777 AIRCRAFT MAINTENANCE MANUAL

GPWS -MODE 3 DESCRIPTION

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GPWS -MODE 3 OPERATION

General
Mode 3 alerts occur between 30 feet and 1333 feet. Mode 3 arms when any of these conditions are true:
*
Airplane goes below 245 feet in landing configuration (gear down, flaps less than 15 units)

*
Airplane goes below 30 feet regardless of flap position.

The LRUs that supply inputs for mode 3 operation are the left, right, and center radio altimeter transceivers and the AIMS cabinets.
The GPWC uses this data to detect mode 3 alerts:
*
Radio altitude

*
Inertial altitude

*
Inertial vertical speed

*
Barometric altitude

*
Barometric altitude rate

*
Flap and gear position.

Mode 3B alerts occur when the altitude is lower than the value stored in a filter in the GPWC. The filter activates at 150 feet and stores approximately 75% of the actual altitude.
The filter stores up to 500 feet altitude for airspeeds below 190 knots (500 feet in the filter represents an altitude of 667 feet).
When the airspeed goes above 250 knots, the upper limit goes to 1000 feet (this represents an altitude of 1333 feet).
Mode 3 changes to mode 4 when the altitude gain filter gets to the upper limits.
Operation
The mode detector uses the inputs to find if there is an alert condition. If there is an alert condition, the mode detector sends a discrete to the ground proximity light. The discrete goes to the speech prom to make the alert aural which goes to the WES. The WES sends the aural to the aural warning speakers.
A discrete also goes to the TCAS computer to inhibit any TCAS advisory messages.
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777 AIRCRAFT MAINTENANCE MANUAL

GPWS -MODE 3 OPERATION

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D633W101-BEJ  Jan 05/2002 

GPWS -MODE 4 DESCRIPTION

General
Mode 4 supplies alerts when the airplane is too close to the terrain and the airplane is not in the landing configuration.
Mode 4 has these two submodes:
*
4A

*
4B.

Description
The GPWC gives a mode 4A alert when the landing gear is up. Mode 4A gives different types of aural messages. The aurals are TOO LOW GEAR, and TOO LOW TERRAIN. The ground proximity light will come on for either of these aural messages.
The GPWC gives a mode 4B alert when the gear is down and the flaps are less than 15 units. Mode 4B can have two aural messages depending on airspeed. The aurals are TOO LOW FLAPS, and TOO LOW TERRAIN. The ground proximity light comes on for either of the aural messages.
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777 AIRCRAFT MAINTENANCE MANUAL

GPWS -MODE 4 DESCRIPTION

EFFECTIVITY BEJ ALL  34-46-00 
Page 39 
D633W101-BEJ  Jan 05/2002 

GPWS -MODE 4 OPERATION

General
Mode 4 alerts occur between 30 feet and 1000 feet. The LRUs that supply inputs for mode 4 operation are the left, right, and center radio altimeter transceiver and the AIMS cabinets.
The GPWC uses this data to detect mode 4 alerts:
*
Radio altitude

*
Computed airspeed

*
Flap and gear position.

The mode 4A aural message TOO LOW-GEAR... changes to the aural TOO LOW-TERRAIN... when the airspeed goes more than 190 knots when the flaps are less than 15 units.
If the flaps are more than 15 units, the aural messages change at 159 knots.
Mode 4B aural message TOO LOW-FLAPS... changes to TOO LOW-TERRAIN...when the airspeed goes more than 159 knots.
Mode 4 changes to mode 3 if the airplane goes below 245 feet in landing configuration (flaps and gear down).
Operation
The mode detector uses the inputs to find if there is an alert condition.
If there is an alert condition, the mode detector sends a discrete to turn on the ground proximity light. The alert discrete goes to the speech prom to make the alert aural.
The alert aural goes to the WES which sends it to the aural warning speakers.
The alert discrete also goes to the TCAS computer to inhibit any TCAS advisory messages.
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D633W101-BEJ  Sep 05/2002 

777 AIRCRAFT MAINTENANCE MANUAL

GPWS -MODE 4 OPERATION

EFFECTIVITY BEJ ALL  34-46-00 
　
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