When can a previously validated Mode C not be used for separation?

A previously validated Mode C may not be used for separation when an exceptional rate indicator X is displayed. This indicator signifies that there is an issue with the aircraft's altitude reporting, which can compromise the integrity of altitude data being provided. In air traffic control, accurate altitude information is crucial for maintaining safe vertical separation between aircraft. If the Mode C transponder is not functioning correctly, as indicated by the exceptional rate indicator, air traffic controllers cannot rely on that data, potentially leading to unsafe situations if separation is based on inaccurate altitude information.

In contrast, other options do not inherently invalidate the use of Mode C for separation. For instance, reaching cruising altitude does not affect the validity of Mode C, as the system continues to report the aircraft's altitude regardless of its vertical status. Similarly, drifting outside of a flight plan does not necessarily indicate a malfunction in altitude reporting. Lastly, while a communication delay can complicate situational awareness, it does not directly invalidate the use of validated Mode C data for separation unless accompanied by other issues affecting altitude reporting.