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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. Airspace is standardized by the International Civil Aviation Organization (ICAO) and breaks down into seven classes:
- In the United States, 'Class A' airspace exists only at high altitudes (18,000' above mean seal level or MSL and above). In some countries, 'class A' airspace also exists around very busy airports (for example, London Heathrow). 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 an altitude-encoding (Mode-C) transponder is mandatory. Canada has classified all airspace above 12,500' to 18,000' as Class B as well. This may be an added safety measure as flight above this altitude requires either a pressurized plane or an oxygen breathing system. However most small private planes are not capable of flying above 12,500'. (See also Class B Airports)
- 'Class C' airspace exists around moderately busy airports -- generally the primary airports for major cities though not major hubs (for example, San Jose). An altitude-encoding transponder is mandatory. All aircraft must be in two-way communication with air traffic control. An explicit clearance is not needed to enter the airspace. (See also Class C Airports)
- '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.
- 'Class F' special use airspace exists for areas reserved for non standard flight operations or other restrictions. Class F Advisory allows general use with specified limitations. Class F Restricted only allows aircraft approved by the controlling agency responsible for the airspace. Examples of special use airspace are military practice areas, parachute jumping and national security areas. The designation of Class F is not used for special use airspace in the United States.
- 'Class G' uncontrolled airspace exists close to the ground and is considered to be all airspace that does not fit into the previous categories. Both VFR and IFR flight is permitted and communication with air traffic control is not required for VFR flight. VFR flight in class G can be conducted in lower visibilities then Class E. Radar seperation of traffic is not available in Class G.
This internationally standardized system of controlled airspaces has replaced most countries' own systems.
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