What is the order of priority for power sources?

• A. APU Generator, External Power, Main Generators
• B. External Power, Main Generators, APU Generator
• C. Main Generators, External Power, APU Generator
• D. #1 and #2 Main Generators, APU Generator, External Power

Answer: (D)

Power sources are prioritized to keep the aircraft self-contained while ensuring reliability. The engine-driven main generators are the primary source because they are integrated with the electrical system and power the buses as the normal, independent source. The APU generator is the next source; it provides electrical power when the engines aren’t supplying power or when an additional source is needed, without needing to run the main engines. External power is used as a last resort on the ground when available, since it depends on an external unit and isn’t always readily available or desirable to rely on during flight or startup.

So the preferred sequence is: main generators first, then the APU generator, and finally external power.