How should a controller determine the altitude of an aircraft operating below the lowest usable flight level?

Determining the altitude of an aircraft operating below the lowest usable flight level can be effectively accomplished by using the phrase "SAY ALTITUDE." This method is part of the standard phraseology used in aviation communication to request a pilot to report their altitude. When a controller issues this request, it prompts the pilot to provide their current altitude, which is essential for ensuring safe separation and maintaining situational awareness around the aircraft.

This phrase is not just a suggestion but a recognized protocol that facilitates clear and concise communication between the controller and the pilot. It helps avoid misunderstandings and ensures that altitude information is conveyed accurately, which is critical for flight safety, especially in congested airspace or when many other aircraft are operating nearby at varying altitudes.

While requesting a flight level could be relevant for aircraft operating at higher altitudes, it is not appropriate for those below the lowest usable flight level, as they cannot be cleared to those levels safely. Involve checking with the pilot can provide altitude information, but it is less efficient than using standardized phraseology to directly ask for it. Determining current height is not a standard practice for controllers, as height can vary widely based on terrain and configuration, making it unreliable without a direct altitude report from the aircraft.