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 C.  A selector panel, two automatic pressure controllers and an outflow valve provide system control. The selector panel supplies signals to the controller. The controller then opens or closes the outflow valve to maintain safe cabin pressures. The pressurization control system provides three independent control paths for the outflow valve. The controller's built in test function (BITE) provides system integrity checks.
 2.  Component Details_________________
 A.  Automatic Pressure Controller (Fig. 2)
_____________________________
 (1)  
Two identical automatic pressure controllers control airplane pressurization by modulating the outflow valve. One controller remains in a standby mode, monitoring system operation. The standby controller takes over system control if the selected controller fails. Both controllers receive identical signals. Each controller contains the sensory and electronic logic, and the control circuit for one outflow valve control channel. 115 Vac powers each E/E rack-mounted controller.

 B.  
Cabin Pressure Selector Panel

_____________________________
 (1)  
The pilots overhead panel (P5) contains the cabin pressure selector panel. The cabin pressure selector panel consists of a mode selector switch, autorate input selector, landing (field) altitude input selector, valve position indicator and manual momentary contact switch.

 (a)  
The mode selector switch allows selection of AUTO 1, AUTO 2 or MAN.

 (b)  
The autorate input selector allows the crew to select the rate of cabin pressurization. Range of selection is 50 to 2000 FPM for climb, and 30 to 1200 FPM for descent. The index position indicates 500 FPM climb and 300 FPM descent. The ratio of climb to descent is always 5 to 3.

 (c)  
The landing (field) altitude input selector allows landing altitude selection by the crew for controller information. Landing altitude range selection is -1000 to +14,000 ft.

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A 757 MAINTENANCE MANUAL
PRESSURIZATION
POSITIVE NEGATIVE ALTITUDE
AUTO 1AUTO 2  MANUAL PRESSURE PRESSURE WARNINGRELIEF RELIEF
CABIN PRESSURE SELECTOR
OUTFLOW VALVEEICAS COMPUTERS
AUTO 1AUTO 2
NEGATIVE PRESSURE 
RELIEF VALVES  POSITIVE PRESSURE 
RELIEF VALVES 
VENT PORT 
CABIN ALTITUDE  STATIC PRESSURE 
AIR DATA  WARNING SWITCH  PORT 
COMPUTERS  21-33-01 

AUTO CABIN
PRESSURE CONTROL
UNITS

Pressurization Control System Figure 1
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

SEE A
CABIN ALTITUDE CONTROL VALVE CLIMB MANUAL
AUTO RATE
MIN MAX
DESCEND
LDG ALT AUTO INOP FWD
02970
MODE SELECTAUTO 2
AUTO 1 MAN
PRESSURE CONTROLLERS A
CABIN
10  0  2  25 20 0  4 2  0 1 2 
8  4  15  6 

1
6 108 2
FPM X 1000 FT X
DIFF PSI ALT RATE  1000
R ALTITUDE CABIN
PRESS DIFF LIMIT: TO/LDG .125 PSI
Automatic Pressure Controller and Cabin Pressure Selector Panel Figure 2
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A

757 MAINTENANCE MANUAL
 (d)  
The valve position indicator shows outflow valve position. A momentary contact switch allows manual control of the outflow valve.

 C.
Outflow Valve (Fig. 3)

_____________
 (1)  
The outflow valve is a double-door thrust-recovery type valve. The doors are aluminum with a Teflon coating to prevent binding and excessive friction. Connecting rods on each side of the valve frame link the forward and aft doors together. The valve controls the flow of cabin air overboard by modulating the valve doors. In the full open position the valve has an effective flow area of 108 sq in. The valve mounts on the left underside of the airplane immediately aft of the bulk cargo compartment.
　
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