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757 MAINTENANCE MANUAL
 AUXILIARY POWER UNIT (APU) FUEL FEED SYSTEM - DESCRIPTION/OPERATION
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 1.  General_______
 A.  The center fuel tank contains all APU fuel feed system components except the APU fuel supply line and shroud. The APU fuel supply line and shroud lies between the center fuel tank and the APU. The DC pump supplies the APU with fuel upon loss of AC power or boost pump failure.
 B.  APU Fuel Feed System
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 (1)  
The engine fuel feed system (Ref 28-22-00) normally supplies fuel to operate the APU whenever AC power is available on the airplane. The DC fuel pump takes over upon loss of AC power on the airplane or due to boost pump failure. The APU fuel feed system supplies the APU with fuel.

 (2)  
The APU fuel feed system consists of a DC fuel pump, check valve, fuel shutoff valve, DC fuel pump pressure switch, APU fuel shutoff pressure switch, two APU flame arrestors, an APU sump drain valve, and a fuel supply line surrounded by a shroud. The DC fuel pump draws fuel from the left main fuel tank and only operates on AC power loss or boost pump failure. The check valve allows the engine fuel feed system to supply the APU with fuel and prevents a back flow of fuel to the engine feed manifold when the DC pump operates. The shutoff valve controls fuel flow to the APU. The DC pump pressure switch senses pump output pressure. The shroud prevents any fuel vapor from the APU supply line from entering the airplane cabin.

 (3)  
The APU fuel shutoff pressure switch senses fuel pressure in the fuel feed manifold. The switch interrupts power to the dc fuel pump if the pressure in the manifold decreases below 4 psi.

 (4)  
The Engine Indicating and Crew Alerting System (EICAS) (AMM 31-41-00/001) provides caution and warning messages for selected system failures.

 2.  Component Details_________________
 A.  DC Fuel Pump (Fig. 2)
 (1)  
Description

 (a)  
The DC fuel pump, a positive displacement type, consists of a pump and driving motor within a housing. The pump housing, within the fuel tank, contains a check valve and a plunger valve. The housing outlet contains the check valve. The housing inlet contains the plunger valve. The driving motor, 28VDC powered, includes a thermal fuse, a check valve, a discharge valve, and a pressure switch. The pump and drive motor mount on the outside of the fuel tank. The motor turns at 6600 rpm and the pump provides a continuous flow of 3.1 GPM at 18 psi.

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