Formal:
* Airworthy: This is the most precise and technical term, often used in official aviation contexts.
* Aeronautically sound: This emphasizes the aircraft's suitability for flight from a technical standpoint.
* Flightworthy: This is a more general synonym for "fit to fly."
Informal:
* Good to go: This is a casual way of saying someone or something is ready for flight.
* Ready for takeoff: This implies that all necessary preparations have been made for flight.
* Cleared for flight: This refers to the official authorization for an aircraft to take off.
Medical:
* Medically cleared: This is used when referring to a person's fitness for flight.
* Cleared by aviation medicine: This is a specific term used in aviation medicine.
The best synonym for "fit to fly" will depend on the specific context and the desired level of formality.