P
LEFT FWD BOOST PUMPLEFT FWD FUELSWITCH BOOST PUMP
L FWD FUEL
PRESSURE SWITCH
FUEL MANAGEMENT CONTROL PANEL
PUMP (Y)
APU FUEL
A
P
VALVE (Y) DC FUEL PUMP
ON (W)
L/R EICAS COMPUTER
DC FUEL PUMP
APU FUEL PUMP PRESSURE SWITCH
TRANSFER RELAY
MP
DC FUEL PUMPDC FUEL PUMP
CONTROL RELAY
1
PREFERRED
PUMP SYSTEM PRESS.SWITCH - LEFT
2
ALTERNATE
APU Fuel Feed System Schematic
Figure 5
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A
757
MAINTENANCE MANUAL
A. Functional Description
(1)
APU Fuel Feed
(a)
The APU fuel feed system supplies fuel for starting the APU. The engine fuel feed system normally feeds fuel to the APU. However, during AC power loss or faulty boost pump operation, the APU fuel feed system takes over. The APU fuel feed system uses the dc fuel pump to supply fuel to start or run the APU. With the APU switch selected to START, the APU fuel shutoff valve opens. The dc fuel pump draws fuel from the left main fuel tank and pumps it to the APU through the open APU shutoff valve. An APU fuel check valve prevents fuel pumped from the dc fuel pump from flowing into the engine feed manifold. When the APU starts, ac power is available and the dc fuel pump automatically shuts down, after the boost pumps begin to operate. The boost pump(s) then feed the APU. The APU fuel check valve allows fuel to flow from the engine feed manifold into the APU fuel supply line.
(2)
DC Pump Operation - On Ground
(a)
The left forward boost pump is dedicated for APU fuel feed. The L FWD PUMP switchlight, on the pilot's overhead panel P5, controls the left forward boost pump. With ac power on, the L FWD PUMP switch on, and the APU selector ON, the left forward boost pump supplies the APU with fuel. If the APU fuel shutoff pressure switch senses a pressure in the engine fuel feed manifold less than 4 psi, the dc fuel pump automaticaly comes on to supply fuel to the APU.
(b)
With AC power on, the L FWD PUMP switchlight, on the P5 panel, off, and the APU selector ON, the left forward boost pump supplies the APU with fuel. A time delay in the left forward pump start relay allows the boost pump to produce enough pressure to supply fuel to the APU and then times out. The left forward boost pump pressure switch then provides the ground for the left forward boost pump to operate. If the boost pump did not produce enough pressure during the time delay (10 seconds) the dc fuel pump starts.
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