Ground Proximity Warning System
(GPWS)
Revised 24-Mar-2001
 
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The Ground Proximity Warning System alerts the flight crew when one of the following thresholds are exceeded between 50 and 2450 feet radio altitude.

Inputs to the ground prox computer are radio altitude from the No1 radio altimeter, barometric altitude rate and mach from an air data computer, glide slope deviation signals from capt side, landing gear and flap positions.

The loss of one of these inputs will deactivate only the affected mode or modes.
Aural alerts and warnings for modes 1 through 4 are accompanied by red PULL UP lights

Modes Explained (Note that these diagrams are simplified)

Mode 1 - Excessive Decent Rate
Has two boundaries and is independent of airplane configuration. Penetration of the first boundary generates the repeated aural alert of SINK RATE. Penetrating the second boundary causes the repeated aural warning of WOOP WOOP PULL UP, until the rte of decent has been corrected.


Mode 1 Envelope

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Mode 2 - Excessive Terrain Closure Rate
Monitors mach number, radio altitude and radio rate of change, barometric altitude and airplane configuration. Has two boundaries, the first causes an aural alert of TERRAIN repeated twice, followed by the repeated aural warning of WOOP WOOP PULL UP. After leaving the PULL UP area the repeating TERRAIN message will be herd while in that portion of the envelope..
If both boundaries are penetrated while in the landing configuration, only the repeating TERRAIN aural alert will occur.
If the landing gear and wing flaps are not in the landing position when leaving the PULL UP area, 300 ft of barometric altitude must be gained before the aural TERRAIN alert is silenced.
As the mach number increases from .35 to .45 with the gear up, the highest altitude at which a mode two alert warning will occur is increased to 2450 ft. This higher portion is inhibited with the flap inhibit switch in the FLAP INHIBIT position.


Mode 2 Envelope

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Mode - 3 Altitude Loss After Take off Or Go Around
Provides an alert if a decent is made during initial climb or go around. The aural alert is a voice message "DON'T SINK", repeated until the condition is corrected. It is effective between 50 and 700 feet radio altitude and generates the alert when accumulated barometric loss equals approx. 10% of the existing radio altitude.
Mode 3 does not arm during decent until below 200 ft radio altitude.


Mode 3 Envelope

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Mode - 4 Two modes 4A and 4B

4A - Unsafe Terrain Clearance With Landing Gear Not Down
This is the terrain clearance mode with the gear retracted, is armed after take off upon climbing through 700 ft radio altitude. When this envelope is penetrated at less than 0.35 mach the aural alert TOO LOW, GEAR is sounded. When the envelope is penetrated at more than 0.35 mach, the aural alert TOO LOW TERRAIN is sounded and the upper boundary of the envelope is increased to 1000 ft rad alt. The message is repeated until the flight condition has been corrected.


Mode 4A Envlope

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MACH -IAS Conversion

MACH
SL
5000'
8000'
10000'
0.34
232
211
200
193
0.45
298
272
258
249

4B - Unsafe Terrain Clearance With The Flaps Not In Landing Position
This mode provides an alert when the gear is down and the flaps are not in landing position. If the envelope is penetrated at less than 0.28 mach with the flaps not in the landing position, the aural alert TOO LOW FLAPS is sounded. When the envelope is penetrated at more than 0.28 mach, the aural alert of TOO LOW TERRAIN is sounded and the upper boundary is increased to 1000 ft rad alt. The voice messages continue to occur until the flight condition has been corrected.
The TOO LOW GEAR alert take priority over TOO LOW FLAPS. The TOO LOW FLAPS and associated TOO LOW TERRAIN alert are inhibited with the flap inhibit switch when moved to the FLAP INHIBIT position

Mode 4B Envlope

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MACH -IAS Conversion

MACH
SL
5000'
8000'
10000'
0.34
185
169
160
154
0.45
298
272
258
249

Mode 5 - Below Glideslope Deviation Alert
This mode alerts you of a descent of more than 1.3 dots below an ILS glideslope. The envelope has two areas of alerting, soft and loud.
In both areas is a repeated aural warning of GLIDESLOPE and illumination of the pilots BELOW G/S lights. The voice message amplitude is increased when entering the loud area In both areas, the aural warning repetition rate is increased as glideslope deviation increases and radio altitude decreases.

The mode is armed when a valid signal is being received by the capts glideslope receiver and the radio altitude is 1000 feet or less.
This mode may be canceled or inhibited by pressing either pilots BELOW G/S light while below 1000 ft radio altitude. The mode will re arm when climbing above 1000 ft rad alt.

Mode 1 through 4 aural alerts and warnings have priority over mode 5 aural alerts, though both PULL UP and BELOW G/S lights could be illuminated at the same time.

Mode 5 Envlope

 

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