On the ground, which system provides electrical power and pneumatic air for starting engines when ground power is unavailable?

• A. Auxiliary Propulsion Unit; provides in-flight thrust as a backup.
• B. Auxiliary Power Unit; supplies electrical power and pneumatic air for engine starts when ground power is unavailable.
• C. Aircraft Power Module; powers cabin lighting during ground operations.
• D. Auxiliary Electrical System; stores electrical energy for emergencies.

Answer: (B)

On the ground, aircraft need a reliable source of power and a way to start the engines when external power isn’t available. The Auxiliary Power Unit is a small turbine built into the aircraft that can be started to supply electrical power to the aircraft systems and to provide bleed air (pneumatic air) for engine start via the air-start system. This lets you start the engines without relying on ground power units or air-bridges.

That’s why this option is the best. It matches the APU’s real role: providing both electrical power and pneumatic air for engine starts when ground power isn’t available. Other descriptions either refer to components or purposes that don’t align with standard practice (like an APU that provides propulsion in flight, which isn’t how the APU is used), or describe functions that aren’t how aircraft power and starting systems operate at ground.