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 49-21-00
 ALL  ú ú 02 Page 609 ú Jul 12/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
APU FUEL SYSTEM - DESCRIPTION AND OPERATION
___________________________________________
 1.  General
_______
 A.  The APU fuel system supplies the fuel for the APU from the airplane fuel system. The No. 1 fuel tank of the airplane is the source for the fuel to the APU. The APU fuel system automatically monitors the fuel flow rate to keep the APU at a constant speed for different load conditions. The components of the APU fuel system are:
 (1)
the APU fuel valve

 (2)
the low-pressure fuel filter

 (3)
the fuel control unit

 (4)
the APU control thermostat (Airplanes without the ETC)

 (5)
the fuel solenoid valve

 (6)
the combustor unit

 (7)
the fuel drain

 (8)
the fuel heater.

 2.  APU Fuel Valve
______________
 A.  The APU fuel valve connects the APU fuel system to the No. 1 fuel tank of the airplane. The APU fuel valve is on the rear spar of the left wing. The APU fuel valve is controlled by the APU master switch. For a more detailed description of the APU fuel valve, refer to AMM 28-22-00/001.
 3.  Low Pressure Fuel Filter (Fig. 1)
________________________
 A.  The low-pressure fuel filter keeps unwanted material from the fuel control unit. The low-pressure fuel filter is a full-flow filter with a filter element. The filter element can be replaced. A bypass valve in the filter permits fuel to be supplied to the fuel control unit when the filter element is clogged. The bypass valve opens when the differential pressure across the low-pressure fuel filter is approximately 14 psi
 (96.5 kPa). The low-pressure fuel filter is installed on the fuel control unit.
 4.  Fuel Control Unit (Fig. 1)
_________________
 A.  The fuel control unit supplies the pressurized fuel to the combustion chamber. The fuel control unit has a fuel pump, low-pressure fuel filter, acceleration limiter valve, flyweight governor, bypass valve and, two relief valves. The fuel control unit also has the fuel inlet ports, fuel outlet port, seal drain line, and control air connection. The fuel control unit is installed on the gearbox. The gear train in the gearbox turns the fuel control unit at approximately 10% of the APU engine speed.
 B.  The fuel pump in the fuel control unit is a gear-type pump that supplies the fuel pressure for the APU fuel system. The fuel pump housing has a pressure relief valve that is spring loaded. When the fuel pressure is too high, the pressure relief valve permits fuel to go back to the fuel inlet port. The high-pressure fuel filter is found downstream of the fuel outlet port. The high-pressure fuel filter keeps all unwanted material from the flyweight governor.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 49-31-00
 ALL  ú ú 02 Page 1 ú Mar 15/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES /MAINTENANCE MANUAL ////////////////////////
LOW-PRESSURE
CHECK VALVE FOR FUEL LINE
THE FUEL HEATER
FUEL
FUEL HEATER CONTROL
UNIT LOW-PRESSURE FUEL FILTER
FUEL
CONTROL AIR SOLENOID
SEE B LINE VALVE
SEE A SEAL DRAIN FOR THE DRIVE SHAFT
FWD
A1
HIGH-PRESSURE FUEL FILTER
HIGH-PRESSURECHECK VALVE FORLOW-PRESSURE FUEL LINETHE FUEL HEATERFUEL LINE FUEL HEATER
FUEL
CONTROL  LOW-PRESSURE
UNIT  FUEL FILTER
FUEL HIGH-PRESSURE
SOLENOID FUEL FILTER
VALVE
HIGH-PRESSURE FUEL LINE
FWD 1 AIRPLANES WITHOUT THE TAFCU
A2
2 AIRPLANES WITH THE TAFCU
APU Fuel System - Component Location
Figure 1 (Sheet 1)

EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
　
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