(2)  
When the APU speed is more than 95%, the 3-way solenoid valve is energized closed and the APU control thermostat works as a load control thermostat. The APU control thermostat operates as an overload protection for the APU. For a more detailed description of this function of the APU control thermostat, refer to 49-52-00/001.

 E.  The fuel atomizer sprays the atomized fuel into the combustion chamber in a constant pattern. The constant pattern is not changed by different fuel pressures. Low pressure fuel flows to the small orificed plate for full atomization. When the fuel pressure increases, the flow divider valve opens and permits the fuel to flow to the two orificed plates. This will correctly atomize the high-pressure fuel.
 F.  When the APU speed is 100%, the governor controls the fuel flow to keep the APU engine speed constant for different load conditions. When the APU speed is more than 100%, the governor flyweights move outward to open the bypass valve. This will decrease the fuel supply to the fuel atomizer. When the APU engine speed decreases too much, the governor flyweights move inward to close the bypass valve. This will increase the fuel flow to the fuel atomizer and let the APU speed increase.
 G.  When the APU master switch is moved to the OFF position, electrical power is supplied to the overspeed oscillator of the speed switch. The 110% contact in the speed switch closes. This gives a signal that is the same as an overspeed condition. Electrical power is removed from the fuel solenoid valve and the APU fuel valve. The flow of fuel to the fuel atomizer is stopped. The APU slows down and stops. All the unwanted fuel in the combustion chamber drains through the fuel drain.
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 49-31-00
 ALL  ú ú 03 Page 9 ú Jul 15/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
APU FUEL SYSTEM - MAINTENANCE PRACTICES
_______________________________________
 1.  General
_______
 A.  This section gives the procedures to do the checks of the APU fuel
 system. The procedures are:

 (1)  
Low Pressure Bleed

 (2)  
High Pressure Bleed

 (3)  
Fuel Leakage Check

 B.  The low pressure bleed makes sure the air is removed from the fuel line
 to the fuel control unit. The high pressure bleed makes sure the air is
 removed from the fuel line to the fuel atomizer.

 C.  The fuel leakage check for the APU makes sure the APU fuel system does not have leakage. This procedure does the check from the APU fuel valve to the inlet of the fuel control unit. If it is necessary to do a leakage check from the No. 1 fuel tank of the airplane to the primary APU fuel-feed check valve, the bypass check valve or the pressure relief valve in the APU fuel-feed line, then do this task: "Check Valve Leakage Troubleshooting" (AMM 28-22-00/201).
 TASK 49-31-00-872-001
 2.  Low Pressure Bleed (Fig. 201)
__________________
 A.  Standard Tools and Equipment
 (1)  Container - for fuel, 2-gallon (8 liter) capacity
 B.  References
 (1)  AMM 24-22-00/201, Manual Control
 C.  Access
 (1)  
Location Zones
 101 Control Cabin (Left)
 205 Electronic Compartment
 215 Left Main Wheel Well
 703 APU Compartment

 (2)  
Access Panels
 1201 Electronic Equipment Access Door
 3804 APU Cowl Door

 D.  Procedure
 S 862-002
 (1)  Set these switches and the circuit breaker:
 (a)  
Set the APU master switch, on the P5 forward overhead panel, to the OFF position and attach a DO-NOT-OPERATE tag.

 (b)  
Set the No. 1 AFT FUEL PUMP switch, on the P5 forward overhead panel, to the OFF position.

 (c)  
Open this circuit breaker and attach a DO-NOT-CLOSE tag: 1) E3-3 Electrical Shelf, APU Control Unit

 S 012-004
 (2)  Open the APU cowl door:
 (a)  Open the latches for the APU cowl door.
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 ALL  ú ú 02 Page 201 ú Nov 15/97
　
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