-around. (Mode 3)

GLIDESLOPE sound clip GND PROX (G/S INHIB) Mode 5 - Below Glide Slope Deviation

Mode 5

Deviation of more than 1.3 dots below glide slope. (Mode 5)

Volume and repetition rate increase as deviation increases.

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

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.

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

SINK RATE sound clip GND PROX (G/S INHIB) Excessive Descent Rate

Mode 1 - Excessive Descent Rate

Excessive descent rate. (Mode 1)


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

Mode 4b - No Land Flap

Unsafe terrain clearance at low airspeed with flaps not in landing configuration. (Mode 4)

Note: Pushing the GND PROX FLAP OVRD switch to OVRD inhibits the 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

Mode 4a - No Gear

Unsafe terrain clearance at low airspeed with landing gear not down. (Mode 4)

Note: Pushing the GND PROX GEAR OVRD switch to OVRD inhibits the alert.

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

Mode 3

Follows DON’T SINK if another descent is initiated after initial alert and before climbing to the altitude where the initial descent began. (Mode 3)

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

Mode 4b - No Land Flap

Unsafe terrain clearance at low airspeed with either landing gear not down or flaps not in landing position. (Mode 4)

Note: Pushing the GND PROX FLAP OVRD switch to OVRD inhibits the alert, when the alert is due to flaps not in landing position.

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

Mode 4a - No Gear

Unsafe terrain clearance at low airspeed with either landing gear not down or flaps not in landing position. (Mode 4)

Note: Pushing the GND PROX GEAR OVRD switch to OVRD inhibits the alert, when the alert is due to gear not down.

Look-Ahead Alerts and Display

Enhanced GPWS provides look-ahead alerts for potentially hazardous flight conditions involving impact with the ground. GPWS monitors terrain proximity and generates a display from a world-wide terrain data base in the GPWS computer. The data base contains detailed terrain data near major airports and data in lesser detail for areas between airports.

Airplane horizontal position is determined using the:

Barometric altitude errors induced from temperature extremes or from non-standard pressure altitudes are minimized. Airplane vertical position is determined using a blended solution calculated from the following:

  • GPS altitude
  • barometric altitude
  • radio altitude
  • static air temperature

Look-Ahead Displays

When the EFIS control panel terrain (TERR) display select switch is pushed on, the TERR annunciation is displayed on the HSI and terrain contours may be displayed.

GPWS look-ahead displays are enabled by depressing the weather/terrain (WXR/TERR) select switch on the EFIS control panel. Selecting the respective terrain (TERR) display select switch to ON will change the HSI display to GPWS look-ahead data. Depending on airplane altitude terrain contours may be displayed. Selecting the weather (WXR) display select switch to ON will return the weather radar display with WXR annunciated on the HSI. Reselecting the active (illuminated WXR ON or TERR ON) switch will deselect both weather and look-ahead data providing a black HSI background.

GPWS look-ahead data and Weather Radar returns cannot be displayed simultaneously on an HSI. If either pilot selects terrain while the other selects weather radar, each display updates on alternating sweeps. All other navigation displays (TCAS, etc) can be simultaneously displayed with terrain data.

When the airplane is lower than 2,000 feet above the terrain, terrain within 2,000 feet of airplane barometric altitude is displayed on the HSI. Non-threat terrain is depicted as several densities of dot patterns in green, amber, or red depending on relative vertical distance between the airplane and the terrain.

When the airplane is higher than 2,000 feet above the terrain or within 400 feet of the nearest airport runway elevation, terrain is not displayed.

Note: The GPWS look-ahead display is not designed to be used as an independent navigation aid.

Look-Ahead Alerting

The enhanced GPWS computer continuously computes clearance envelopes looking down and ahead of the airplane. Estimated time to impact is calculated from airplane position, barometric altitude, present track, vertical path, and ground speed. FMC VNAV or LNAV (Refer to Chapter 11, Flight Management, Navigation) paths are not considered in the estimated time to impact.

When the terrain clearance boundaries are crossed the GPWS issues alerts. Alert levels, warning or caution, are based on estimated time to impact. Look-ahead alerts will cause the GPWS look-ahead awareness display to "pop-up" when:

  • the HSI display is in an appropriate mode (see tables)
  • neither terrain (TERR) display select switch is ON
  • the EFIS control panel WXR/TERR has been selected ON

GPWS look-ahead warning alerts are accompanied by:

  • visual alerts (see tables)
  • voice aural alerts (see tables)
  • solid red terrain displayed on HSIs

GPWS look-ahead warnings are accompanied by master WARNING light illumination.

If illuminated, pushing a Master WARNING/CAUTION Reset switch resets the Master WARNING lights but does not inhibit the GPWS warning.

GPWS look-ahead caution alerts are accompanied by:

  • visual alerts (see tables)
  • voice aural alerts (see tables)
  • solid amber terrain displayed on HSIs

The master CAUTION lights do not illuminate for look-ahead caution alerts.

Note: Terrain ahead of the airplane may exceed available climb performance. A GPWS caution or warning alert 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.

Note: Unless otherwise stated, the EGPWS 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 alerts OBSTACLE PULL UP and CAUTION OBSTACLE are generated.

Look-Ahead Alerting - Warnings

Aural
Alert
Visual Alert Description Display
TERRAIN,
TERRAIN,
PULL UP

PULL UP light (red)

Master WARNING lights.

Solid red terrain on HSI

NOT STC A/C Red TERRAIN message on both HSI (all modes).

20 to 30 seconds from projected impact with terrain shown solid red on the HSI.

Pop-up look-ahead display is only available in the following modes:

  • VOR, APP, MAP and CTR MAP

Note: Pushing the GND PROX TERR OVRD switch to OVRD inhibits the alert.

EGPWS Warning

Look-Ahead Alerting - Cautions

Aural
Alert
Visual Alert Description Display
CAUTION
TERRAIN

Solid amber terrain on HSI.

GND PROX light

NOT STC A/C Amber TERRAIN message on both HSI (all modes)

40 to 60 seconds from projected impact with terrain shown solid amber on the HSI.

Pop-up look-ahead display is only available in the following modes:

  • VOR, APP, MAP and CTR MAP

Note: Pushing the GND PROX TERR OVRD switch to OVRD inhibits the alert.

EGPWS Caution
TOO LOW,
TERRAIN
GND PROX light

Terrain Clearance Floor (TCF) alert indicating unsafe terrain clearance based on current airplane location, nearest runway center point and radio altitude. Similar to Mode 4 but available in all flight modes.

Note: Pushing the GND PROX TERR OVRD switch to OVRD inhibits the alert.

Note : Although some manufacturers may show TOO LOW TERRAIN to be a caution, most companies dictate that the response is as for a warning.

GPWS Terrain Display

Color 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

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 emphasizing 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 colors 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 color as the highest terrain color pattern on the display, and the "lowest" terrain number is shown in the color of the lowest terrain color 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 color dot patterns and terrain elevation threshold values are shown in the following table:

(Bold Italics indicate new functionality in Peaks Mode)

Color 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 colors 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 color dot patterns if the aircraft display hardware supports the color cyan. The Peaks Mode display is prioritized such that higher-level colors and densities override lower color and densities for maximum situational awareness.

GPWS Callouts