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of the APU generator adapter oil. Fuel that has passed through the fuel/oil heat exchanger is
returned to the right main tank.
Selection of the APU PWR FUEL switch/light activates the APU fuel pump. This selection also
opens the APU’s fuel shutoff valve and negative-G shutoff valve. Insufficient APU fuel pump
output pressure is indicated by an APU PUMP caution message, and illumination of the PUMP
FAIL switch/light legend on the APU panel.
APU Fuel Shutoff Valve
The APU fuel shutoff valve (SOV) opens when the PWR FUEL switch/light is pressed in, and
closes when it is pressed out. If the valve does not match the switch/light position within
5 seconds, an APU SOV caution message is displayed.
When the APU FIRE PUSH switch/light is pressed in, the APU fuel shutoff valve and the APU
negative-G shutoff valve close. Successful closing of the valves is indicated by an APU SOV
CLSD advisory message, and illumination of the green SOV CLOSED switch/light legend on the
APU panel. If the APU SOV fails to close after APU FIRE PUSH switch/light selection, an APU
SOV caution message is displayed.
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−8
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
APU Fuel Pump and Shutoff Valve
Figure 05−15−5
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−9
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
APU Fuel Control Unit
The APU fuel control unit (FCU) is driven by the accessory gearbox, and meters fuel to the
combustor under command of the electronic control unit (ECU). The APU fuel control unit is
equipped with an internal solenoid-operated fuel shutoff valve that is commanded open and
closed by the APU ECU, controlling fuel to the APU fuel control unit. In addition to controlling
fuel shutoff, the ECU regulates fuel flow output from the fuel control unit, ensuring proper APU
operating speed and temperature ranges.
During normal or automatic APU shutdown, a simulated overspeed signal is generated within
the ECU, commanding closure of the fuel control unit’s solenoid shutoff valve. Closing this valve
causes an isolation of fuel supply to the APU’s fuel control unit, resulting in APU shutdown.
APU Negative-G Shutoff Valve
The APU negative-G shutoff valve opens and closes when the PWR FUEL switch/light is
pressed. If the valve fails to close when the PWR FUEL switch/light is pressed out, the APU NEG
G SOV caution message is displayed.
If a negative-G condition exists or the APU fuel pump fails, a one-way check valve, that
operates based on positive differential pressure, allows fuel from the negative-G shutoff valve to
feed the APU. An additional one-way check valve prevents this alternate fuel from entering the
fuel/oil heat exchanger.
In the event of an actual APU fire, the negative-G shutoff valve is commanded closed when the
APU FIRE PUSH switch/light is activated. If the valve fails to close, the left engine fuel feed
manifold continues to supply fuel to the APU fuel control unit, and the APU NEG G SOV caution
message is displayed. Under this condition, it is necessary to shutdown the left engine in order
to shut off the fuel supply to the APU.
NOTE
If the APU fuel pump is not operable, the APU has an operating time
limit from start-up until shutdown, due to insufficient cooling of the
generator adapter oil. Refer to FUEL ATA 28 of the Minimum
Equipment List for the allowable operating time limit.
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−10
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
Negative−G Shutoff Valve and FCU
Figure 05−15−6
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−11
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION
Description
APU start and ignition is automatically controlled by the APU electronic control unit.
Components and Operation
APU Battery
The 24-volt, 43-ampere-hour battery is located below the APU enclosure, and provides cranking
power to turn the APU starter. A minimum of 22 volts is required for starter motor operation. In
the event of APU battery rundown, external DC power may be used to supply the starter via the
APU battery direct bus.
Electronic Control Unit
The electronic control unit (ECU) is powered from two 28V inputs: primary, via the battery bus,
and secondary, via the main battery direct bus. The PWR FUEL switch/light controls the primary
　
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