D.
Outflow Valve Actuator
______________________
(1) The outflow valve actuator mounts directly on the valve frame and drives both doors simultaneously through a control arm and linkage. The actuator consists of two identical 115-Vac 400-Hz motors, a 28 Vdc motor and a gearbox. The gearbox includes a feedback potentiometer and limit switches.
3. Operation_________
A. Functional Description
______________________
(1)
Pressure Control System (Fig. 4)
(a)
The pressure controller modulates the outflow valve to maintain the desired cabin pressure. Opening the valve allows more air to exhaust from the airplane. This reduces cabin pressure. Closing the valve increases cabin pressure. Modulating the valve between the full open and full close extremes, allows the desired cabin pressure to be maintained.
(b)
Before takeoff the crew selects the desired automode, landing (field) altitude and the autorate. Placing the mode selector to AUTO 1 or AUTO 2 releases control of the pressurization system to the selected controller. The other controller goes into a standby mode and monitors the selected controller.
(c)
The airplane's two air data computers (ADC) supply ambient pressure signals and barometric corrections to each pressure controller. The controller's integral pressure sensor measures cabin pressure.
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A
757 MAINTENANCE MANUAL
SEE A
BSEE ACTUATOR OUTFLOW VALVE
AC MOTOR CONNECTOR ELECTRICAL (4 PLACES)
AFT DOOR DC MOTOR GEAR BOX CONTROL ARM AC MOTOR
FOR CLARITY AIRPLANE DIRECTION VALVE ROTATED FROM
FORWARD DOOR A FWD
COMPOUND GEAR PLANETARY GROUND DC MOTOR
AC MOTOR POTENTIOMETER
TACHOMETER LIMIT SWITCHES
STOP
OUTPUT
Cabin Pressure Outflow Valve Figure 3 B OUTFLOW VALVE ACTUATOR GEAR TRAIN
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A
757 MAINTENANCE MANUAL
(d)
A throttle advanced signal from the throttle microswitch pack assembly greater than 10.5-degrees from idle, puts the controller in the takeoff mode. At rotation the controller receives a signal from the air/ground relay, and enters the flight mode. Once in the flight mode, the controller creates a cabin pressure autoschedule. The autoschedule is the ideal cabin-to-ambient differential pressure for varying airplane altitudes. The selected landing altitude and the maximum cabin-to-ambient pressure ratio (delta P = 8.6 psi) limit the autoschedule.
(e)
The error between the cabin pressure and the cabin pressure command determines the cabin pressure change command. The selected autorate limits the rate of change. In cruise, the cabin pressure follows the cabin pressure command.
(f)
The controller positions the outflow valve to maintain the correct cabin pressure and rate of cabin pressure change. A tachometer on the outflow valve actuator provides a feedback signal to the controller. This improves the speed response of the actuator.
(g)
GUN 001-003, 008-999; If the cabin altitude reaches 11,000 ft, the controller automatically closes the outflow valve to increase cabin pressure.
PILOTS OVERHEAD PANEL, P5
CONTROLLER AUTO 2
Pressure Control System Figure 4
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INCLUDES ANEROID SW (BOTH CONTROLLERS)
AIR/GRD
SW
c P
TACH FEEDBACK
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