Class A Airspace Altitude Class A begins at Flight Level 180 (18,000 feet) and extends vertically up to and including Flight Level 600 (60,000 feet) in the continental United States and Alaska. This includes airspace overlying the waters within 12 nautical miles of the mainland and Alaskan coast.

In the United States and several other jurisdictions, the upper limit for Class A airspace is Flight Level 600 (FL600), which corresponds to an altitude of approximately 60,000 feet above mean sea level. Class A airspace officially begins at 18,000 feet MSL (FL180) and extends up to this 60,000-foot ceiling. Within this vast vertical corridor, all operations are conducted under Instrument Flight Rules (IFR), and pilots must be in constant communication with Air Traffic Control (ATC). This airspace is where the majority of commercial jetliners and high-performance military aircraft operate. Above FL600, the airspace transitions back to Class E (Uncontrolled). This high-altitude region is sparsely populated, occupied only by specialized research aircraft, high-altitude balloons, and advanced supersonic or space-bound vehicles. The FL600 limit exists because standard altimeter settings and air traffic separation rules become less practical at the extreme edges of the atmosphere. For most pilots, FL600 represents the "edge of the world," beyond which traditional navigable airspace ends and the fringes of "near space" begin, requiring entirely different sets of aeronautical and physical considerations.