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3.  Pressure Controller
A.  The pressure controller is an electronic unit, which receives inputs from the pressurization control panel, airplane ambient pressure and cabin pressure, then positions the outflow valve to maintain proper differential pressure (Fig. 2).
B.  Two completely independent pressure control system networks are contained within the pressure controller. The automatic system operates as a function of destination landing field altitude and cruise altitude while the standby system operates according to the selected cabin altitude and selected rate of cabin pressure change.
C.  Inputs to the pressure controller for the automatic and standby systems are shown
schematically on a block diagram (Fig. 2).

D.  All outputs from the pressure controller operate the cabin pressurization outflow valve. The controller will operate either under the automatic system or the standby system, but never both systems simultaneously. The FLT-GRD switch is used to modify both outputs for cabin pre-pressurization on the ground. When the switch is moved to FLT position with the selector in AUTO the valve will move to a position which will cause cabin pressure to rise slightly above (approximately 0.1 psi.) that of the takeoff field. With the selector in STBY the valve will be positioned to control the pressure to the selected standby cabin altitude. With mode selector switch in either AUTO or STBY and FLT-GRD switch is moved to GRD (airplane on ground), the outflow valve will move to the full open position at the rate required to completely depressurize the airplane at rate limit.
E.  On the front of the controller, there are test points under a cover which are used for trouble shooting the pressurization control system. On the rear of the controller, the upper connection is used only for bench test.
F.  The pressure controller is installed in the electronic equipment compartment on the forward top shelf. There is a delta P module installed adjacent to the pressure controller, which provides pressurization control for maximum operating altitude of 37,000 ft.
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21-31-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/77 

K17016
5C8  Pressurization Control System Block Diagram 
May 01/99  Figure 2  21-31-0 
Page 5 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

G.  On some airplanes, there is a dual delta P module installed adjacent to the pressure controller, which provides a maximum of 7.45 psi with the pressurization mode selector in AUTO and 28,000 ft or below selected with the FLT ALT selector. A 7.80P is provided when FLT ALT above 28,000 ft is selected with the pressurization mode selector in AUTO. STANDBY maximum pressure differential is 7.8 psi.
4.  Cabin Pressurization Outflow Valve
A.  The outflow valve is a thrust recovery, rotating gate valve which is driven by either a rotary dc electrical actuator or a rotary ac electrical actuator. Each actuator may be connected to the gate shaft with an electrically operated spring-loaded clutch. AUTO and AC MANUAL modes operate the ac actuator. STANDBY and DC MANUAL modes operate the dc actuator. When either actuator is in operation the clutch to the other actuator is disengaged.
B.  With no electrical power to the clutches, the dc actuator clutch is spring loaded disengaged and the ac actuator clutch is spring loaded engaged. With electrical power to the clutches, the dc actuator clutch is engaged and the ac actuator clutch is disengaged. The clutches are energized when operating from the standby system or the manual dc system.
C.  There is a limit switch installed on each end of the valve gate shaft. The switches assure that the forward outflow valve (Ref 21-43) does not affect the cabin pressurization control system. The forward outflow valve closes when the cabin pressurization outflow valve is within 0.5 degrees of the full-closed position. The forward outflow valve opens when the cabin pressurization outflow valve opens approximately 3.5 degrees.
D.  The outflow valve is installed so its outlet is flush with the body skin. The valve is located to the right of the airplane centerline, at the underside of the airplane, approximately opposite the aft entry door.
WARNING:  OUTFLOW VALVE IS MOTOR-OPERATED. DO NOT INSERT HAND OR TOOLS IN OUTLET DURING ANY GROUND OPERATION OR INJURY TO PERSONNEL MAY OCCUR.
5.  Forward Outflow Valve
A.  The forward outflow valve is located at the forward left side of the forward cargo compartment. The forward outflow valve is a component of the cargo heating system (Ref 21-43) and does not affect the cabin pressurization control system. This is assured by an electrical circuit that closes the forward outflow valve whenever the cabin pressurization outflow valve is nearly closed (Fig. 4). There is a limit switch installed in each actuator of the cabin pressurization outflow valve. The forward outflow valve will close when the cabin pressurization outflow valve is within 0.5 degrees of closed and will open when the pressurization outflow valve opens approximately 3.5 degrees. A position switch in the forward outflow valve actuator controls a F OUTFLOW CLOSED warning indicator on the overhead air conditioning panel.
　
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