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The APU Battery must have at least 22 volts for starter motor operation.
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
5-12 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
Operating Sequences
Start Sequence
With the Battery Master switch ON, pushing the PWR FUEL switch/light opens the
APU fuel shutoff valve and negative-G shutoff valve, turns on the APU fuel pump,
arms the START/STOP switch/light, and displays the APU gauges on the EICAS.
Pressing in the START/STOP switch/light powers and signals the ECU to initiate the
start sequence. The start is automatically controlled.
• 10% - Ignition activated
• 60% - Starter cutout
• 95% - Ignition deactivated
• 95% + 4 secs (approx.) - APU AVAIL (green) on
• 100% + 15 secs (approx.) - APU GEN OFF (amber) message on EICAS
When the APU has stabilized and is ready for use, a green AVAIL light illuminates.
The APU GEN OFF (amber) message indicates that the APU is stabilized, operational,
and the APU generator has not been selected.
If time permits, allow APU rpm to stabilize at 100% for two
minutes before applying bleed air load.
Shutdown Sequence
The normal shutdown sequence is as follows:
• press out the START/STOP switch/light
• wait until RPM and EGT gauges wind down through the 6 o’clock position
• press out the PWR FUEL switch
Pressing out the START/STOP switch/light causes the internal FCU fuel shutoff valve
to close and the APU to shut down. When the PWR FUEL switch is pressed out, ECU
power is removed, the APU fuel shutoff valve and negative-G shutoff valve are closed,
and the APU fuel pump turns off. The gauges will remain in view for approximately
60 seconds with RPM and EGT indications reading zero.
If the PWR FUEL switch is not deselected at the end of the APU
shutdown sequence, the internal runtime clock of the APU
continues to register as if the APU was still operating.
NOTE
NOTE
AUXILIARY POWER UNIT
For Training Purposes Only
Sept 04
5-13
P I LOT T R A I N I N G GU I D E
Protective Shutdowns
Automatic protective shutdowns occur for the following preprogrammed conditions:
All protective shutdowns generate EICAS messages except for the Overcurrent
condition.
The cause of the shutdown is also annunciated on the APU FAULT indication panel
(except for APU FIRE) by magnetic trip indicators or LEDs. Starting with aircraft
5492 and subsequent, the fault panel has LED fault indicators (see Figure 5-9).
The panel also includes an IND RESET button to clear all trip indicators and an APU
STOP button for emergency shutdown of the APU from outside the airplane. If the
APU STOP button is used, the OVERSPEED magnetic indicator will show a fault
condition.
CONDITION EICAS INDICATION
FIRE Master warning, fire bell, APU fire light, APU FIRE warning
message
OVERSPEED Master warning, triple attenson, APU OVERSPEED warning
message
OVERTEMP Master warning, triple attenson, APU OVERTEMP warning
message
LOW OIL PRESSURE Master warning, single attenson, APU OIL PRESS caution
message
HIGH OIL TEMPERATURE Master caution, single attenson, APU OIL TEMP caution
message
OVERCURRENT Nil
GENERATOR ADAPTER LOW OIL PRESSURE Master caution, single attenson, APU OIL PRESS caution
message
GENERATOR ADAPTER HIGH OIL
TEMPERATURE
Master caution, single attenson, APU OIL TEMP caution
message
Protective Shutdowns Table
Table 5-1
AUXILIARY POWER UNIT
5-14 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
Pneumatic Supply
Description
APU compressor bleed air is used to pressurize the left side of the 10th-stage bleed air
manifold to provide pneumatics for engine starting and air conditioning. Reverse
airflow protection is provided by a one-way check valve and the interlock control
logic of the 10th-stage bleed air switch/lights (for additional information on the 10thstage
bleed air manifold, see Chapter 19 Pneumatic System).
Components and Operation
The APU pneumatic control system consists of two components, the load control
valve (LCV) and the surge control valve (SCV).
Bleed Air Supply
The APU delivers its pneumatic output to the 10th-stage manifold through the load
control valve. When the load control valve is opened, a portion of the compressor
discharge air is routed to the left side of the 10th-stage bleed air manifold. This diverts
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