How often should the oil in the C-5 APU be checked?

The correct choice emphasizes the importance of routine checks of the oil in the C-5 APU. Regular inspection of the oil ensures that the APU operates smoothly and efficiently, which is crucial for the overall performance of the aircraft. Checking the oil during scheduled maintenance allows for the identification of any potential issues before they escalate, helping to maintain safety and reliability.

Additionally, checking the oil before each flight adds a layer of precaution. It ensures that the APU has adequate lubrication, which is essential for its operation, especially during the initial phases of a flight when the APU is crucial for providing power and environmental control.

In contrast, the other choices do not reflect the best practices for maintenance. Checking every 20 flight hours or once a month may not be sufficient for a system that is under constant operational demands. Only checking oil during emergencies lacks proactive maintenance and could lead to serious issues if problems go unnoticed.