When is a radar service typically discontinued for aircraft?

Radar service is typically discontinued for aircraft when they proceed into a non-radar coverage area. This is because radar tracking relies on a continuous line of sight between the radar installation and the aircraft. Once an aircraft enters an area where radar signals are not available or cannot effectively track the aircraft, radar services can no longer be provided. As a result, controllers must terminate radar service since they can no longer monitor the aircraft's position accurately.

In contrast, radar service is often maintained while the aircraft is at cruising altitude and within controlled airspace, as these conditions generally ensure consistent radar coverage. Completing a flight plan does not necessarily correlate with the termination of radar services; a flight plan can remain open even if radar services are underway. Therefore, focusing on the conditions that affect radar coverage directly helps clarify why the chosen answer is the most appropriate.