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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. 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 APU 150
Vol. 2 05−15−12
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION (CONT'D)
APU Start and Ignition
Figure 05−15−7
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−13
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 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
• 100% rpm + 15 seconds (approx) – APU GEN OFF caution message on EICAS
(indicating that the APU generator has not been selected on).
The green AVAIL legend on the START/STOP switch/light indicates that the APU is stabilized
and ready for use.
NOTE
If time permits, allow APU rpm to stabilize at 100% for two minutes
before applying electrical or bleed air loads.
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−14
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION (CONT'D)
APU Start
Figure 05−15−8
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−15
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
START AND IGNITION (CONT'D)
Normal Shutdown
The APU GEN switch is selected to the OFF/RESET position prior to shutting down the APU.
NOTE
To reduce thermo cycles, shutdown the APU in the last running
configuration. Do not change the bleed load before shutting down
the APU.
Pressing out the START/STOP switch/light results in the following:
• AVAIL legend out;
• Internal FCU shutoff valve closes; and
• APU shuts down.
Pressing out the PWR FUEL switch/light results in the following:
• ECU primary power is removed;
• APU fuel shutoff and negative-G shutoff valves close;
• APU fuel pump turns off; and
• APU GEN OFF caution message out as the RPM and EGT decay.
The RPM and EGT gauges display actual values and remain in view for approximately 60
seconds.
NOTE
The battery master switch should not be selected off, until the RPM
and EGT gauges are removed from the EICAS. This interval allows
the ECU time to complete its shutdown functions, and to power itself
down.
ECU secondary power is removed when all ECU internal checks are completed, to avoid battery
drain.
AUXILIARY POWER UNIT
Description APU 150
Vol. 2 05−15−16
　
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