What is the priority of power sources for each AC bus?

• A. Onside Generator; APU Generator; Opposite Generator; External Power
• B. External Power; Onside Generator; APU Generator; Opposite Generator
• C. APU Generator; Onside Generator; External Power; Opposite Generator
• D. Onside Generator; External Power; APU Generator; Opposite Generator

Answer: (A)

Power to an aircraft’s AC buses is arranged in a defined hierarchy so the most reliable, closest source feeds the bus first, with backups ready if that source fails. The highest priority is the generator on the same side as the bus because it provides the most direct, stable feed with the least potential for cross-feed issues or voltage instability. If that onside generator isn’t delivering, the next source in line is the APU generator, which can supply essential power when the main engines aren’t providing it. If the APU generator isn’t available either, the opposite-side engine generator serves as the next backup. External power is kept as the last resort, typically used only when all internal sources are unavailable or during certain ground operations, ensuring internal generation is preferred whenever possible.