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A red TERRAIN message shows on the ND
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The terrain display shows on both NDs if not selected on either one
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The terrain threat on the ND changes from dots to a solid red color
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The master warning lights come on.
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777 AIRCRAFT MAINTENANCE MANUAL
GPWS -TERRAIN AWARENESS
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GPWS -TERRAIN CLEARANCE FLOOR
Description
Terrain clearance floor (TCF) alerts the flight crew when the airplane descends too low on approach. TCF uses airplane position and a runway database to determine if an alert condition exists.
Terrain Clearance Floor Inputs
The ground proximity warning system (GPWS) receives airplane data from these systems:
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Global positioning system (GPS)
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Air data inertial reference system (ADIRS)
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Radio altitude (RA) system.
TCF uses this data:
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Latitude
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Longitude
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Radio altitude.
TCF uses GPS for latitude and longitude. It uses flight management computing function (FMCF) data if GPS data is not valid.
Terrain Clearance Floor Logic
The GPWC stores a runway database in memory. This database contains the location of all hard surface runways in the world that are 3,500 feet or more in length. TCF makes a terrain clearance envelope around the runway. The altitude of the envelope increases as the distance from the airport increases. GPWC compares airplane latitude, longitude, and radio altitude with TCF envelope data. If the airplane descends through the floor of the envelope, GPWC makes an alert.
TCF makes an alert even if the airplane is in landing configuration.
Terrain Clearance Floor Caution Alert
If the GPWC determines the airplane is below the TCF, it makes this caution alert:
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Aural message TOO LOW TERRAIN. This message repeats for each 20 percent loss of altitude.
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The ground proximity warning light comes on and stays on until the airplane climbs above the TCF.
There is no TCF warning alert.
The GPWC inhibits TCF alerts for any of these conditions:
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Airplane is on the ground
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Less than 20 seconds after takeoff
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Less than 30 feet radio altitude.
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GPWS -TERRAIN CLEARANCE FLOOR
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GPWS -NORMAL DISPLAYS
Terrain Awareness Display Data
The navigational displays (NDs) show this GPWS data:
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Terrain awareness display
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GPWS system messages
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GPWS alert messages.
Terrain Awareness Display
The terrain awareness display on the NDs uses dots to show terrain ahead of the airplane. Dot color and dot pattern density compare terrain altitude with airplane altitude. These are the dot colors and patterns the terrain display uses:
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High density red -Terrain more than 2,000 feet above airplane altitude.
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High density yellow -Terrain 1,000 feet to 2,000 feet above airplane altitude.
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Low density yellow -Terrain 500 feet below to 1,000 feet above airplane altitude. Gear down changes the 500 feet to 250 feet.
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High density green -Terrain 500 feet below to 1,000 feet below airplane altitude. Gear down changes the 500 feet to 250 feet
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Low density green -Terrain 1,000 feet below to 2,000 feet below airplane altitude.
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Black -Terrain more than 2,000 feet below airplane altitude.
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Magenta -Unknown terrain.
When the GPWS detects a terrain caution alert, the threat terrain changes from dots to a solid yellow color. When the GPWS detects a terrain warning alert, the threat terrain changes from dots to a solid red color.
On final approach, terrain near the runway does not display.
GPWS System Messages
These GPWS system messages show in cyan on the left side of the ND:
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TERR shows when terrain data displays
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TERR TEST shows when the GPWS is in self test mode.
GPWS Alert Messages
These GPWS alert messages show on the right side of the ND:
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TERRAIN (amber) shows when a terrain awareness caution occurs
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TERRAIN (red) shows when a terrain awareness warning occurs
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WINDSHEAR (red) shows when a predictive windshear warning occurs.
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