Which phraseology is appropriate for determining the altitude of an aircraft operating below the lowest usable flight level?

The phrase "Say altitude" is appropriate for determining the altitude of an aircraft operating below the lowest usable flight level because it employs standardized aviation communication that prompts the pilot to provide their current altitude directly. This phraseology is concise and clear, making it easy for pilots to understand the action required. It also reflects the urgency and importance of obtaining this information in a safe and efficient manner.

In air traffic control communication, clarity and brevity are crucial, especially when addressing critical safety information such as altitude. The choice of "Say altitude" aligns with established procedures that ensure pilots respond promptly with their altitude data. This prompt allows air traffic controllers to manage traffic more effectively and maintain safe separation.

The other phrases such as "State your altitude," "Report altitude," and "Indicate current altitude" may convey a similar meaning, but they could introduce unnecessary complexity or verbosity into the communication, which is not ideal in high-stakes situations where clarity is paramount. Therefore, "Say altitude" is the most appropriate and effective choice for this scenario.