H59381
540  Forward Outflow Valve Control Schematic 
May 15/77  Figure 3 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  21-31-0 Page 7 

5.  Operation
A.  The automatic mode is the normal operation of the pressurization control system. The destination landing field altitude and desired cruise altitude are set on the control panel, converted to voltage signals and input to the cabin pressure controller. The controller also receives signals from the No. 1 air data computer (No. 2 air data computer on airplanes with  SB 21-1119), barometric correction potentiometer, outflow valve position feedback, and from the landing gear ground safety sensor. The controller also detects cabin and ambient air pressure and converts these to voltage signals. The controller provides voltage signals to the outflow valve causing the valve to modulate the flow of cabin air to maintain cabin pressure at the desired level.
B.  Pressurization control is automatically transferred from automatic mode to standby if the cabin pressure rate of change exceeds 1.0 psi per minute or cabin altitude exceeds 13,895 feet. In the event of a power failure to the automatic system, control will automatically be transferred to the standby system provided that power is available to that system. Upon transfer, the cabin pressure will change to the selected standby cabin altitude at the selected standby rate of change. During standby control the cabin pressure is limited to 14,625 feet by a pressure switch in the controller. This signals the dc servomotor, which modulates the outflow valve. The AUTO FAIL and STANDBY lights come on when the automatic system has been superseded by the standby system. The automatic and standby systems provide inputs to each other so that a smooth transition may be made between the two modes of control. The automatic mode of operation may be selected after an automatic transfer to standby. If any fault remains in the automatic mode, the system will automatically transfer back to the standby mode.
C.  The standby system may be utilized, although the automatic change to standby has not occurred. By moving the selector switch to STBY after setting the cabin altitude and pressure rate of change, the standby system will function as above. The STANDBY light will come on. The possible indicator light conditions for the standby mode are:
(1)  
When standby mode is selected, only the standby light will be on.

(2)  
When an automatic transfer to standby has taken place and the selected mode is still automatic the STANDBY and AUTO FAIL lights will be on. The AUTO FAIL light will go out when standby is manually selected.

(3)  
If an automatic fault occurs and the standby mode is inoperative, only the AUTO FAIL light will be on.

542 
21-31-0 Page 8  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/99 

D.  Manual operation of the outflow valve occurs when the selector switch is moved to either MAN AC or MAN DC. In either case electrical power is switched to the manual control outflow valve toggle switch and the valve can be opened or closed as desired. A single toggle will change the valve position and cause a cabin pressure change in flight. When the switch is released the valve will hold the position obtained. During manual control the cabin altimeter and differential pressure indicator must be monitored for obtaining the desired cabin condition. The MANUAL light above the control panel comes on during manual control.
E.  The FLT-GRD switch is used to control cabin pressurization during takeoff and after landing in order to minimize the pressure fluctuations as the controller transfers from the ground to flight modes of operation. When the controller is functioning in AUTO and the FLT-GRD switch is placed in the FLT position, the minimum differential pressure control circuit will provide an overriding signal to pressurize the cabin to approximately 0.1 psi above ambient (altitude approximately 200 sea level feet below field elevation). When the airplane leaves the ground and the auto control signal is transferred from the depressurized ground signal to the climb schedule by the landing gear safety switch, the minimum differential control will maintain the cabin pressure at 0.1 psi above ambient until the climb schedule cabin pressure exceeds the 0.1-psi pressure differential. During descent, FLT-GRD switch in FLT position, the automatic system schedules cabin descent pressure to maintain a slightly higher pressure on landing (approximately 0.15-psi differential). After landing when FLT-GRD switch is moved to GRD, the outflow valve will move to the full open position at the rate required to completely depressurize the airplane at rate limit.
　
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