*
Flap override switch

*
Terrain override switch

*
Gear override switch.

Ground Proximity Light/Ground Proximity Glideslope Inhibit Switch
This combination light/switch has two functions. The amber ground proximity light shows ground proximity alerts for modes 1 through 5.
Push the switch to prevent mode 5 (below glideslope) alerts.
Override Switches
The flap override and the gear override switches simulate a flaps down condition and a gear down condition. The use of these switches prevent alerts when the flight crew makes flaps up or gear up approaches intentionally. The flight crew uses the terrain override switch for any of the following conditions:
*
The crew determines position data is not valid

*
Terrain awareness alerts show when it is obvious no terrain threats exist.

The switches have guards to keep them in the normal position.
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777 AIRCRAFT MAINTENANCE MANUAL

GPWS -CONTROL COMPONENTS

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GPWS -MODE 1 DESCRIPTION

Purpose
Mode 1 supplies alerts and warnings for large descent rates when the airplane is near the terrain. This mode is independent of flap or gear positions.
Description
Mode 1 alerts or warnings occur between 2450 feet and 30 feet. The rate of descent sets the point at which mode 1 alerts occur.
The alert consists of these annunciations:
*
Aural message SINK RATE

*
Ground proximity light on the landing gear panel.

If the descent rate does not decrease, the alert changes to a warning. These are the warning annunciations:
*
A red PULL-UP message on the PFD

*
Red master warning lights

*
The aural message PULL-UP.

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GPWS -MODE 1 DESCRIPTION

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GPWS -MODE 1 OPERATION

General
Mode 1 alerts and warnings occur between 30 feet and 2450 feet radio altitude. The LRUs that supply inputs for mode 1 operation are the left, right, and center radio altimeter transceivers and the AIMS cabinets.
The GPWC uses this data to detect mode 1 alerts and warnings:
*
Radio altitude

*
Inertial vertical speed

*
Barometric altitude.

The mode detector in the GPWC calculates the descent rate from the inertial vertical speed input. If it is not available, the mode detector uses barometric altitude rate.
The type of annunciation depends on the rate of descent and the radio altitude.
The first annunciation is an alert. If the rate of descent does not decrease, the annunciation changes to a warning.
Operation
The mode detector uses the inputs to see if there is an alert or warning condition.
If there is an alert condition, the GPWC sends a discrete to the ground proximity light. The alert discrete goes to the speech prom to make the alert aural. The alert aural goes to the warning electronic units (WEUs) which amplify and send it to the aural warning speakers.
The alert discrete also goes to the TCAS computer to inhibit any TCAS advisory messages.
If there is a warning condition, the GPWC sends a PULL-UP warning discrete to the AIMS cabinets to show the message PULL UP on the PFDs. The warning discrete goes to the speech prom to make the warning aural. The warning aural goes to the warning electronic units (WEUs) which amplify and send it to the aural warning speakers.
The warning discrete also goes to the TCAS computer to inhibit any TCAS advisory messages.
The AIMS cabinets send the warning discretes to the warning electronic units (WEUs) to make the master warning lights come on.
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GPWS -MODE 1 OPERATION

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GPWS -MODE 2A DESCRIPTION

Purpose
Mode 2 supplies alerts and warnings if the closure rate to the terrain is too large. Mode 2 has two submodes (mode 2A and 2B).
The position of the flaps determine if it is a mode 2A or a mode 2B alert.
Description
Mode 2A gives annunciations for a large closure rate with the flaps up (less than 15 units). Mode 2A gives an alert and a warning. This is the alert:
　
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