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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.
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.
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−10
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
Negative−G Shutoff Valve and FCU
Figure 05−10−6
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−11
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 in the aft equipment
bay. The APU battery provides the cranking power required 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 (ECU)
The ECU monitors APU sensors and switches, and sets the appropriate fuel and temperature
schedules. In addition, the ECU relays APU operating data to the flight crew through the data
concentrator units (to EICAS) and to the APU fault indication panel located on the left aft
fuselage (next to the ground air adapter).
The ECU senses changes in shaft or bleed load and adjusts the fuel supply to maintain a
constant rpm at 100%. If EGT operating limits are exceeded, the ECU will reduce the bleed load
by cycling the load control valve closed.
The ECU is powered by the battery bus when the APU PWR FUEL switch/light is pressed in.
APU Starter
The APU starter motor is DC powered by the APU battery direct bus. It transfers cranking power
to the APU drive shaft through the accessory gearbox. The white START legend in the top half
of the START/STOP switch/light illuminates whenever the starter motor is engaged.
Ignition System
The ignition system ignites the fuel/air mixture during start.
The ignition system is energized after the START/STOP switch/light is pressed and APU rpm
reaches 10%. Ignition is de-energized when rpm reaches 95%.
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−12
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION (CONT'D)
APU Start and Ignition
Figure 05−10−7
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−13
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION (CONT'D)
APU Start
With the battery master switch ON, pressing the PWR FUEL switch/light results in the following:
• APU fuel shutoff and negative-G shutoff valves open;
• APU fuel pump energizes; and
• Displays the APU gauges on EICAS.
Pressing the START/STOP switch/light enables power to the ECU and signals the ECU to
initiate the start sequence. The starter engages, the start legend is illuminated on the APU
panel, and the following events occur:
• 10% rpm – Fuel and ignition energized (a rapid increase in temperature, as shown on
the EGT gauge, indicates light-off);
　
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