In flight, the APU generator primarily provides which capability?

• A. Pneumatic hydraulic power to actuate flight controls.
• B. Purely mechanical backup for engine start.
• C. Cabin entertainment power.
• D. Electrical power as a backup source (and can provide pneumatic bleed air for engine starting when required).

Answer: (D)

The APU generator’s job is to provide electrical power as a backup source for the aircraft’s electrical system, and it can also supply pneumatic bleed air for engine starting when needed. In flight, the primary electrical power comes from the engine-driven generators, but the APU generator kicks in if an engine generator fails or if additional electrical load is required, keeping critical systems and avionics powered. The APU also provides bleed air that can be used to start engines with the pneumatic starting system, or to supply bleed air to pneumatic systems when needed. This combination makes electrical power backup the main capability, with bleed air as a secondary function for starting engines; it’s not primarily about hydraulic power for flight controls or a purely mechanical engine-start backup, and while cabin power (including entertainment) can be supported, that is not the defining role.