TM 55–2835-208-23
1 –13. FUEL SYSTEM
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INLET FUEL FILTER Fuel from the aircraft system is supplied to the inlet of a disposable
10–micron fuel filter.
FUEL PUMP. The fuel pump is a positive-displacement, gear-type pump. A wire-mesh,
25-micron filter is installed in the top of the pump housing.
ACCELERATION CONTROL ASSEMBLY. The acceleration control assembly is mounted
on, and shaft-coupled to the fuel pump. The acceleration control meters the amount of fuel
supplied to the start fuel nozzle and main fuel manifold.
PCD TUBE ASSEMBLY. Compressor discharge pressure (PCD) is supplied to the
differential pressure regulating valve of the acceleration control. PCD increases as the speed
of the APU increases. As the PCD increases, the fuel metering valve in the acceleration control
opens. This results in additional fuel flow to the turbine.
START FUEL VALVE ASSEMBLY. The start fuel valve is a solenoid activated valve. The
valve is normally closed and is energized by a signal from the electronic sequence unit (ESU).
At 5 percent speed, the start fuel valve opens to allow fuel flow to the start fuel nozzle.
RESTRICTOR (all except –100). The restrictor controls the amount of air flow that purges
fuel from the start fuel nozzle.
START FUEL NOZZLE. Fuel is sprayed into the combustor and is ignited by an igniter plug.
MAIN FUEL VALVE ASSEMBLY. The main fuel valve is a solenoid activated valve. The
valve is normally closed and is energized by a signal from the electronic sequence unit (ESU).
At 14 percent speed, the main fuel valve opens to allow fuel flow to the six fuel nozzles.
FUEL MANIFOLD ASSEMBLY. The six fuel nozzles are mounted in bosses on the
combustor assembly. These six fuel nozzles are connected to form the fuel manifold assembly.
COMBUSTOR DRAIN CHECK VALVE. The check valve, spring loaded open, opens when
the APU is shut down and drains unburned fuel from the combustor.
PURGE VALVE (-100 ONLY). The purge valve, in the “purge” position, allows compressor
discharge (PCD) airflow through the start fuel nozzle to clear nozzle of residual fuel.
FILTER TO PUMP INLET TUBE ASSEMBLY. Connects the fuel filter to the fuel pump
inlet.
1-8 Change 5
