What is the maximum permissible time for ground operation with DC power only?

• A. 3 minutes
• B. 5 minutes
• C. 7 minutes
• D. 10 minutes

Answer: (B)

The idea being tested is how long you can operate the airplane on DC power alone while on the ground. When you’re using DC power only, you’re relying on a limited, non-generator source for the aircraft’s electrical needs. That source is designed to handle essential systems and a portion of the loads for a short period, but it can be drained quickly if too much is drawn. To protect the power source and ensure you can still recover power if something goes wrong, there’s a safety cap on how long you can run on DC power alone. Five minutes is the chosen limit because it gives enough time to perform essential preflight checks and configurations without risking deep discharge of the power source. If you need more time, you would bring in another power source (like external AC power) or start the engines/APU to provide sustained electrical power. The other time options would either be insufficient for typical preflight tasks or would push the DC source toward unsafe depletion.