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MAINTENANCE MANUAL
12. APU Isolation Valve (Fig. 16)
___________________
A. The APU isolation valve is installed in the left body gear wheel well in the upper outboard aft corner. The valve is a 4.5 inch diameter butterfly valve. It opens or closes to control the supply air from the APU to the air supply distribution system.
(1)
The valve is operated by a 115v ac motor.
(2)
The valve has a manual override drive that permits you to lock the valve in the open or closed position.
13. APU Check Valve (Fig. 16)
_______________
A. The APU check valve is installed in the left body gear wheel well in the lower inboard forward corner. The valve is a 6-inch dual flapper type check valve. The air from the APU goes through the APU isolation valve before it goes through the check valve. The check valve will not let air flow from the engines or the ground air source into the APU.
(1)
The check valve is made up of two semi-circular flappers that are hinged on a pin. The pin goes through the center of the valve body and through the flappers.
(2)
The air pressure from the APU will force the helical torsion spring to drive the check valve flappers open until contact is made with the check valve stop pin. The flappers will close to prevent reverse air flow back into the APU.
14. Air Supply Control and Test Unit (ASCTU) (Fig. 4)
________________________________________
A. The ASCTU is installed in the main equipment center aft of the nose gear wheel well on the E2-4 rack. The ASCTU gives control and indication data for the pneumatic system components. The ASCTU has a Built In Test (BIT) function that gives status information to EICAS and the central maintenance computer (CMC). The redundant BIT logic will continuously monitor the air supply components, but does not supply inputs for the operation or control of the pneumatic system. The ASCTU has the following components:
-four control logic cards
- one isolation card
-
two monitor cards
- one power supply card.
B.
ASCTU Operation
(1) The ASCTU receives inputs from the bleed control module. It makes an analysis of this information and sends the appropriate command signals to the pneumatic system components. The ASCTU monitors the position of the components and the pressure and the temperature of the system. This information is processed and sent to EICAS. If the ASCTU determines that there is a system failure, the BIT part of the ASCTU will send the component fault to the CMC. The indication part of the ASCTU will send a signal to the SYS FAULT light on the bleed control module and to EICAS.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
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MAINTENANCE MANUAL
APU CHECK
VALVE
SEE B
APU ISOLATION
VALVE, V121
SEE A
APU ISOLATION VALVE, V121
APU CHECK
VALVE MANUAL OVERRIDE
DRIVE
FWD
APU ISOLATION VALVE, V121
A
INBD
FWD
APU CHECK VALVE B
APU Isolation Valve and APU Check Valve Figure 16
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
15. Operation
_________
A. Functional Description
(1)
High Pressure Controller/PRV Controller Operation (Fig. 17)
(a)
The power that is required for the high pressure controller and the PRV controller is 28v dc. The high pressure controller and the PRV controller use pneumatic air to control the position of the HPSOV and the PRV. The controllers are controlled by the ASCTU.
(b)
When the engines are started the ASCTU commands the PRV to close. Failure of the PRV to close may result in the following indications:
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