During a generator failure, which part of the aircraft's power system is responsible for maintaining power to critical equipment?

• A. Main battery only
• B. External power source
• C. Ground power unit
• D. Essential bus and standby power sources

Answer: (D)

During a generator failure, power to critical equipment is kept by the essential bus and the standby power sources. The essential bus is the distribution path that remains energized by backup sources when the main generators fail, so vital flight instruments, controls, and communications keep operating. Standby power sources—such as a standby battery and related power devices—automatically energize to support the essential bus when normal generation is lost. The main battery alone cannot sustain all critical loads for an extended period, and external power on the ground isn’t available in flight. So the combination of the essential bus and standby power sources is what maintains power to the critical equipment.