What should be done if a Mode C readout cannot be validated?

When a Mode C readout cannot be validated, the appropriate action is to avoid using the Mode C information for separation. This is crucial because the validity of altitude data is essential for maintaining safe separation between aircraft. If a readout cannot be confirmed, relying on that potentially inaccurate information could lead to unsafe flight situations or possible collisions.

Mode C transponders provide altitude data that controllers depend on for maintaining vertical separation. If this data is unreliable, the decision to disregard it ensures that safety is prioritized. By not using the unvalidated Mode C info, controllers can implement alternative methods of separation, such as visual separation or relying on other information like the aircraft's intended altitude from clearances or the last known altitude.

Using the last known altitude, contacting the pilot, or waiting for another transmission might seem like viable options; however, these methods do not directly address the immediate need for validated altitude data in the context of maintaining safe vertical separation among aircraft.