A concise proword used in aviation radio communications to reject an air traffic control clearance or request. Pilots state this when an assigned altitude, heading, or speed is unsafe, outside the aircraft's performance envelope, or hampered by weather. It immediately signals the controller to formulate an alternative plan without requiring a lengthy explanation.
Heard on the job
Center, Delta niner two is unable to cross waypoint Alpha at flight level three zero zero due to weather.
Insider tip
Never say 'I can't do that' on the radio. A simple 'unable' tells the controller everything they need to know to issue a new clearance.