The United States organizes its airspace into classes labeled A through G. The classes differ in how much air traffic control (ATC) service is provided and what is required of a pilot to operate there. Classes A, B, C, D, and E are controlled airspace; Class G is uncontrolled. (There is no Class F in the United States.)
The controlled classes
- Class A — From 18,000 feet MSL up to and including flight level 600. All operations must be conducted under instrument flight rules (IFR). There is no VFR flight in Class A.
- Class B — Surrounds the busiest airports, shaped like an upside-down wedding cake. An ATC clearance is required to enter, and the airspace carries specific equipment and pilot-certificate requirements.
- Class C — Surrounds moderately busy airports with an operating control tower and radar approach control. Two-way radio communication must be established before entry, and the aircraft must have an operating transponder.
- Class D — Surrounds airports with an operating control tower that are not busy enough for Class C or B. Two-way radio communication must be established before entry.
- Class E — All other controlled airspace that is not A, B, C, or D. It often begins at the surface, 700 feet, or 1,200 feet above ground level and supports instrument approaches and en route IFR traffic.
Uncontrolled airspace
Class G is uncontrolled airspace in which ATC has no authority or responsibility to control air traffic. It typically exists close to the ground in areas without other airspace overlying it. Pilots are still responsible for seeing and avoiding other traffic and for complying with the applicable visibility and cloud-clearance minimums.
What changes between classes
As you move from G toward A, two things generally increase: the level of ATC service and the requirements placed on the pilot and aircraft (clearances, radio contact, transponders, and weather minimums). Knowing which class you are in tells you exactly what is expected before and during entry.