APU Flashcards
The APU supplies bleed air and electrical power.
Electrical power has priority over bleed air.
The APU can be started and operated to the airplane maximum operating altitude.
Electrical power is available throughout the airplane operating envelope.
With the APU SEL in the OFF position;
the APU bleed air isolation valve is closed, a normal shutdown is initiated and the auto shutdown fault logic is reset.
ON is the APU operating position.
Moving the APU SEL to the ON position
opens the APU fuel valve and inlet door,
activates an AC or DC fuel pump and powers the APU controller.
START is a momentary position
spring-loaded to return to ON. It initiates the automatic start sequence.
The APU FAULT light is illuminated if
an APU fault and / or fire is detected,
if the APU has shutdown due to a fault and / or fire,
or momentarily during the APU controller self-test.
The APU is started either by
an electric start motor or an air turbine starter.
APU Battery
Main Battery for APU
APU Battery powers Electric Starter
Main Battery
Inlet Door
Fuel Valve
Fire Detection System
The air turbine starter uses
bleed duct pressure or ground air.
Starter selection is automatic.
The air turbine starter has
priority over the electric start motor when there is sufficient bleed air duct pressure.
APU fuel is supplied from
the left fuel manifold by any operating AC fuel pump or the DC fuel pump.
With AC power available and the APU SEL in the ON position,
the left forward fuel pump operates automatically.
If AC power is not available or no AC pump pressurizes the left fuel manifold
the DC pump in the left main tank provides APU fuel.
When the APU air inlet door reaches the full open position
the starter engages.
When the APU reaches approximately 50 % RPM
the starter disengages and ignition is turned off.
If the start fails,
the APU shuts down automatically.
The EICAS message APU SHUTDOWN is displayed.
In flight, if both AC transfer busses lose power,
the APU automatically starts, regardless of APU SEL position.
The APU can be shutdown by positioning the SEL to ON, then OFF
Rotating the APU SEL to OFF begins the shutdown cycle by
closing the APU bleed air valve.
The APU continues running for a cooldown period.
The EICAS memo message APU COOLDOWN is displayed during the cooldown period.
When the cooldown period finishes, the APU shuts down.
The APU has attended and unattended operating modes.
The attended mode operates
when either engine is running or starting, or when the airplane is in flight.
The unattended mode operates at all other times on the ground.
In the attended mode, any of the following faults cause the APU to shutdown immediately
APU fire or
inlet over-temperature,
overspeed or
loss of overspeed protection,
an APU controller failure or
speed droop.
There is no cooldown period. The EICAS advisory message, APU SHUTDOWN, displays.
In the attended mode For APU high EGT,
high oil temperature or
oil pressure indications,
the APU continues to operate and the EICAS caution message APU LIMIT is shown.
There is no cooldown period when the APU is shutdown after the APU LIMIT message is displayed.
In the unattended mode, any of the following faults cause the APU to shutdown immediately
APU fire or inlet over temperature,
overspeed or loss of overspeed protection,
high EGT, low oil pressure, high oil temperature,
generator oil filter approaching bypass,
intake door failure,
APU controller failure,
speed droop,
no combustion on start,
no acceleration on start or load compressor reverse flow.
There is no cooldown period.
When a limit is exceeded & the APU is shutdown with the selector switch:
The APU SEL is turned OFF.
This attempts to reset the APU controller.
