Definition
A pilot or controller transmission indicating an inability to comply with a specific instruction, request, or clearance. When used, it signals that the issuing party must offer an alternative or that the receiving party will not execute what was asked.
Plain English
A short radio word meaning 'I can't do that.' It tells the other person to stop expecting the action and offer something else.
Context Anchor
Used on the radio between pilots and air traffic control when a pilot or controller cannot do what was requested or assigned.
Why Pilots Care
Using the exact word prevents confusion and keeps the conversation clear when safety depends on precise communication.
Intuition Check
“Unable” does not mean “I do not feel like doing it” or “I am being difficult.” In aviation radio use, it means the action cannot be complied with safely, legally, or practically.
Example Sentence 1
Center, Cessna Three-Four-Alpha, unable Flight Level Two-Three-Zero due to icing; requesting lower.