Which statement correctly describes the behavior of the essential buses under total generator loss in flight?

Which statement correctly describes the behavior of the essential buses under total generator loss in flight?

• A. Essential buses are de-energized
• B. Essential buses are powered only by the APU
• C. Essential buses are powered by external power
• D. Batteries to supply the Essential Buses (in-flight only)

Answer: (D)

When all generating sources fail in flight, the aircraft’s emergency power arrangement switches the essential buses to battery power. This keeps the critical flight deck systems—instruments, flight controls, and essential avionics—operational for a limited time, allowing the crew to maintain control and complete a safe landing. External power is not available in flight, and relying on an APU or other power source isn’t the guaranteed mode during a total generator loss, so the designed, automatic source for essential buses in this scenario is the batteries.

When all generating sources fail in flight, the aircraft’s emergency power arrangement switches the essential buses to battery power. This keeps the critical flight deck systems—instruments, flight controls, and essential avionics—operational for a limited time, allowing the crew to maintain control and complete a safe landing. External power is not available in flight, and relying on an APU or other power source isn’t the guaranteed mode during a total generator loss, so the designed, automatic source for essential buses in this scenario is the batteries.