Auxiliary Power Unit (APU): Functions, Control, and Protection
Purpose and Function
The APU is a small self-contained gas turbine engine, typically located in the tail cone of the aircraft.
- Primary Outputs:
- Electrical Power: Drives a generator (typically 115V AC, 400Hz) to supply aircraft systems on the ground or in flight (backup).
- 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.