APU System Presentation

The Auxiliary Power Unit (APU), is located in the unpressurized tail cone.

It is a self contained unit which can make the aircraft independent of external pneumatic and electrical power sources.

The APU may be started using electrical power supplied by:

• Only the aircraft batteries, or
• Only the external power source, or
• The engine generator source.

The APU provides:

• AC electrical power for the aircraft electrical system
• Bleed air for engine start and/or air conditioning.

The APU has an electrically operated air intake flap which, when it is opened, brings external air to its compressor inlet.

Now, we will see how information is presented to the pilots.

In the cockpit you can monitor the following APU parameterson the ECAM APU page:

• EGT and compressor speed
• APU generator
• APU bleed
• AIR INTAKE Flap position.

Let’s see the different controls that the pilots use to operate the APU. These controls are installed in various places on the overhead panel. The APU MASTER SW and the START pb are installed on the APU control panel.

The APU GEN pb-sw is located on the ELEC panel, and is used to control the APU generator.

The APU BLEED pb-sw is located on the AIR COND panel, and controls the APU bleed system.

During the APU start sequence, at approximately 95% N speed, the APU AVAIL memo is displayed on the EWD. This is to indicate that the APU parameters are normal and the APU is available to supply electrical power and bleed air.

Note: The AVAIL indication also appears in green on the ECAM APU page, on the START pb the ON light goes off, and the green AVAlL light comes on.

As soon as the APU BLEED pb-sw is set to ON, the green APU BLEED memo replaces the APUAVAlL memo on the EWD. This is to indicate that the APU bleed valve is open with bleed air pressure available.

APU fire controls are installed on the FIRE panel. For additional information, refer to the ATA 26 - FIRE PROTECTION chapter.

There is also:

• An APU SHUT OFF pb and an associated APU FIRE warning light located on the external power panel (refer to the ATA 26 - FIRE PROTECTIONchapter).

This panel is located in front of the nose gear bay.

The APU parameters are monitored by an Electronic Control Box (ECB) which in turn controls the start, the operation and the shutdown of the APU.

The ECB initiates a normal shutdown when the MASTER SW is set to OFF.

Note: According to the version, an APU cool down period may be initiated after the closure of the APU BLEED valve.

The ECB initiates an auto shutdown when a monitored parameteris out of limits (for instance: a slow acceleration during start or a fire has been detected on ground only).

Note :

• The APU will stop immediately.
• The AIR INTAKE FLAP will close 15 minutes:
  • After the APU speed is below 7%, or
  • After lift off (if the auto shutdown occurred before takeoff).

The ECB initiates an EMER shutdown when an APU FIRE PUSHbutton is pushed on APU FIRE panel or an APU SHUT OFF pb is used on EXTERNAL POWER panel.

Note: the APU will stop immediately.

Notice that in both cases of shutdown the ECAM action is to set the MASTER SW to OFF in order to switch off the FAULT light and to de-energize the ECB.

Module completed

Video study

• Watch the video available on YouTube