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747-400 MAINTENANCE MANUAL
CABIN PRESSURE CONTROL SYSTEM - DESCRIPTION AND OPERATION
_________________________________________________________
1. General (Fig. 1)_______
A. The cabin pressure control system (CPCS) maintains a safe and comfortable cabin pressure by controlling the size of the openings where air leaves the cabin. The air leaves through two outflow valves which are driven more open to decrease cabin pressure or more closed to increase cabin pressure. The CPCS also limits the rate of change of cabin pressure and controls the cabin-to-ambient pressure differential.
B. The CPCS consists of a selector panel, two cabin pressure controllers, two interface control units, two outflow valves, and manual control relays. The forward overboard valve is a component of the ventilation system (Ref 21-26-00), but is primarily controlled by the CPCS. Pressure relief valves (Ref 21-32-00) prevent overpressurization and negative pressurization of the airplane.
C. Normally the CPCS operates in automatic mode, with cabin pressure maintained according to a preset schedule in the cabin pressure controller software and data from the air data computers and flight management computers. The cabin pressure controller sends an outflow valve positioning command to the two interface control units, which activate the outflow valve drive mechanisms.
D. The CPCS can be operated in manual mode independently of the automatic control. In manual mode the cabin pressure controllers are bypassed and a separate circuit and drive motor are used to position the outflow valves under direct control from the flight deck.
Cabin Pressure Control System Block Diagram
Figure 1
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747-400
MAINTENANCE MANUAL
Cabin Pressure Control System Component Location
Figure 2 (Sheet 1)
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747-400
MAINTENANCE MANUAL
INTERFACE CONTROL
AFT UNIT
SEE D
SEE C DC MOTOR
C PRESSURIZATION PANEL AUXILIARY COMPONENTS
OUTFLOW VALVE AFT A TACHOMETER, AND BRAKE AC MOTOR,
POTENTIOMETERS FEEDBACK
A FLAPPER DOOR GEAR TRAIN HOUSING
D
NEARLY CLOSED FULL OPEN (USUAL FLIGHT POSITION) (GROUND POSITION)
OUTFLOW VALVE FLAPPER DOORS
A-A
NOTE: LEFT SIDE PRESSURIZATION COMPONENTS SHOWN. RIGHT SIDE COMPONENTS SIMILAR.
Cabin Pressure Control System Component Location Figure 2 (Sheet 2)
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747-400
MAINTENANCE MANUAL
ENGINE/APU/CARGO FIRE CONTROL PANEL
Cabin Pressure Control System - Component Location
Figure 2A
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C20263
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747-400
MAINTENANCE MANUAL
2. CPCS Selector Panel (Fig. 2)
___________________
A. The CPCS selector panel M181 on the P5 pilots' overhead panel permits selection of CPCS mode and manual landing altitude, allows manual control of the outflow valves, and displays the position of the outflow valves.
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