What does it indicate if Mode C has reported the aircraft at an assigned altitude but deviation is 300 feet or more above assigned altitude?

When Mode C reports that an aircraft is at an assigned altitude but shows a deviation of 300 feet or more above that assigned altitude, this situation signifies a potential loss of separation. This deviation can create risks in maintaining safe distances between aircraft, particularly in controlled airspace, where precise altitude adherence is crucial for avoiding collisions.

In air traffic control, maintaining separation between aircraft is vital for safety. If an aircraft is not at the correct altitude, it can unintentionally move into the altitude of another aircraft, leading to a situation where the required minimum vertical separation is violated. Thus, air traffic controllers must be alert to such deviations and take necessary actions, which may include issuing commands to rectify the aircraft’s altitude or rerouting traffic to ensure safety.