Class A Airspace — Controlled airspace — IFR only, ATC clearance required, full separation provided.

Class A Airspace is the most restrictive class of controlled airspace. Only IFR flights are permitted, and all aircraft must have an ATC clearance. ATC provides separation between all aircraft.

In most countries, Class A extends from FL180 (18,000 ft) to FL600 (60,000 ft). All aircraft must be equipped with a transponder and two-way radio. VFR flight is not permitted in Class A airspace.