What are some points to start counting from when considering an aircraft overdue?

When assessing whether an aircraft is overdue, the most relevant point to start counting from is the estimated time of arrival (ETA) over a compulsory reporting point or clearance limit. This is because compulsory reporting points are specific locations along a route that pilots are required to report to air traffic control. If an aircraft does not reach its ETA at such a point, it raises immediate concerns regarding its status, as it may indicate that the aircraft is in distress or has deviated from its intended flight path.

Using the ETA over a reporting point is crucial as it aligns with air traffic management protocols that prioritize safety and timely responses to potential emergencies. This measure provides a clear and agreed-upon benchmark for when to consider an aircraft overdue, ensuring that appropriate actions can be taken based on established aviation safety practices.

Other points such as the actual departure time, flight plan submission time, or maintenance completion time do not serve as reliable indicators for determining if an aircraft is overdue. Actual departure time may vary due to delays, and flight plan submission may occur well before departure, which does not account for the ongoing operation of the flight once airborne. Maintenance completion time is also irrelevant in this context, as it does not relate to the aircraft's operational status during flight.