Home » Commercial » Airbus » A320 » Airbus A320 Cockpit Tour: #27 – APU Panel: What do all those buttons do?!

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The Auxiliary Power Unit (APU) is a self-contained unit that supplies the aircraft with pneumatic and
electrical power.
On the ground
‐ It supplies bleed air for starting the engines and for the air conditioning system
‐ It supplies electrical power to the electrical system.
During takeoff
‐ It supplies bleed air for air conditioning (for example, when optimum aircraft performance is
In flight
‐ It backs up the electrical system
‐ It backs up the air conditioning
‐ It can be used to start the engines.
The APU may obtain power for starting from the aircraft’s batteries or normal electrical system, or
from ground service.
APU starting is permitted throughout the normal flight envelope (Refer to LIM-APU Operational

(1) MASTER SW pb-sw
This switch controls the electric power supply for the operation of the APU and its protective features.
It also controls the starting and shutdown sequences.
ON : The blue ON light comes on.
Electric power goes to the APU system; the ECB performs a power-up test.
The APU air intake flap opens.
The APU fuel isolation valve opens.
If no fuel tank pump is running, the APU fuel pump operates.
If the aircraft has ground power or main generator power, the APU page appears on
the ECAM display.
OFF : Manual shutdown sequence.
‐ The ON light on the MASTER SW pb-sw, and the AVAIL light on the START pb go
‐ If the aircraft was using APU bleed air, the APU keeps running for a cooling period
of 60 to 120 s.
‐ At 7 % the air inlet flap closes.
FAULT lt : Depending on version of the ECB, this amber light comes on, and a caution appears
on ECAM, when an automatic APU shutdown occurs, which can happen in case of:
Fire (on ground only)
Air inlet flap closed
No acceleration
Slow start
EGT overtemperature
No flame
Reverse flow
Low oil pressure
High oil temperature
DC power lost (BAT OFF when aircraft on batteries only)
Sensor failure
IGV failure
ECB failure
No speed
Loss of overspeed protection
Oil system shutdown
Inlet overheat
Clogged oil filter
Loss of EGT thermocouples
Note: In the case of an automatic, non-emergency shutdown, the air inlet flap closes 15 min after
the APU speed is lower than 7%. If an automatic, non-emergency shutdown happens on
ground, the 15 min countdown starts after liftoff.

(2) START pb-sw
ON : Blue ON light comes on.
‐ When the flap is completely open, the starter is energized.
‐ 1.5 s after the starter is energized, the ignition turns on.
‐ When N = 60 %. The APU starter is de-energized. The ignition is turned off.
‐ 2 s after N reached 95 %, or when N is above 99.5 %:
The ON light on the START pb goes out.
The APU may now supply bleed air and electrical power to the aircraft systems.
‐ 10 s later, the APU page disappears from the ECAM display.
AVAIL lt : This green light comes on when N is above 99.5 % or 2 s after N reaches 95 %.

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