28-25-00
ALL ú ú 02 Page 5 ú Jun 10/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
7. APU DC Fuel Boost Pump Control Relay
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A. The APU dc fuel boost pump control relay controls the operation of the APU dc fuel boost pump. The relay is located in the P414 power distribution center - left. The relay receives power from the APU control switch and a ground through the APU dc fuel boost pump control pressure switch. When the relay is energized, power is supplied to the boost pump.
8. APU Fuel Shutoff Valve (Fig. 1)
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A. The APU fuel shutoff valve controls fuel flow to the auxiliary power unit. The valve assembly consists of two replaceable subassemblies consisting of an electric actuator located on the aft side of No. 2 main tank rear spar and a valve body located inside the tank. The valve is controlled by the APU control switch on the pilots' overhead panel P5. For description of the APU fuel valve control circuit, refer to 49-31-00, APU Engine Fuel System.
B. The actuator consists of a dc motor, gears, limit switches, electrical connector, a splined drive shaft, and a manual override handle incorporated with a valve position indicator. Operation of the override handle will not cause either the limit switches or the actuator to be out of phase with the control circuit. The actuator is coupled to the valve body by inserting the splined drive shaft through the spar web into the splined receptacle of the valve.
C. The valve body consists of a valve housing, rotating type valve disk, seals, thermal relief, valve ports for tubing connections and a mounting surface sealed to prevent fuel leakage around the actuator shaft opening in the spar web. Thermal relief in the valve is from line to tank and not across the shutoff element.
D. Valve position can be determined from manual override lever position.
9. APU DC Fuel Boost Pump Control Pressure Switch (Fig. 1)
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A. The APU dc fuel boost pump control pressure switch supplies automatic changeover from the APU dc boost pump to the aft boost pumps for the No. 2 and No. 3 main tanks. The change over occurs when the APU has started and ac power is available. The switch is installed on the rear spar and is connected to a sensor found in the fuel discharge line from No. 2 aft main boost pump.
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28-25-00
ALL ú ú 18 Page 6 ú Feb 15/98
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
10. Operation
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A. Functional Description (Fig. 2 and 3)
(1)
When the APU control switch on the P5 pilots' overhead panel is put in the ON position, the APU fuel control relay R7200 is energized. Power is supplied from the battery to open the APU fuel shutoff valve and energize the No. 2 and No. 3 aft main boost pump control relays.
(2)
When the APU is switched on, dc power is also supplied to the APU fuel boost pump control relay. While the APU fuel boost pump control pressure switch senses low pressure in the No. 2 main aft boost pump fuel manifold, the relay is energized. Power is supplied to the APU dc fuel boost pump.
(3)
If the APU dc fuel boost pump pressure switch senses low pressure after the APU dc fuel boost pump is commanded on, the APU FUEL advisory is displayed on the EICAS main screen and the APU FUEL PUMP status message is recorded.
(4)
After the APU has started and the ac ground service transfer bus is energized, the No. 2 and No. 3 aft main boost pumps will begin to operate. Fuel pressure from this pump actuates the APU dc fuel boost pump control pressure switch, removing the ground for the APU dc fuel boost pump relay and removing power from the APU dc fuel boost pump.
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