If battery voltage is less than 21.0 V, what procedure should be followed?

When the battery voltage is less than 21.0 V, it indicates that the aircraft's electrical system is not functioning properly, which can lead to significant operational issues during flight. The safest procedure in this situation is to report the irregularity and refrain from departing until the issue is addressed.

This protocol ensures that any potential electrical failures are resolved before attempting to operate the aircraft, thereby maintaining the safety of the flight and its passengers. Low battery voltage can lead to insufficient power for critical systems, which are necessary for safe operation. It is essential to ensure that the aircraft is in proper working condition before takeoff. Hence, taking precautionary measures by reporting the irregularity and not departing is crucial for flight safety.