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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’s motive flow
continues to supply fuel to the APU fuel control unit, and the APU NEG G SOV caution EICAS
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
Vol. 2 05−10−10
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
APU FUEL DISTRIBUTION (CONT'D)
Negative−G Shutoff Valve and FCU
Figure 05−10−6
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−11
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
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
power, and secondary power is directly connected via circuit breaker. Secondary power is used
by the ECU as a backup when the ECU’s primary voltage drops below 22V. The ECU’s internal
logic disconnects the secondary 28V input (removing power from the ECU) after shutdown, and
when all ECU shutdown tasks are completed.
The ECU monitors APU sensors and switches, and sets the appropriate fuel and temperature
schedules. The ECU senses changes in shaft or bleed load, and adjusts the fuel supply to
maintain a constant RPM at 100%. In addition, the ECU sends APU operating data to the data
concentrator units for EGT and RPM display on the EICAS page. The ECU also provides
operating data for fault identification on the APU FAULT panel, located on the left aft fuselage
(next to the ground air adapter).
APU Starter
The APU starter motor is DC-powered by the APU battery direct bus, and transfers cranking
power to the APU drive shaft through the accessory gearbox. The white START legend in the
upper 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 system is energized after the
START/STOP switch/light is pressed in and APU rpm reaches 5%. Ignition is de-energized
when the rpm reaches 99%.
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−12
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
START AND IGNITION (CONT'D)
APU Start and Ignition
Figure 05−10−7
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−13
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
START AND IGNITION (CONT'D)
APU Start
With the battery master switch ON, pressing the PWR FUEL switch/light results in the following:
• APU ECU is powered, and after a 3-second BITE test, the APU EGT indication shows
actual EGT (ambient temperature), and the rpm shows actual percentage;
• APU fuel shutoff and negative-G shutoff valves open; and
• APU fuel pump energizes.
Pressing the START/STOP switch/light 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:
• 5% rpm – Fuel and ignition energized (a rapid increase in temperature, as shown on the
EGT gauge, indicates light-off);
• 35−45% rpm – Starter cutout – START (white legend) out;
• 95% rpm + 4 seconds (approx) – APU AVAIL (green legend) on;
• 99% rpm – Ignition deactivated; and
　
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