The Auxiliary Power Unit (APU) is a small gas turbine engine, typically located in the tail section of an aircraft, designed to supply electrical and pneumatic power to various aircraft systems when the main engines are not running.

Functions and Operation:

1. Electrical Power Supply:
   The APU drives an AC generator that provides three-phase electrical power for essential aircraft systems such as avionics, flight controls, lighting, and environmental control systems (ECS).
2. Pneumatic Power (Bleed Air):
   The compressor section of the APU delivers compressed air through the pneumatic system, which is used for:
   • Starting the main engines
   • Running the air conditioning and pressurization systems
3. Emergency Backup:
   In case of failure of the main electrical or pneumatic systems during flight, the APU can serve as an independent emergency power source, ensuring continued operation of critical systems.

Main Components:

• Gas Turbine Core: Includes compressor, combustion chamber, and turbine
• Electrical Generator: Produces AC power for onboard systems
• Gearbox and Accessories: Transmits mechanical power to the generator and pumps
• Electronic Control Unit (EECU or ECU): Automatically controls start-up, fuel flow, and overall APU operation
• Fuel and Lubrication Systems: Supply and maintain the APU’s power and cooling needs

Importance and Maintenance:

Because the APU plays a vital role in pre-flight preparation and main engine start-up, any malfunction—especially in its pneumatic or electrical subsystems—can lead to flight delays or cancellations.
Therefore, APUs are routinely monitored using Health Monitoring Systems (HMS) and Predictive Maintenance programs to ensure reliability and safety.