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case.
Functional Description
The combustor drain is a 0.060 inch diameter orifice that drains
fluid. During APU operation, the orifice lets a small quantity of
air go out from the combustor.
APU FUEL SYSTEM - APU COMBUSTOR DRAIN ORIFICE
EFFECTIVITY
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APU FUEL SYSTEM - APU COMBUSTOR DRAIN ORIFICE
APU FUEL SYSTEM - APU COMBUSTOR DRAIN ORIFICE
EFFECTIVITY
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APU FUEL SYSTEM - APU COMBUSTOR DRAIN ORIFICE
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The APU fuel system supplies fuel for combustion. The fuel
control unit also provides regulated fuel pressure for the inlet
guide vane actuator(IGVA) and the surge control valve (SCV)
operation. The APU electronic control unit (ECU) controls the
APU fuel system.
Fuel Supply
GUN 001-099, 101-115
The fuel system boost pumps supply pressurized fuel or the
APU fuel control unit suction supplies fuel through the fuel
shutoff valve to the APU. The shutoff valve opens when the APU
switch is put in the ON or the START position.
GUN 201-999
The fuel system AC boost pumps or the DC APU boost pump
supplies fuel through the fuel shutoff valve to the APU. The
shutoff valve opens when the APU switch is put in the ON or the
START position.
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Fuel Control Unit
The fuel control unit (FCU) has these functions:
* Pressurizes the fuel
* Cleans the fuel
* Controls the fuel pressure
* Controls fuel flow.
ECU Control
The ECU logic controls the following fuel feed components:
* Fuel shutoff valve
* Fuel solenoid valve
* Fuel metering valve
* Flow divider solenoid.
The fuel shutoff valve opens when the APU switch is in the ON
or START position. During the APU start, the ECU sends a
signal to the fuel solenoid valve to open when the APU RPM is
more than 7 percent. The ECU controls the fuel metering valve
when the APU speed is more than 7 percent. The ECU also
controls the flow divider solenoid valve on the fuel flow divider.
The ECU uses start-up control logic to control the fuel metering
valve when the speed is less than 95 percent. This logic
schedules fuel flow to start the APU quickly and to keep the EGT
low. Start-up fuel flow logic uses these inputs:
* APU speed (N)
* Inlet pressure (P2)
* Inlet temperature (T2)
* Exhaust gas temperature (EGT).
The ECU uses the same inputs for the on-speed logic above 95
percent speed.
APU FUEL SYSTEM - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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49-30-00-007
The flow meter in the FCU sends a fuel-flow feedback signal to
the ECU. The ECU uses this signal to make sure the APU gets
the necessary fuel.
Fuel Flow Divider
The fuel flow divider gives fuel to the primary manifold during
start of the APU. At 25-40 percent speed or approximately 120
psi, the fuel flow divider gives fuel to both the primary and
secondary fuel manifolds for APU operation.
The fuel flow divider has a normally open flow divider solenoid.
The ECU signals the flow divider solenoid closed above
approximately 25,000 feet (7,620 meters) altitude to make sure
the internal check valve for secondary fuel does not open and
cause the APU speed to decrease.
The flow divider solenoid also improves start capability in cold
weather.
The ECU uses P2, T2, and speed signals to control the fuel flow
divider solenoid valve.
Actuator Pressure Regulator
The FCU also supplies pressurized fuel to operate the IGVA and
the SCV. The pressure is regulated at 225 to 275 psi. Return fuel
from these valves goes back to the pump inlet.
GUN 201-999
DC APU Boost Pump
The DC boost pump operates when the APU switch is in the ON
or START position and the fuel pressure from the ac pumps that
could provide fuel to the APU are low. When the ac boost pump
pressure goes high, the dc boost pump goes off.
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