Auxiliary Power Unit (APU): Functions, Control, and Protection

021-11-06Pablo Asensio Martínez2026-03-271 min

Purpose and Function

The APU is a small self-contained gas turbine engine, typically located in the tail cone of the aircraft.

  • Primary Outputs:
    1. Electrical Power: Drives a generator (typically 115V AC, 400Hz) to supply aircraft systems on the ground or in flight (backup).
    2. Pneumatic Power (Bleed Air): Supplies high-pressure air for:
      • Main Engine Starting.
      • Air Conditioning (Environmental Control System).
      • Wing Anti-icing (in some aircraft, though rare).
  • Benefits: Makes the aircraft independent of ground support equipment (GPU / Air Start Unit).

Operation

  • Fuel Source: Draws fuel from the aircraft's main tanks (e.g., Left Main Tank).
  • Starting:
    • Usually started electrically via the Aircraft Battery or Ground Power.
    • Envelope: Must be certified for starting and operation at altitude.
  • Combustion: Operates at constant speed (governed) to maintain generator frequency (400Hz).

Controls and Indication

  • Master Switch:
    • Energizes the electronic control box (ECB/FADEC).
    • Opens the Air Intake Flap (to ingest air).
    • Opens the Fuel Isolation Valve.
  • Start Button:
    • Energizes the starter motor.
    • Activates ignition systems.
    • Initiates automatic accelerating sequence.

Automatic Shutdown Protection

The APU is capable of protecting itself by shutting down automatically (especially on the ground) for conditions such as:

  • Overspeed (Critical protection).
  • Low Oil Pressure.
  • High Oil Temperature.
  • EGT Overtemperature (Exhaust Gas Temperature).
  • Fire.
  • reverse flow, Underspeed, etc.

Note: In flight, some protective shutdowns (like high oil temp) may be inhibited to prioritize electrical power availability in an emergency.