Identify two main functions of an auxiliary power unit (APU) on a narrowbody.

• A. Provide electrical power and bleed air on the ground or when engines are not running; assist starting engines.
• B. Provide electrical power only.
• C. Assist starting engines only.
• D. Provide electrical power and bleed air only on the ground.

Answer: (D)

The main idea is that an APU provides power and pneumatic air to the aircraft when the main engines aren’t running, helping with ground operations and engine starts. The two primary functions are delivering electrical power to run systems and supplying bleed air for starting engines and for cabin conditioning and air systems on the ground. The ability to assist starting engines is a key additional capability that supports fast, independent engine starts when the aircraft is not connected to ground power or external air. The description that matches this most closely notes providing electrical power and bleed air on the ground or when engines aren’t running, with engine-start assistance included as part of the APU’s role. Saying these functions occur only on the ground isn’t accurate, since on some aircraft the APU can supply electrical power in flight or provide bleed air in specific situations as well.