Controlled airspace

Controlled airspace exists in areas where air traffic control is capable of providing traffic separation. These would often 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.

In the United States, most airspace that is more than 1,200 feet above ground level (AGL) is controlled airspace. Exceptions include remote and mountainous areas where radar coverage and radio communications may not be available except 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' 'Class Alpha' airspace exists only at high altitudes (18,000' above mean sea 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' 'Class Bravo' airspace exists around the very busiest airports in the world -- generally major air carrier hubs (for example, Los Angeles International). All aircraft must have a clearance to enter the airspace and an altitude-encoding (Mode-C) transponder that automatically reports the aircraft altitude to air traffic control is mandatory. Canada has classified all airspace from 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. (See also Class B Airports)

• 'Class C' 'Class Charlie' airspace exists around moderately busy airports -- generally the primary airports for major cities though not major hubs (for example, San Jose, California). An altitude-encoding transponder is mandatory. All aircraft must be in two-way communication with air traffic control. A clearance is not needed to enter the airspace. (See also Class C Airports)

• 'Class D' 'Class Delta' airspace exists around airports with an operational control tower and that are not busy 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' 'Class Echo' airspace exists almost everywhere else (in the United States) except close to the ground. Both VFR and IFR flight is permitted and communication with air traffic control is not required for VFR flight.

• 'Class F' 'Class Foxtrott' 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 in the United States.

• 'Class G' 'Class Golf' uncontrolled airspace is all airspace that has not been designated in one of the previous categories. Both VFR and IFR flight is permitted and communication with air traffic control is not required. VFR flight in class G can be conducted in lower visibilities then Class E. Radar separation of traffic is not available in Class G.

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