Until what condition does ATC assume obstacle clearance for an aircraft?

Air Traffic Control (ATC) assumes obstacle clearance for an aircraft particularly when navigational guidance is provided through radar vectors. This means that ATC actively monitors and directs the aircraft's path, thereby ensuring that it is safely separated from obstacles and other traffic. Radar vectors allow ATC to provide real-time, precise adjustments to an aircraft’s flight path, which is crucial for maintaining safety, especially in complex or congested airspace.

When ATC guides an aircraft with radar vectors, they calculate and manage separation from terrain and other obstructions, allowing for continuous assessment of the aircraft's safety as it maneuvers. This proactive management is essential for effective communication and ensuring that the aircraft remains on a safe trajectory.

In contrast, conditions such as reaching a cruising altitude or departing from controlled airspace do not inherently guarantee obstacle clearance, as they do not involve active monitoring and direction from ATC. Similarly, simply initiating a turn does not ensure that an aircraft is following a safe trajectory, as that action would still require ATC guidance to ensure safety.