777 AIRCRAFT MAINTENANCE MANUAL
APU FUEL FEED -COMPONENTS
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APU FUEL FEED -FUEL SUPPLY LINE AND SHROUD
APU Fuel Supply Line and Shroud
The APU fuel supply line is flexible rubber and kevlar. It has an aluminum shroud. The fuel supply line exits from the center dry bay. It passes through the right wheel well, and goes under the floor to the APU.
Shroud Drain Line
The shroud drain line connects the fuel line shroud to the APU shroud drain mast. If there is a leak in the fuel supply line, the fuel is contained by the shroud and flows overboard through the drain mast.
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APU FUEL FEED -FUEL SUPPLY LINE AND SHROUD
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APU FUEL FEED -FUNCTIONAL DESCRIPTION -FUEL FLOW
General
The APU fuel feed system uses the main pumps or the DC pump to supply fuel to the APU. The system can also supply fuel to the left engine feed manifold.
APU Fuel Feed From Main Pumps
The APU normally receives fuel from the left engine feed manifold. The engine feed manifold receives fuel from any of the main or center tank fuel pumps. The fuel flows through a check valve and the APU fuel shutoff valve to the APU fuel line.
APU Fuel Feed From DC Fuel Pump
During APU starts and operation, if there is no fuel pressure in the left engine feed manifold, the APU dc fuel pump automatically turns on. This supplies fuel through the APU fuel valve to the APU fuel line.
Main Engine Feed From DC Fuel Pump
The APU dc fuel pump also turns on when there is no pressure in the left engine feed manifold and the left engine is not operating. Fuel flows through the APU isolation valve into the left engine feed manifold.
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APU FUEL FEED -FUNCTIONAL DESCRIPTION -FUEL FLOW
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777 AIRCRAFT MAINTENANCE MANUAL
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APU FUEL FEED -FUNCTIONAL DESCRIPTION -APU DC FUEL PUMP & ISOLATION VALVE
General
The ELMS automatically controls the APU dc fuel pump and the APU isolation valve. When you start the APU on the ground and ac power is not available, the ELMS starts the APU dc fuel pump. During flight, if there is an engine flameout, the ELMS can start the APU dc fuel pump and open the APU isolation valve. This lets the dc fuel pump supply fuel to the left engine when the ac boost pumps do not operate.
APU DC Fuel Pump
The APU dc fuel pump gets power from battery bus #2. The P310 EEU sends a signal to turn on the pump. This energizes the APU dc pump control relay to supply power to the pump.
Isolation Valve
The APU isolation valve gets power to open or close from the captain’s flight instrument bus. The P310 EEU sends a signal to open the valve. This energizes the APU isolation valve relay to supply power to open the valve.
Power to operate the valve goes through limit switches to the actuator. The actuator moves the valve to the commanded position. When the valve is in the commanded position, the limit switches remove power from the actuator and send power to the ELMS valve agree status logic.
Control
For APU fuel feed, the P310 EEU sends the dc pump command signal when these conditions occur:
*
No pressure in the left engine feed manifold
*
APU running, or APU selector in START or ON position.
For left engine fuel feed, the P310 EEU sends the dc pump command signal and the isolation valve command signal when these conditions occur:
*
No pressure in the left engine feed manifold
*
left engine N2 is less than the engine run speed
*
Left engine fuel control switch is in the RUN position
*
The airplane is in the air.
The ELMS uses signals from the ac fuel pump pressure switches and the crossfeed valves to find if there is pressure in the left engine feed manifold. The P110 and P210 EEUs send the signals on the systems ARINC 629 buses to the P310 EEU. The APU signals come from the APU control unit and the APU selector.
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