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 compute terrain clearance envelopes ahead of the aircraft. Terrain alerts are given if there is any disagreement between the boundaries of these envelopes and the terrain elevation data that is stored in the terrain database.
 Two envelopes are computed for this function: a terrain caution alert envelope, and a terrain warning alert envelope.
 When the aircraft enters the caution envelope, an aural CAUTION TERRAIN message is heard and an amber TERRAIN message shows on the ND. The conflicting terrain areas are also shown in solid yellow on the terrain display.
 When the aircraft enters the warning envelope, an aural TERRAIN TERRAIN, PULL UP message is heard, a red TERRAIN message shows on the ND, and the red PULL UP indication shows and master warning lights come on. The conflicting terrain area shows in solid red on the terrain display. The PULL UP aural message is repeated until the aircraft exits the envelope.
 2)  EGPWS terrain display data is shown on the captain's and first officer's ND. When the terrain display is visible to the flight crew, it replaces the weather radar display. The terrain display can be selected any time by pressing the TERR switch on the EFIS control panel. If neither TERR switch is selected, the terrain data will be automatically displayed if a terrain caution or warning condition exists. a) The "peaks" feature shows the highest and lowest
 elevations for the terrain being displayed. The graphical terrain map is displayed at all altitudes. (Without peaks, the display is blank for elevations greater than 2000 feet above terrain.)
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 b)  The "obstacles" feature utilizes a database for obstacle conflict alerting and display that is not yet worldwide. This feature provides caution and warning visual and audio alerts when an obstacle conflict is detected.
 (11) Ground Proximity Warning Computer Function (Fig. 3)
 (a)  
The ground proximity warning computer receives input signals from several aircraft sensors and systems. These input signals are used to compute the airplane flight status. The flight status is compared with the warning and caution mode boundaries to determine if the warning and caution criteria are met. The computer generates the aural and visual messages to the warning and caution lights, displays, speakers, and the EICAS computer.

 (b)  
Signals from the following systems are used to develop the warning and alerting modes:

 -Air Data System
 -EFIS/EICAS Integrated Display System (IDS)
 -Flight Management System
 -Inertial Reference System
 -Instrument Landing System (ILS) or ILS MMR
 -Radio Altimeter System
 -Stall Warning System

 (c)  
Signals from these systems may also be used:

 -Global Positioning System Multi-Mode Receiver (MMR), if installed and connected
 -Weather Radar System, if connected
 1) Air Data System

 The GPWS uses true airspeed, computed airspeed, corrected barometric altitude, uncorrected barometric altitude, static air temperature, and altitude rate from the Air Data System.
 2)  EFIS/EICAS Integrated Display System (IDS)
 The GPWS uses decision height and PFD discrete word data (terrain select, range) from the IDS system.
 3)  Flight Management System
 The GPWS uses latitude, longitude, and magnetic track angle data from the Flight Management System.
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TERRAIN
AWARENESS
ALERTING AND
DISPLAY
ALGORITHMS

 

AUDIO ALERT MESSAGES

FLIGHT DECK
SPEAKERS AND
INTERPHONE

VISUAL ALERT MESSAGES

ALERT LAMPS
AND EFIS
DISPLAY

TERRAIN
EFIS NAV
DISPLAY DATA

DISPLAY OR
Wx RADAR
INDICATOR

AIRCRAFT
PARAMETERS
　
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