What is the function of the APU and what are typical limits on its use during ground operations and flight?

• A. Provides electrical power and bleed air when engines are off or on the ground; limitations include fuel consumption and certain operations restricted during takeoff/landing or in specific configurations
• B. Controls fuel flow to engines and starts engines
• C. Replaces the primary electrical system permanently
• D. Used only on the ground to taxi

Answer: (A)

The APU’s job is to be a compact power source for the aircraft, supplying electrical power and bleed air when the main engines aren’t running or aren’t providing those services. On the ground, it keeps the aircraft’s systems alive and can supply bleed air for air conditioning and for starting the engines. In flight, it’s used only when needed, such as when extra electrical power or bleed air is required or when the main generators or engine bleeds aren’t available.

Because the APU runs on fuel, there are practical limits on how long you use it: it adds fuel burn and wear, so operators restrict its use to what’s necessary. There are also restrictions during certain phases of flight or configurations—often you avoid or limit APU bleed air during takeoff and landing, or you disable it when engine bleeds or environmental control system demands require a specific setup.