Controlled airspace exists in areas where air traffic control is capable of providing traffic separation. These would be areas where radar coverage is available, or at high altitudes where VFR flight is prohibited. This does not mean that air traffic control actually provides services to all flights in the airspace, only that such service is possible.

Most airspace that is more than 1,200 feet above ground level (AGL) is controlled airspace. Exceptions include mountainous terrain where radar coverage and safe IFR flight are only possible at higher altitudes. Controlled airspace is standardized by the International Civil Aviation Organisation (ICAO) and breaks down into five classes:

  • In the United States, 'Class A' airspace exists only at high altitudes (18,000 msl and above). In some countries, 'class A' airspace also exists around very busy airports. Only IFR flight is permitted in 'class A' airspace. All other classes of controlled airspace permit both IFR and VFR flight. There is no speed limit in class A airspace (except the sound barrier over land).

  • 'Class B' airspace exists around the very busiest airports in the world -- generally major air carrier hubs (for example, Los Angeles International). All aircraft must have an explicit clearance to enter the airspace and radar coverage is mandatory.

  • 'Class C' airspace exists around moderately busy airports -- generally the primary airports for major cities though not major hubs (for example, San Jose). Radar coverage is mandatory. All aircraft must be in two-way communication with air traffic control. An explicit clearance is not needed to enter the airspace.

  • 'Class D' airspace exists around airports with an operational control tower and that are not large enough to warrant a 'class B' or 'class C' airspace designation. Radar coverage may exist but is not mandatory (pilot position reports and tower binoculars are usually sufficient). The tower is responsible for sequencing takeoffs and landings.

  • 'Class E' airspace exists almost everywhere else except close to the ground where Uncontrolled Airspace exists. Both VFR and IFR flight is permitted and communication with air traffic control is not required for VFR flight.

This internationally standardized system of controlled airspaces has replaced most countries' own systems.