When unable to approve a pilot's request for deviation from the filed flight plan, what should the controller say?

The proper response when a controller cannot approve a pilot's request for deviation from the filed flight plan is to say "UNABLE" and provide a reason if time allows. This response is correct as it communicates to the pilot that the requested deviation cannot be granted while also maintaining transparency in operations. Providing a rationale aids in the pilot's understanding of the situation, potentially mitigating confusion or frustration.

Using "UNABLE" conveys a polite and professional tone, ensuring that communication remains clear and respectful. When time permits, giving a reason enhances situational awareness for the pilot, which is crucial for safe aviation operations and maintaining effective communication between the air traffic controller and the flight crew. This practice aligns with standard operational procedures that emphasize clarity and safety in aviation communications.