Class A Airspace

Class A airspace is controlled airspace. It extends from 18,000 feet MSL through FL 600 over the continental United States and Alaska, including coastal waters out to 12 nm miles from the coast.

The altitudes above 18,000 feet are usually cruising altitudes for high performance airplanes. These airplanes typically cruise at high speeds, making it difficult to see and avoid other air traffic. Because of this, an instrument rating and air traffic control clearance is required to operate inside Class A airspace. This allows air traffic control to maintain separation between all aircraft inside the Class A.

ยง 91.135 Operations in Class A airspace.