Which of the following is a requirement when the tank is full according to the LAVS INOP indicator?

When the tank is full according to the LAVS INOP indicator, the requirement to discontinue lavatory use is crucial for maintaining operational safety and hygiene on the aircraft. The LAVS INOP indicator signals that the lavatory waste tank has reached its capacity, which means that waste cannot be properly processed or stored. Allowing continued use of the lavatory under these conditions could lead to overflows or malfunctions, posing a health risk to passengers and crew, and potentially result in environmental hazards.

Discontinuing lavatory use helps to ensure that the cabin remains safe and comfortable, preventing any unpleasant situations that could arise from a malfunctioning lavatory system. In addition, it allows for the necessary actions to be taken, such as notifying maintenance personnel to address the issue and empty the tank when possible.

The other options, while potentially relevant, do not directly address the immediate requirement that becomes critical when the tank is full. For instance, emptying the tank is indeed a necessary maintenance action, but that occurs after lavatory use has been discontinued. Checking water levels and notifying personnel can be part of a broader maintenance routine, but the most immediate action required when the indicator shows the tank is full is to stop using the lavatory to avoid complications.