What is the preferred phraseology to request an aircraft to change altitude?

The preferred phraseology for altitude change requests is designed to ensure clear and effective communication in aviation operations. The correct choice reflects a format that prioritizes the clear intent of the request: the air traffic controller is asking the pilot to achieve a specific altitude.

The use of "Request" in the phrase signals that the aircraft is being asked to change its altitude but does not imply a directive. By incorporating "one five thousand," it concisely presents the altitude in a format that pilots are accustomed to. This is vital for maintaining clarity and precision in communication, which is crucial for safety in an environment where multiple aircraft may be listening and responding.

The other options either employ phrases that may indicate different intentions or lack the clarity and standardization expected in aviation communication. Using the word "descend" implies a specific action that may not be appropriate in all scenarios, while "switch to" doesn't accurately convey the request for altitude change and could lead to misunderstandings. Additionally, the phrasing involving "altitude fifteen thousand" is less standard and may not be as instantly recognizable to pilots as the preferred format.