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 8.  Operation (Fig. 2, Fig. 3)
_________
 A.  The GPWS is operational when load control centers P18 and P6 are energized and the following system circuit breakers are closed: ground proximity warning, No. 1 air data, No. 1 VHF navigation, No. 1 inertial reference, No. 1 digital analog adapter, stall warning, flight management, flight interphone, and EFIS.
 B.  Signal inputs to the GPWC consist of radio altitude information from LRRA No. 1, mach and altitude rate information from central air data computer No. 1, and glide slope deviation from the VHF NAV system.
 (1)  
The LRRA system provides the GPWS with radio altitude, and an altitude trip when the airplane descends below the decision height set on the EFIS control panel. LRRA signals are used in the control of modes 1 thru 6. Mode 2 requires a radio altitude rate signal (terrain closure rate) which is derived by the computer from the radio altitude signal. The radio altitude signal is a dc voltage which is a function of airplane height above ground level and is useful to approximately 2500 feet.

 (2)  
The CADC system provides mach and barometric altitude rate signals to the GPWS computer. Altitude rate is used for the computations of mode 1 conditions, and is used to derive altitude loss, used in the computation of mode 3 conditions. The barometric rate signal is ac with a scale of 250 mv/1000 fpm altitude change. This rate signal is in phase with the 26-volt ac reference from the CADC during ascent and out-of-phase during descent. The mach signal is used to set warning thresholds for mode 2 and mode 4 computations. It is a three wire synchro output calibrated from zero degrees for 0.2 mach indication and increasing to one synchro revolution for a 1.2 mach indication.

 (3)  
The VHF navigation system provides glide slope deviation signal which is used in the mode 5 computation. The glide slope deviation signal provides a low level input with a sensitivity of 75 mv/deviation dot.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

FOR A CONSTANT RA CLIMB RATE, THIS BOUNDRY IS DEFINED AS:__t -t
11.25 X ( - 1 + e )
15 15

WHERE X= CLIMB RATE IN FEET (RA) PER SECOND t= TIME IN SECONDS; WHERE t= 0 AT THE POINT WHERE RADIO ALTITUDE FIRST EXCEEDS 150 FEET
* ONCE A WARNING/ALERT HAS
OCCURRED, LOWERING THE RA
BELOW 30 FEET WILL TURN OFF THE WARNING/ALERT WITHIN
1.5 SECONDS. (UPON COMPLETION OF MESSAGE)
BARO ALTITUDE (FT) (ALTITUDE LOSS)

Mode 3 - Descent After Takeoff
Figure 6

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

25002000
1500
HEIGHT
ABOVE
TERRAIN
(FT) 1000
500
500
00.1 0.2 0.3 0.4 0.5 0.6 0.7
MACH NUMBER
MODE 4A
2500
2000
1500
HEIGHT
ABOVE
TERRAIN
(FT) 1000

500
245
300

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
MACH NUMBER
MODE 4B
Mode 4 - Proximity to TerrainFigure 7

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 C.  Binary inputs to the GPWC consist of a landing gear and landing flap position signal and 28-volt dc radio altimeter valid, glide slope valid and baro rate computer valid signals.
 (1)  
The landing gear lever switch actuation is determined by the position of the landing gear control lever. When the lever is moved to the down detent position, the microswitch is actuated. Switch actuation provides a ground to the ground proximity warning system, indicating that the landing gear lever is in the down position. The ground from the landing gear lever switch is also provided to the landing gear accessory module.
　
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