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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 designations are standardized by the International Civil Aviation Organization (ICAO) and break down into seven classes:

  • In the United States, 'Class A' airspace exists only at high altitudes (18,000 feet above mean sea level and above). In some countries, 'class A' airspace also exists around very busy airports (for example, London Heathrow). Only flight under Instrument Flight Rules (IFR) 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 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' 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' 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' 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' is Advisory Airspace. This exists when some form of positive separation is offered between certain flights, but where the traffic environment is not fully known to Air Traffic Control. It is therefore Uncontrolled Airspace . Many countries use Class F airspace on low-traffic routes, around low-traffic airports and for military and other special-use airspace. Class F airspace is not used in the United States.


  • 'Class G' 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. In the Unitd States, radar separation of traffic is not available in Class G.


This internationally standardized system of controlled airspaces has replaced most countries' own systems, however the narrative here describes how this is applied to airports in the United States; other countries apply these airspace classifications differently.


AIRSPACE SIMPLIFICATION


In 2004 ICAO launched a project to simplify the number of classes of airspace to three. The European agency EASA has been working on this proposal for the European region, and has given the three classes the terms N, K and U, standing for intentions-known, known-traffic and unknown-traffic airspace. These roughly correspond to the existing classes C, E (with mandatory mode S transponder carriage) and G. From 2008 it is intended that all European airspace will be one of these three classes.