On the ground, when external power is not connected, which source is commonly used as a backup to supply electrical power?

• A. External power
• B. Standby battery
• C. APU generator
• D. Main engine generators

Answer: (C)

When external power isn’t available on the ground, the aircraft uses the APU generator as the backup electrical source. The APU (auxiliary power unit) is a self-contained turbine that can be started on the ground and drives an electrical generator to supply the aircraft’s systems, enabling avionics, lighting, environmental control, and more without needing the main engines. The standby battery is designed to provide power for only essential systems for a limited time during an electrical fault, not to run the whole electrical load. The main engine generators only produce power once the engines are running, so they aren’t usable when the aircraft is still on the ground with engines off.