What is the primary purpose of an aircraft APU?

What is the primary purpose of an aircraft APU?

• A. To replace main engines for propulsion.
• B. To power the aircraft only during flight.
• C. To provide reliable secondary power for starting engines and to supply electrical and pneumatic power for aircraft systems on the ground or when main engines are unavailable.
• D. To provide primary hydraulic power.

Answer: (C)

An auxiliary power unit (APU) serves as a self-contained, on-board power source. Its main job is to provide electrical and pneumatic power when the main engines aren’t running or when the aircraft is on the ground. This allows starting the main engines using bleed air supplied by the APU’s air system (air-start) and keeps essential systems—avionics, lighting, environmental controls, and other aircraft systems—operational without needing ground power or running the main engines. That’s why this option is the best: it describes delivering reliable secondary power for engine starts and for electrical and pneumatic needs when on the ground or when the main engines aren’t available. It’s not used to propel the aircraft, and while it can contribute bleed air in flight or for backup purposes, its primary role is not primary hydraulic power.

An auxiliary power unit (APU) serves as a self-contained, on-board power source. Its main job is to provide electrical and pneumatic power when the main engines aren’t running or when the aircraft is on the ground. This allows starting the main engines using bleed air supplied by the APU’s air system (air-start) and keeps essential systems—avionics, lighting, environmental controls, and other aircraft systems—operational without needing ground power or running the main engines.

That’s why this option is the best: it describes delivering reliable secondary power for engine starts and for electrical and pneumatic needs when on the ground or when the main engines aren’t available. It’s not used to propel the aircraft, and while it can contribute bleed air in flight or for backup purposes, its primary role is not primary hydraulic power.