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 5.  Outflow Valve (Fig. 2)
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 A.  The outflow valves function as variable size openings to control the rate of discharge of conditioned air from the pressurized cabin. This provides the pressure drop necessary to maintain required cabin pressure. The outflow valves are located on the underside of the fuselage in the area aft of the bulk cargo compartment.
 B.  Each outflow valve has 2 flapper doors which are positioned by a motor driven actuator and linkage. Mechanical stops on the actuator limit flapper door travel. Spring-loaded teflon wipers on the edges of the flapper doors seal between the doors and the valve frame. A metal screen over the valves prevents debris from entering the mechanisms.
 C.  The outflow valve actuator consists of a gearbox with a 115 vac motor and motor brake, a tachometer, a 28 vdc motor, a control feedback potentiometer and a position indicator potentiometer. The individual motors or the complete actuator may be removed with the valve installed, or the entire valve assembly may be removed as a unit.
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A
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MAINTENANCE MANUAL

 D.  The actuator ac motor is operated by the cabin pressure controller via the ICU when the CPCS is in the normal automatic mode. When manual control of the outflow valve is enabled, the actuator dc motor is operated by the OUTFLOW VALVES OPEN-CLOSE switch on the CPCS selector panel.
 6.  Forward Overboard Valve (Fig. 2)
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 A.  The forward overboard valve (Ref 21-26-00) is a butterfly valve which promotes air circulation in the forward part of the cabin. The valve is located on the inside of the fuselage to the left of the nose gear wheel well. The valve is opened or closed for different phases of flight on command from the ECS miscellaneous card.
 7.  Engine/APU/Cargo Fire Control Module (Fig. 2A)
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 A.  The Engine/APU/Cargo Fire Control Module on the overhead P5 panel is used to depressurize the airplane when there is a main deck cargo compartment fire.
 B.  When the main deck fire suppression system is armed and you push the DEPRESS switchlight, a signal of depressurization is sent to the pressure controllers A and B and relay R8538 is energized. With the relay R8538 energized, the 11,000 ft cabin altitude limit system in the ICUs is deactivated. Output from the pressurization controller (A or B, whichever is in control), will open the left and right outflow valves to depressurize the cabin to 25,000 feet at a controlled rate of 2,500 sea level feet per minute.
 C.  The system is reset when you push the DEPRESS switchlight again. Relay R8538 is deenergized and the 11,000 foot cabin altitude limit system in the ICUs will drive the two outflow valves to the closed position to restore normal pressurization operation.
 8.  Manual Control Relays
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 A.  With the outflow valve under manual control, the dc motor of the valve actuator is energized by means of 4 relays. Manual control relays and auxiliary components for the outflow valve motors are mounted on the air pressurization components auxiliary panel (P213 - left, P212 - right) located near the outflow valve.
 9.  Operation (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7)
 A.  Functional Description
 (1)  
The CPCS has 2 basic functional states, automatic and manual. In automatic, the primary automatic pressure controller computes and controls outflow valve position. In manual, the flight crew directly controls outflow valve position.

 (2)  
Automatic

 (a)  When the MAN L and MAN R switch-lights are off, the CPCS will be in the auto mode. In the auto mode there are different CPCS control modes which correspond to different phases of flight -on ground, takeoff, climb, cruise, descent, and climb abort. The CPCS selects the appropriate control equations based upon the airplane's flight phase.
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