Which statement about the system power sources is true?

• A. The battery is the primary power source during normal operation; the generator provides back-up.
• B. The alternator feeds DC directly to the distribution without rectification.
• C. The system is powered by a battery and an alternator/generator.
• D. The system uses external power only on the ground.

Answer: (C)

In aircraft electrical systems, power comes from two sources that work together: a battery and an alternator (generator). The alternator, driven by the engine, supplies the normal electrical load and charges the battery as you fly. The battery provides starting power and serves as a backup for essential systems if the alternator fails or during transient conditions. The alternator’s output is converted to the proper DC level by rectifiers and controlled by a regulator before it feeds the distribution bus. So the system isn’t powered solely by the battery in normal operation, and power isn’t delivered directly as DC from the alternator without rectification. External power may be used on the ground, but it isn’t the only source in flight.