S 864-014
 (2)  
Remove the DO-NOT-OPERATE tag from the APU control switch on the P5 overhead panel.

 S 414-015

 (3)  
Close the left APU access door, 317AL, and right APU access door, 318AR:

 (a)  
Remove the two quick-release pins, disconnect the two hold-open rods and put the two hold-open rods in the two clips on the right access door.

 (b)  
Install the two quick-release pins in the two brackets on the right APU compartment bulkhead.

 (c)  
Lift the right access door until you can install the lanyard on the pin at the aft APU compartment bulkhead.

 NOTE: The lanyard will hold the right access door on the
____
 APU compartment bulkhead.

 (d)  
Remove the two quick-release pins, disconnect the two hold-open rods and put the two hold-open rods in the two clips on the left access door.

 (e)  
Install the two quick-release pins in the two brackets on the left APU compartment bulkhead.

 (f)  
Make sure the insulation covers are correctly attach to the APU compartment firewall.

 (g)  
Lift the left access door until the two APU access doors are approximately aligned.

 (h)  
Close the two APU access doors.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 (i) Close the six latches on the right access door.
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 ALL  ú ú 04 Page 406 ú Oct 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 APU DIVERTER AND IGV CONTROL SYSTEM - DESCRIPTION AND OPERATION
_______________________________________________________________
 1.  General
_______
 A.  The control systems for the diverter valve system and the inlet guide vane (IGV) system work together to maintain the outlet flow in the load compressor at certain levels according to airplane pneumatic demands.
 B.  The diverter control system maintains a minimum outlet corrected flow in the load compressor, when airplane flow demand is reduced, by opening the diverter valve to dump the delivery flow into the APU exhaust. The system consists of a diverter valve, diverter valve control, and a rate sensor.
 C.  The control system for the inlet guide vane (IGV) modulates the air flow to the load compressor according to airplane pneumatic demand. The system consists of an IGV actuator which is linked to the vanes by a connecting rod.
 2.  Diverter Valve (Fig. 1)
______________
 A.  The diverter valve consists of an aluminum butterfly valve driven by a half-area type pneumatic actuator. The valve is sized for compressor flow requirements. Compressor discharge pressure (P3) is provided on one side of the actuator. A pressure signal generated by the area ratio between a fixed orifice in the valve and the nozzle area of the diverter valve control is provided to the other side of the actuator. The difference between the two pressure signals and a spring load determines the diverter valve position.
 3.  Diverter Valve Control (Fig. 1)
______________________
 A.  The diverter valve control (DVC) sets the diverter valve modulated pressure in response to compressor flow and commands from the APUC. A venturi tube located in the load compressor diffuser senses flow through the diffuser as a differential pressure (P2-Ps2). The Ps2 pressure, taken at the throat of the venturi tube, is lower than the static (P2) pressure. As flow through the diffuser increases, Ps2 decreases and the resultant pressure differential increases. A lever pivoting on ball bearings is positioned as a function of the pressure differential P2-Ps2 through the use of two opposing diaphragms. As the pressure differential increases (increased compressor flow), the change in the lever force balance will act to close the pressure bleed nozzle. This increases modulated pressure and closes the diverter valve.
 B.  In addition, the APUC is capable of trimming the diverter valve control to closely match the surge limits of the compressor. Pressure regulated fuel is supplied to the large end of a reset piston in the diverter valve control. The pressure on the piston causes a plunger to move which applies pressure to a trim spring. The spring creates a force imbalance on the lever arm which repositions the flapper and provides air pressure to open the diverter valve. Commands from the APU controller position a torque motor armature, which acts as a flapper valve to effectively control the fuel pressure (Pr). The armature modulates the pressure (Pr) which controls the reset piston position. Changing the signals from the APU controller varies the fuel pressure, thus moving the piston in either direction.
　
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