Which source provides essential DC power during a primary-source failure, if the other sources are not available?

• A. Battery provides essential DC power.
• B. External power provides essential DC power.
• C. Ram Air Turbine provides essential DC power.
• D. A separate auxiliary cable provides essential DC power.

Answer: (A)

The essential idea is backup power for the aircraft’s DC systems. When the main power sources fail, the aircraft battery is designed to directly supply DC power to the essential DC bus, keeping the most critical instruments, flight controls, and cockpit avionics alive for a minimum period. External power isn’t available in flight during a failure, so it can’t be relied on. The RAM Air Turbine can provide emergency power, but its primary role is to deliver power (often AC) through alternative paths and it isn’t guaranteed to feed the essential DC bus in all configurations. A separate auxiliary cable is simply a means to connect external power on the ground, not a standalone DC source during flight. Therefore, the battery is the correct source for essential DC power during a primary-source failure.