What is the primary function of an Auxiliary Power Unit when the engines are shut down?

What is the primary function of an Auxiliary Power Unit when the engines are shut down?

• A. To start the aircraft engines.
• B. To supply direct current power to the aircraft when engines are shut down.
• C. To provide air conditioning only on the ground.
• D. To power emergency braking systems.

Answer: (B)

The main idea here is that the Auxiliary Power Unit provides electrical power to the aircraft when the engines are not running. By supplying direct current power, the APU keeps essential systems—such as avionics, lighting, and cabin electricals—operational without needing the main engines or a ground power unit. This enables the aircraft to stay powered on the ground and to start the engines later if needed. While the APU can support air conditioning on the ground and other functions, those are secondary to its role as a portable power source in this situation.

The main idea here is that the Auxiliary Power Unit provides electrical power to the aircraft when the engines are not running. By supplying direct current power, the APU keeps essential systems—such as avionics, lighting, and cabin electricals—operational without needing the main engines or a ground power unit. This enables the aircraft to stay powered on the ground and to start the engines later if needed. While the APU can support air conditioning on the ground and other functions, those are secondary to its role as a portable power source in this situation.