AUXILIARY POWER UNIT

AIRCRAFT SYSTEMS - APU

The Auxiliary Power Unit (APU) is a self-contained unit that makes the aircraft independent of
external pneumatic and electrical power supplies.

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, thus avoiding a reduction in engine thrust caused by the
use of engine bleed air for this purpose when optimum aircraft performance is required.

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 alone or in combination with the
external power, or from ground service.

APU starting is permitted throughout the normal flight envelope.

The ECAM displays APU parameters.




APU ENGINE

The basic element of the APU is a single-shaft gas turbine that delivers mechanical shaft power for
driving the accessory gbx (electrical generator, starter, etc.) and produces bleed air (engine starting and pneumatic supply).

ELECTRONIC CONTROL BOX

The Electronic Control Box (ECB) is a full-authority digital electronic controller that performs the bulk
of the APU system logic for all modes of engine operation, such as :
‐ Sequences the start and monitors it.
‐ Monitors speed and temperature.
‐ Monitors bleed air.
‐ Sequences the shutdown.
‐ Controls the automatic shutdown.

AIR INTAKE SYSTEM
The air intake and an electrically operated flap allow external air to reach the compressor inlet.

STARTER
The ECB controls the electric starter.
The starter engages if the air intake is fully open and the
MASTER SW and the START pushbutton are ON.

FUEL SYSTEM

The left fuel feed line supplies the APU.
The required pressure is normally available from tank pumps.
If pressure is not available (batteries only or pumps off) the APU FUEL PUMP starts automatically.
The ECB controls the fuel flow.
OIL SYSTEM

The APU has an integral independent lubrication system (for lubrication and cooling).

INLET GUIDE VANES (IGV)

The IGVs control bleed air flow, and a fuel-pressure-powered actuator positions the IGVs.
The ECB controls the actuator in response to aircraft demand.

AIR BLEED SYSTEM

The air bleed system is fully automatic.
The APU speed is always 100 % whatever the air bleed system demand and the ground/flight
configuration are.

CONTROLS

The flight crew uses the controls on the APU panel for routine shutdown. For emergency shutdown :
‐ the flight crew can push the APU FIRE handle, or
‐ the ground crew can push the APU SHUT OFF pushbutton on the interphone panel under the
nose fuselage.

GROUND OPERATION SAFETY DEVICES

The APU may run without cockpit crew supervision when the aircraft is on the ground. In case of fire
in the APU compartment :
‐ APU fire warnings operate in the cockpit.
‐ A horn in the nose gear bay sounds.
‐ The AVAIL light goes out.
‐ The FAULT light in the MASTER SW lights up.
‐ The APU shuts down.
‐ The APU fire extinguisher discharges.

OVERHEAD PANEL




(1) MASTER SW pb-sw
This switch controls the electrical 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 out.
‐ The APU keeps running for a cooling period of 120 s at N 100 % speed.
‐ At 7 % the air inlet flap closes.
FAULT lt : This amber light comes on, and a caution appears on ECAM, when an automatic
APU shutdown occurs, which happens in case of:
- Fire (on ground only)
- Air inlet flap not open
- Overspeed
- No acceleration
- Slow start
- EGT overtemperature
- No flame
- Underspeed
- Reverse flow
- Low oil pressure
- High oil temperature
- DC power loss. (BAT OFF when aircraft on batteries only)
- ECB failure
- Loss of overspeed protection

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 APU starter is energized.
‐ 1.5 s after the starter is energized, the ignition is turned ON.
‐ When N = 55 %. 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 %.


EXTERNAL CONTROLS





ECAM APU PAGE





(1) AVAIL - Displayed in green when APU N is above 99.5 % or 2 s after N is above 95 %.

(2) APU bleed air valve position
Inline-Green : The APU bleed air valve is not closed.
Crossline-Green : The APU bleed air valve is closed.
Crossline-Amber : The APU bleed air valve is closed and the APU bleed is ON.
XX-Amber : The APU bleed air valve status information is not avail, or theAPU BLEED pb status is not avail.

(3) APU bleed air pressure
This box displays the relative bleed air pressure in green.
It shows an amber XX when the ADIRS1 is not available or selected OFF or the data from the
ECB are invalid or not transmitted.

(4) APU GEN line contactor indication - Displayed in green when the APU GEN contactor is closed.

(5) APU GEN parameter - Identical to the APU GEN parameters on the ELEC page.

(6) FUEL LO PR - Displayed in amber if the APU fuel pressure gets low.

(7) FLAP OPEN - Displayed in green when APU air intake flap is fully open.

(8) APU N
‐ Displays APU speed in green.
‐ Becomes amber when N ≥ 102 %.
‐ Becomes red when N ≥ 107 %.

(9) APU EGT



‐ Displays APU EGT in green.
‐ Becomes amber when EGT ≥ EGT MAX See * -33 °C.
‐ Becomes red when EGT ≥ EGT MAX See * (automatic shutdown begins).

* ECB calculates EGT MAX and transmits it to the ECAM. It is a function of N duringstart, and a function of ambient
temperature when the APU is running.
Maximum EGT during start: 982 °C.
Maximum EGT with APU running:
‐ 682 °C during at least 5 s.
‐ or, 700 °C to 742 °C depending on the ambient temperature.
(10) LOW OIL LEVEL
Adv: Displayed if the ECB detects a low APU oil lvl when the acft is on the gnd and the APU is not running.


MEMO DISPLAY


APU AVAIL appears in green when APU N is above N > 99.5 % or 2 s after N is above 95 %.







PERFORMANCE LOADING FUEL

APU START/SHUTDOWN DURING REFUELING/DEFUELING

Applicable to: ALL

APU starts or shutdowns are permitted during refuel/defuel procedures. If it is necessary to operate
the APU, the limits that follow apply :

a. An APU start is not permitted during a refuel/defuel procedure if the APU has failed to start or an
automatic shutdown has occured

b. A normal APU shutdown must be completed if a fuel spill has occured during the refuel defuel
procedure.

‐ Electrical transients (caused by switching among the APU, the external and the engine electrical
supply) during automatic refueling may stop the process. If the automatic refueling process is
stopped, it is necessary to re-enter the Preselected Fuel Quantity.