777 AMM 飞机维护手册 导航 Navigation 1(68)

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Description
Windshear is the effect of large volumes of air that change direction quickly. The most dangerous type of windshear for airplanes is the microburst. When the airplane is near the terrain, a downward microburst gives a pilot little time to respond to the effects.
The initial effects on an airplane when it is in a microburst are an increase in airspeed and lift. Because of the increase in lift, the airplane altitude increases.
As the airplane continues through the microburst, the effects quickly change to a loss in airspeed and lift. Because of the decrease in lift, the airplane altitude decreases.
The GPWC uses inputs from airplane systems to detect the changes caused by windshear conditions.
Mode 7 warnings are:
*
Master warning lights come on.

*
Siren then the aural message WINDSHEAR WINDSHEAR WINDSHEAR from the aural warning speakers

* Red WINDSHEAR message on the PFDs.
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777 AIRCRAFT MAINTENANCE MANUAL

GPWS -MODE 7 DESCRIPTION

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GPWS -MODE 7 OPERATION

General
Program pins enable mode 7.
These LRUs supply data for mode 7 operation:
*
Left, right, and center radio altimeter transceivers

*
AIMS cabinets

*
Program pins.

The GPWC uses this data to detect windshear:
*
Radio altitude

*
Inertial vertical speed

*
Pitch and roll angle

*
Pitch rate

*
Longitudinal acceleration

*
Normal acceleration

*
Vertical acceleration

*
Indicated angle of attack (AOA)

*
Corrected AOA

*
Stick shaker AOA

*
Flap angle

*
Minimum operating speed

*
True airspeed

*
Computed airspeed.

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GPWS -MODE 7 OPERATION

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GPWS -TERRAIN AWARENESS

Purpose
The ground proximity warning computer (GPWC) compares the airplane flight profile, flap and gear position, and terrain clearance to determine if an alert or warning condition exists.
Terrain Awareness Inputs
The GPWC receives airplane data from the air data inertial reference system (ADIRS) and the global positioning system (GPS). The terrain awareness function uses this data:
*
Latitude

*
Longitude

*
Barometric altitude

*
Ground track

*
Ground speed

*
Heading

*
Roll attitude

*
Flight path angle (calculated by GPWC).

Terrain Awareness Calculation
The terrain awareness function uses GPS for latitude and longitude. It uses air data inertial reference system (ADIRS) data if the GPS data is invalid or not available. If the GPWC finds the ADIRS data is not accurate enough for the current flight phase, the terrain awareness function fails.
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The GPWC stores a world-wide terrain database in memory. The GPWC looks at airplane position and track, and compares this data to the terrain database. If the GPWC finds there is a terrain threat, it makes an alert.
Terrain Display Output
The GPWC makes a digital map of the terrain forward of the airplane. It sends this digital map to AIMS to show on the navigational displays (NDs). The display uses different colored dots to show terrain altitude relative to airplane altitude.
Terrain Alert Outputs
If the GPWC finds the airplane is 60 seconds from a terrain conflict, it makes a terrain caution alert. These are the caution alert indications:
*
Alert aural message CAUTION TERRAIN

*
Amber message TERRAIN shows on the ND

*
The terrain display shows on both NDs if not selected on either one

*
The threat terrain on the ND changes from dots to a solid yellow color

*
The ground proximity warning light comes on.

If the GPWC finds the airplane is 30 seconds from a terrain conflict, it makes a terrain warning alert. These are the warning alert indications:
* Aural message TERRAIN, TERRAIN PULL UP
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GPWS -TERRAIN AWARENESS

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