What’s Auxiliary Power Unit

[Ref Youtube, Hillary, smartcockpit.com]

Auxiliary Power Unit (APU) is a small gas turbine engine located at the rear part of most of the modern civil aircraft

Functions of APU

  1. Electrical power (230V AC, 150kVA)
  2. Bleedair (for air conditioning and engine start)

Notes: In A359’s APU, electrical generation has priority over bleed air generation!

During different phases of flight, APU are serving for different purposes:

  • On ground- 3 functions – 1) Engine Start, 2) Air Conditioning, 3) Electrical Power
  • During T/O – 2 functions -1) Bleed Air for Air Con. 2) Electrical Power
  • In flight – 3 functions – 1) Engine Start, 2) Back up for Air Conditioning, 3) Back up for Electrical

Also, we should noticed that APU has certain limitations on its operations, using the A359 as an example:

APU for engine start in flight is only allowed at or below 25,000 ft
APU for acting as backup air conditioning in flight is only allowed at or below 22,500 ft


APU Systems 

  1. Electronic Control Box(ECB)
  2. starter/generator system

APU has an Electronic Control Box (ECB) which monitors and controls all APU functions – start sequence, the turbine speed and exhaust temperature, oil and fuel supply etc. In case any failure, the ECB will shut down the APU automatically to prevent further damage.

Starter/Generator system is literally two different modes of APU.


Fire protection system of APU

Just like the bigger main engines – Fire can happen in APU, so APU might has its own fire protection system, which detect and automatically activates internal fire extinguisher.


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