APU Fuel Pump - Block Diagram Figure 003
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 To purge the APU fuel line, it is possible to operate the APU fuel pump by the APU Fuel-Line Vent-Pushbutton 8QC, installed on the APU compartment firewall. The APU fuel pump operates as long as the pushbutton is pushed.
 4.
 Power Supply

____________ (Ref. Fig. 004) For normal operation, the essential busbar 801XP (115V AC) supplies the pump motor. When the busbar is not energized, the static inverter busbar 901XP (115V AC) supplies the pump motor. The APU fuel pump supply relay 9QC (on panel 103VU) monitors the essential busbar and performs the busbar to switch. The pushbutton switch 8QC operates the APU pump on the ground to purge the fuel line. The APU fuel-line vent relay (6QC) is supplied with 28V DC from the DC Busbar 1 (103PP).

 5.
 Interfaces

__________
 The APU fuel pump system has interfaces with:

 -
the APU fire-extinguishing system (Ref. 26-22-00),

 -
the main fuel pump system (Ref. 28-21-00),

 -
the APU LP fuel shut-off system (Ref. 28-29-00),

 -
the APU fuel distribution system (Ref. 49-31-00),

 -
the APU control and monitoring system (Ref. 49-61-00),

 -
the APU emergency shutdown (Ref. 49-62-00).

 6. Component Description
_____________________
 (Ref. Fig. 005)

 A. Pump Element The pump element (4QC) is a centrifugal type impeller which is driven by a 3 phase electric motor, supplied with single phase 115 V AC at 400HZ. The fuel that circulates through the unit, cools and lubricates the pump element. A thermal fuse, set at 175DEG.C (347DEG.F), protects the motor from an overheat condition.
 B. Pump Canister (P/N C49AA0221) (Ref. Fig. 005, 006) The pump canister (9QM) contains the pump and is attached to the wing-box rear spar. A drain gutter collects any fuel leakage and drains it away from the installation area. The engine fuel crossfeed and the APU fuel lines are bolted to the canister within the tank.
 C. Pump Canister (P/N C49AA0222) (Ref. Fig. 007, 008) The pump canister (9QM) contains the pump and is attached to the wing-box rear spar. The engine fuel crossfeed and the APU fuel lines are bolted to the canister within the tank.
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 APU Fuel Pump - Electrical Schematic Figure 004
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R  APU Fuel Pump (P/N C12CB0010)
 (with Canister (P/N C49AA0221))
 Figure 005
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R  APU Fuel Pump (P/N C12CB0011/C12CB0012/C12CB0013)
 (with Canister (P/N C49AA0221))
 Figure 006
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 APU Fuel Pump (P/N C12CB0010)
 (with Canister (P/N C49AA0222))
 Figure 007
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R  APU Fuel Pump (P/N C12CB0011/C12CB0012/C12CB0013)
 (with Canister (P/N C49AA0222))
 Figure 008
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 D. Pressure Switch The pressure switch (7QC) has a cylindrical chamber with a banjo type head. A bolt secures it to a check valve on the rear spar. The fuel pressure in the crossfeed line goes through the check valve and the banjo head to operate a microswitch in the chamber. When the fuel pressure drops below 1.5 bar (22 PSI) the microswitch closes and transmits 28VDC to APU fuel pump relay, to energize the APU fuel pump.
 E. VENT APU FUEL LINE Pushbutton-Switch
 The VENT APU FUEL LINE pushbutton-switch (8QC) permits the APU fuel pump R to operate on the ground. You press and hold the pushbutton-switch to R purge the fuel line during ground maintenance. This energizes the APU R fuel line vent relay (6QC) to start the pump. The pump is supplied with
 115V AC from the essential busbar 801XP. The pump stops when the R pushbutton is released.
 F. APU Inlet Low Pressure Switch The APU inlet low pressure switch (5030QM) is installed in the APU compartment at the fuel inlet connection to the FCU. It operates on fuel absolute pressure in the APU fuel-feed line at the inlet to the FCU. When it operates, it transmits a signal to the Electronic Control Box (ECB) 59KD. The switch closes when the inlet pressure decreases to 1.09 +0.06 -0.06 bar (15.8091 +0.8702 -0.8702 psi). The switch opens when the inlet pressure increases to 1.19 +0.06 -0.06 bar (17.2594 +0.8702 -0.8702 psi).
　
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