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BLEED AIR TEST UNIT CONTROL (P5)
APU Isolation Valve Schematic
Figure 20

A
747-400
MAINTENANCE MANUAL

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747-400
MAINTENANCE MANUAL

 (2)  
PRSOV Operation (Fig. 18)

 (a)  
The PRSOV controls the flow of engine bleed air to the air supply distribution system.

 (b)  
The required power for the valve operation is 28v dc.

 (c)  
To open the PRSOV the engine bleed switch is put in the ON position. The ASCTU will then send a 28v dc pulse to set the valve to the AUTO mode. This lets the valve operate pneumatically.

 (d)  
To close the PRSOV the engine bleed switch is put in the OFF position. The ASCTU will then send a 28v dc pulse to set the valve to the OFF mode. The valve is closed if any of the following conditions occur: 1) An overheat condition - 450 °F (232 °C) 2) The fire switch is operated.

 (e)  
The following conditions will cause the EICAS advisory message BLEED 1 (2, 3, or 4) OFF to be shown: 1) the PRSOV closed position switch is closed 2) the applicable engine core speed (N2) is above 50% 3) the applicable engine bleed switch is selected OFF.

 (3)  
Wing Isolation Valve Operation (Fig. 19)

 (a)  
The power required for valve operation is 115v ac.

 (b)  
There is a left and a right ISLN switch on the bleed control module. When the switch is put in the OPEN position, the actuator opens the valve. When the switch is put in the CLOSE position, the actuator closes the valve. This will isolate the pneumatic duct on the applicable side of the airplane.

 (4)  
Fan Air Modulating Valve Operation

 (a)  
The fan air modulating valve (FAMV) is opened and closed by the signal pressure from the fan air temperature sensor (FATS) and by bleed air pressure from the precooler. When the temperature of the bleed air sensed by the FATS is below 320°F (160°C), the FAMV is closed. When the temperature is above 380°F (193°C), the FAMV is fully open. If the sensed temperature is between 320°F (160°C) and 380°F (193°C), the fan air modulating valve will regulate.

 (b)  
The pneumatic signal changes in proportion to the bleed air temperature. As the temperature increases, the valve moves towards the open position and permits more fan air to go through the precooler. The fan air then flows overboard through a duct in the core cowl.

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 (c)  
The indicated precooler outlet temperature on the EICAS ECS maintenance page may vary between 250°F (160°C) and 380°F (193°C) while the fan air modulating valve regulates between open and closed.

 (5)  
APU Isolation Valve Operation (Fig. 20)

 (a)  
The power required to operate the APU isolation valve is 115 VAC.

 (b)  
When the APU switch is put in the ON position, the valve will open and permit APU bleed air to pressurize the pneumatic system. The following conditions must occur for the control relay to be energized and cause the valve to open:

 -
the APU switch is in the ON position

 -
the APU override relay is not energized

 -
the APU fire switch is in the OFF position

 -
the APU start relay is energized (ON or START position)

 -
the APU is at 95% capacity.

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