21-31-00
ALL ú ú 01 Page 6 ú Jun 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
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 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 altitude is limited to 14,625 feet by a pressure switch in the controller. This signals the dc actuator, which modulates the outflow valve closed. The AUTO FAIL and STANDBY lights come on when the automatic system has been transferred to the standby system and the mode selector is in AUTO. The automatic mode of operation may be reselected after an automatic transfer to standby. If any fault remains in the automatic mode, the system will automatically transfer back to the standby mode.
FORWARD OUTFLOW RECIRC FAN
VALVE ENERGIZED RELAY
AIR CONDITIONING RELAY MODULE
Forward Outflow Valve Control Schematic Figure 3
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21-31-00
ALL ú ú 01 Page 7 ú Jun 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
C.
D.
E.
A
737-300/400/500MAINTENANCE MANUAL
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 control cabin pressure. 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 AUTO 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.Manual operation of the outflow valve occurs when the mode selector switch is moved to either MAN AC or MAN DC. 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, rate of climb, and differential pressure indicators must be monitored for obtaining the desired cabin condition. The MANUAL light above the control panel comes on during manual control.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 a signal the outflow valve to close to pressurize the cabin to approximately 0.1 psi above ambient (altitude approximately 200 sea level feet below field elevation). When the controller is operated in STBY mode, the cabin altitude selector must be set to provide a cabin altitude 200 feet below the takeoff or landing field elevation. When the FLT/GRD switch is placed to FLT the pressurization controller will provide a signal to close the outflow valve and pressurize the airplane to 200 feet below the selected field elevation under rate limit control. 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 under rate limit control.
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