If one power source is degraded, which bus remains powered to critical loads?

If one power source is degraded, which bus remains powered to critical loads?

• A. External power only
• B. Essential AC bus
• C. Main AC bus
• D. No power

Answer: (B)

Redundancy and automatic bus transfer are built to keep the important systems alive even if a power source drops off. In this aircraft, critical loads are tied to the essential AC bus, which is designed to stay energized by any healthy source—generator, APU, or external power—through automatic cross-feed and transfer logic. When one power source degrades, the system automatically routes power from the remaining source to the essential AC bus, preserving authority for instruments, flight control, and other critical equipment. The main AC bus powers nonessential systems and can lose power without compromising the critical loads, which is why the essential AC bus remains powered to those critical loads.

Redundancy and automatic bus transfer are built to keep the important systems alive even if a power source drops off. In this aircraft, critical loads are tied to the essential AC bus, which is designed to stay energized by any healthy source—generator, APU, or external power—through automatic cross-feed and transfer logic. When one power source degrades, the system automatically routes power from the remaining source to the essential AC bus, preserving authority for instruments, flight control, and other critical equipment. The main AC bus powers nonessential systems and can lose power without compromising the critical loads, which is why the essential AC bus remains powered to those critical loads.