If you cannot comply with ATC instructions, what do you say?

The phrase "UNABLE" is the standard and internationally recognized response to indicate that an aircraft cannot comply with Air Traffic Control (ATC) instructions. This term is clear and concise, allowing for effective communication in potentially critical situations.

Using "UNABLE" conveys that the pilot or aircraft is currently incapable of following the specific instruction due to safety, operational, or technical reasons. This terminology is part of standard phraseology endorsed by the International Civil Aviation Organization (ICAO), ensuring clarity and reducing misunderstandings in verbal communications between pilots and ATC.

The other options, while they may seem intuitive, do not conform to the established aviation phraseology and may lead to ambiguity. For instance, "NEGATIVE" might suggest that an instruction is being rejected rather than indicating an inability to comply. "CANNOT COMPLY" and "NOT POSSIBLE" are less precise and may not be immediately understood in the high-pressure context of air traffic communications. Thus, using "UNABLE" is the correct and recommended choice when faced with the need to communicate the inability to follow ATC instructions.