What does an APU provide when the main engines are not operating?

What does an APU provide when the main engines are not operating?

• A. Electric power and compressed air.
• B. Hydraulic power.
• C. Only electrical power.
• D. Only cabin pressurization.

Answer: (A)

When the main engines aren’t running, the APU’s job is to keep essential systems powered. Its primary outputs are electrical power and compressed air (bleed air). The electrical supply keeps aircraft systems, avionics, lighting, and starting equipment operational, while the bleed air allows the APU to start the other engines and to run environmental control systems such as air conditioning and other pneumatic components. Hydraulic power is typically provided by engine-driven pumps, and while some installations may include an APU-driven hydraulic pump, that isn’t the standard function you rely on from the APU. So the APU’s main function in this situation is to provide electric power and compressed air.

When the main engines aren’t running, the APU’s job is to keep essential systems powered. Its primary outputs are electrical power and compressed air (bleed air). The electrical supply keeps aircraft systems, avionics, lighting, and starting equipment operational, while the bleed air allows the APU to start the other engines and to run environmental control systems such as air conditioning and other pneumatic components. Hydraulic power is typically provided by engine-driven pumps, and while some installations may include an APU-driven hydraulic pump, that isn’t the standard function you rely on from the APU. So the APU’s main function in this situation is to provide electric power and compressed air.