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The world's navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO) and described below, though they might use only some of the classes defined below, and significantly alter the exact rules and requirements. Similarly, individual nations may also designate Special Use Airspace with further rules for reasons of national security or safety.
ICAO definitions 
|This section does not cite any references or sources. (March 2011)|
On March 12, 1990, ICAO adopted the current airspace classification scheme. The classes are fundamentally defined in terms of flight rules and interactions between aircraft and Air Traffic Control (ATC). Generally speaking, the ICAO airspaces allocate the responsibility for avoiding other aircraft, namely either to ATC (if separation is provided) or to the aircraft commander (if not).
Some key concepts are:
- Separation: Maintaining a specific minimum distance between an aircraft and another aircraft or terrain to avoid collisions, normally by requiring aircraft to fly at set levels or level bands, on set routes or in certain directions, or by controlling an aircraft's speed.
- Clearance: Permission given by ATC for an aircraft to proceed under certain conditions contained within the clearance.
- Traffic Information: Information given by ATC on the position and, if known, intentions of other aircraft likely to pose a hazard to flight.
- Flight Rules: Aircraft can operate under Visual flight rules (VFR) or Instrument Flight Rules (IFR). There is also an intermediate form, Special visual flight rules (SVFR).
- ICAO adopted classifications
Note: These are the ICAO definitions. Country specific adaptations (such as "two-way communications" instead of "clearance" for Class C in the US) are discussed in the sections below.
- Class A: All operations must be conducted under IFR or SVFR. All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
- Class B: Operations may be conducted under IFR, SVFR, or VFR. All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
- Class C: Operations may be conducted under IFR, SVFR, or VFR. All aircraft are subject to ATC clearance (country specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other and from flights operating under VFR, but VFR flights are not separated from each other. Flights operating under VFR are given traffic information in respect of other VFR flights.
- Class D: Operations may be conducted under IFR, SVFR, or VFR. All flights are subject to ATC clearance (country specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other, and are given traffic information in respect of VFR flights. Flights operating under VFR are given traffic information in respect of all other flights.
- Class E: Operations may be conducted under IFR, SVFR, or VFR. Aircraft operating under IFR and SVFR are separated from each other, and are subject to ATC clearance. Flights under VFR are not subject to ATC clearance. As far as is practical, traffic information is given to all flights in respect of VFR flights.
- Class F: Operations may be conducted under IFR or VFR. ATC separation will be provided, so far as practical, to aircraft operating under IFR. Traffic Information may be given as far as is practical in respect of other flights.
- Class G: Operations may be conducted under IFR or VFR. ATC separation is not provided. Traffic Information may be given as far as is practical in respect of other flights.
The table below provides an overview of the above classes, and the specifications for each.
|Class||Controlled||IFR||SVFR||VFR||ATC Clearance||Separation||Traffic Information|
|A||Controlled||Yes||Yes||No||Required||Provided for all flights||N/A|
|B||Controlled||Yes||Yes||Yes||Required||Provided for all flights||N/A|
|C||Controlled||Yes||Yes||Yes||Required||Provided for all IFR/SVFR||Provided for all VFR|
|D||Controlled||Yes||Yes||Yes||Required||Provided for IFR/SVFR to other IFR/SVFR||Provided for all IFR and VFR|
|E||Controlled||Yes||Yes||Yes||Required for IFR and SVFR||Provided for IFR/SVFR to other IFR/SVFR||Provided for all IFR where possible, to VFR where possible but only when requested|
|F||Uncontrolled||Yes||No||Yes||Not Required||Provided for IFR/SVFR to other IFR/SVFR where possible||Provided where possible|
|G||Uncontrolled||Yes||No||Yes||Not Required||Not provided||Provided where possible|
Use of airspace classes 
Each national aviation authority determines how it uses the ICAO classifications in its airspace design. In some countries, the rules are modified slightly to fit the airspace rules and air traffic services that existed before the ICAO standardisation.
Australia has adopted a civil airspace system based on the United States National Airspace System (NAS):
- Class A is used above FL 180 along the populated coastal areas, and above FL 245 elsewhere.
- Class B is not used.
- Class C is used in a 360° funnel shape in the Terminal Control Zones of the major international airports, extending up to the base of the Class A, generally at FL 180 over these airports. It also overlays Class D airspace at smaller airports.
- Class D is used for the Terminal Control Zones of medium sized airports, extending from the surface up to 2,500 feet (760 m) AGL (depicted in MSL on a chart). Above this, Class C airspace is used, although generally only in a sector, and not 360° around the airport.
- Class E is used along the populated coastal areas, from 8,500 feet (2,590 m) to the base of the overlying Class A or Class C airspace.
- Class F is not used.
- Class G is used wherever other classes are not—almost always from the surface to the base of the overlying Class A, C, D or E airspace.
Transition from GAAP to Class D 
Australia used to have a non-standard class of airspace for use at the capital city general aviation airports, called a General Aviation Airport Procedures Zone (GAAP Zone). A control tower provided procedural clearances for all aircraft inside the zone. Additionally, any aircraft operating within 5 nmi (9.3 km; 5.8 mi) of the zone must obtain a clearance. VFR aircraft arrive and depart using standard arrival and departure routes, while instrument arrival and departure procedures are published for IFR operations. During visual meteorological conditions (VMC), IFR aircraft are not provided with full IFR services. During instrument meteorological conditions (IMC), or marginal VMC, VFR operations are restricted in order to facilitate full IFR service for IFR aircraft.
As of the 3rd of June 2010, all GAAP aerodromes were changed to Class D aerodromes, and the previous Class D procedures were changed. The new Class D procedures are similar to the FAA Class D procedures. VFR Aircraft are no longer required to enter the airspace via set inbound/outbound points, however can be directed there by ATC. VFR and IFR aircraft now require taxi clearance in the "manoeuvring area" of the aerodrome, but can still taxi within set apron areas without a clearance. IFR aircraft now receive slot times and the visibility requirements of Special VFR are reduced from 3000m Visibility to 1600m.
There are seven classes of airspace in Canada, and each is designated by a letter (A through G).
In Germany, Classes A and B are not used at all. Class C is used for Airspace above Flight Level (FL) 100 (or FL 130 near the Alps) up to FL 660. Airspace is divided into lower airspace below FL 245 and upper airspace above FL 245.
- Class A is not used.
- Class B is not used.
- Class C is used for controlled zones above and around airports and for airspace above FL 100 (or FL 130 near the Alps) up to FL 660.
- Class D is used for controlled zones, or above and around airspace Class C designated zones where CVFR is not necessary.
- Class E is used for airspace between usually 2,500 ft (760 m) AGL (around airports 1,000 ft (300 m) or 1,700 ft (520 m) AGL) and FL 100.
- Class F is used for IFR flight in uncontrolled airspace.
- Class G is used below 2,500 ft (760 m) AGL (around airports below 1,000 ft (300 m) AGL, then rises via a step at 1,700 ft (520 m) to 2,500 ft (760 m) AGL). IFR flights in Class G are not allowed in Germany.
In Lithuania, Classes A and B are generally not used at all. Classes C and D are used in the following areas of controlled airspace of the Republic of Lithuania:
- in control zones (CTR);
- in terminal control areas (TMA);
- in control area (CTA);
- in upper control area (UTA).
In the Netherlands, a relatively large part of the country is Class A airspace. Near Amsterdam, the capital of the Netherlands, the airspace is almost completely build up with class A. It starts at 1500' MSL, and ends at FL195. Further away from Amsterdam and its airport Schiphol, Class A starts at a higher altitude. Class B is used a lot as well. Anywhere in the Netherlands, Class A airspace ends at FL195 and changes in to Class C. Dutch CTRs are always class C airspace. Class F is the only class that cannot be found in the Dutch airspace.
Russia adopted a modified version of ICAO airspace classification on November 1, 2010. The division into classes for the airspace of Russia was introduced for the first time in the history of Russia.
The airspace above the territory of Russia is divided as follows:
- Class A applies to airspace above and including 8100 m (the boundary between lower and upper airspace in Russia). All operations in Class A airspace must follow IFR and are separated from each other by ATC. Permanent two-way radio contact with ATC is required. Permission for using airspace is required except for the special cases listed in clause 114 of the Federal rules for using Russian air space.
- Class B is not used.
- Class C airspace is defined below 8100 m and allows IFR and VFR operations. Both IFR and VFR operations are required to have permanent two-way radio contact with ATC. IFR flights are separated from each other and from VFR flights. VFR flights are separated from IFR flights and are provided traffic information about other VFR flights. Permission for using airspace is required except for the special cases listed in clause 114 of the Federal rules for using Russian air space.
- Class D is not used.
- Class E is not used.
- Class F is not used.
- Class G airspace is defined wherever Class A and Class C airspaces are not defined. Class G airspace allows IFR and VFR operations. For altitudes less than 3000 m (9843 ft) the speed must not exceed 450 km/h (280 mph). Flights are provided with flight information service as requested. IFR flights are required to have permanent two-way radio contact with ATC. No separation is provided by ATC. No permission for using airspace is required.
Airspace controlled by Russia outside the territory of Russia has different division into classes and includes redefined Class A and Class G, but no class C airspace.
In Sweden, airspace is divided into airspace class C and G only.
- Class A is not used.
- Class B is not used.
- Class C is used for all controlled airspace.
- Class D is not used.
- Class E is not used.
- Class F is not used.
- Class G is used elsewhere.
United Kingdom 
- Class A:
- All airways up to FL 195 with the exception of airways lying within the Belfast CTR/TMA and the Scottish TMA.
- The Terminal Control Areas (TMAs) around London, Birmingham and Manchester.
- The London Control Zone around Heathrow and the Channel Islands Control Zone; these areas are thus off-limits to VFR flights (however Special VFR is used as a get-around for this).
- The CTAs of Daventry, Clacton, Cotswold and Worthing.
- Airways typically start at FL 70 and routing options become more attractive above FL 140.
- Class B: No Class B in the UK.
- Class C: All UK airspace between FL 195 and FL 660. (The Upper Flight Information Region (UIR) boundary begins at FL 245.)
- Class D:
- Class E: Parts of the Belfast and Scottish TMAs. A clearance is not required for VFR flights within Class E airspace, however pilots are strongly advised to contact the appropriate ATSU.
- Class F: "Advisory Routes" (ADRs), i.e. regularly used routes similar to airways but where traffic levels are not high enough to warrant establishment of an airway.
- Class G: All remaining airspace, comprising by far the largest part of the airspace below FL 195. The UK is unusual in that it has not adopted a widespread class E system of airways for most airspace lower than FL 70. Therefore for light aircraft, IFR flight in Class G airspace is relatively common. Use of a radio or transponder is not required, even in IMC. ATC units may provide an "as far as practical" form of separation between some such flights, but participation in the service is voluntary.
In addition the UK has a couple of special classes of airspace that do not fall within the ICAO classes:
- Aerodrome Traffic Zones (ATZ) are zones around an airport with a radius of 2 nm or 2.5 nm, extending from the surface to 2,000 ft (600 m) AAL (above aerodrome level). Aircraft within an ATZ must obey the instructions of the tower controller (if present), or must make radio contact with the Aerodrome Flight Information Service unit or Air/Ground Communication Service unit for the aerodrome before entering the zone (in the case of an uncontrolled airfield), or must obey ground signals if non-radio.
- Military Air Traffic Zones (MATZ) are zones from the surface to 3,000 ft (900 m) AAL (above aerodrome level) set up for 5 nm around military air bases in class G airspace. Stubs, 4 nm wide and 5 nm in length, extend from the main zone, are orientated with the aerodrome's main runway, and start at 1,000 ft (300 m) and go up to 3,000 ft (900 m). Military aircraft treat these as if they are controlled airspace; civilian traffic are advised but not obliged to do the same.
United States 
The U.S. adopted a slightly modified version of the ICAO system on September 16, 1993, when regions of airspace designated according to older classifications were converted entirely. The exceptions are some Terminal Radar Service Areas (TRSA), which have special rules and still exist in a few places.
- With some exceptions, Class A airspace is applied to all airspace between 18,000 feet (5,500 m) and Flight Level 600 (approximately 60,000 ft). Above FL600, the airspace reverts to Class E. The transition altitude is also consistently 18,000 feet (5,500 m) everywhere. All operations in US Class A airspace must be conducted under IFR. SVFR flight in Class A airspace is prohibited.
- Class B airspace is used around major airports, in an inverted wedding cake shape that is designed to contain arriving and departing commercial air traffic operating under IFR, up to 10,000 feet (3,000 m) above MSL (12,000 feet MSL around Denver, Colorado, and Salt Lake City, Utah). Aircraft must have established two-way radio communications with ATC, and obtained permission to enter the airspace.
- Class C airspace is used around airports with a moderate traffic level.
- Class D is used for smaller airports that have a control tower. The U.S. uses a modified version of the ICAO class C and D airspace, where only radio contact with ATC rather than an ATC clearance is required for VFR operations.
- Other controlled airspace is designated as Class E, this includes a large part of the lower airspace. Class E airspace exists in many forms. It can serve as a surface-based extension to Class D airspace to accommodate IFR approach/departure procedure areas. Class E airspace can be designated to have a floor of 700' AGL or 1,200' AGL, or a customized floor of any other altitude. Class E airspace exists above Class G surface areas from 14,500' MSL to 18,000 MSL. Federal airways from 1,200 AGL to 18,000 MSL within 4 miles (6 km) of the centerline of the airway is designated Class E airspace. Airspace at any altitude over 60,000' (the ceiling of Class A airspace) is designated Class E airspace.
- The U.S. does not use ICAO Class F.
- Class G (uncontrolled) airspace is mostly used for a small layer of airspace near the ground, but there are larger areas of Class G airspace in remote regions.
Airspace classes and VFR 
Authorities use the ICAO definitions to derive additional rules for VFR cloud clearance, visibility, and equipment requirements.
For example, consider Class E airspace. It is possible that an aircraft operating under VFR is not be communication with ATC, so it is imperative that its pilot be able to see and avoid other aircraft (and vice versa). That includes IFR flights emerging from a cloud, so the VFR flight must keep a designated distance from the edges of clouds above, below, and laterally, and must maintain at least a designated visibility, to give the two aircraft time to observe and avoid each other. The low-level speed limit of 250 knots does not apply above 10,000 feet (3,000 m), so the visibility requirements are higher.
On the other hand, in Class B and Class C airspaces, separation is provided by ATC to all aircraft. Now the VFR pilot only needs to see where it is going, so visibility requirements are reduced and there is no designated minimum distance from clouds.
Special use airspace 
Each national authority designates areas of special use airspace (SUA), primarily for reasons of national security. This is not a separate classification from the ATC-based classes; each piece of SUA is contained in one or more zones of letter-classed airspace.
SUAs range in restrictiveness, from areas where flight is always prohibited except to authorized aircraft, to areas that are not charted but are used by military for potentially hazardous operations (in this case, the onus is on the military personnel to avoid conflict). Refer to the external links for more specific details.
- "SVFR in class A – ATP Forum discussion". Retrieved 2011-03-29.[unreliable source?]
- Airfield Guide Lithuania, 29 SEP 2005, ENR 1.1-1
- The Official Site of the Prime Minister of the Russian Federation, March 11, 2010.
- Appendix to the Resolution of the Government of The Russian Federation #138 of March 11, 2010. (Russian)
- Order of the Ministry of Transport of the Russian Federation #199 of September 15, 2010. (Russian)
- Godwin, Peter (1987–2004). The Air Pilot's Manual: Aviation Law And Meteorology. Cranfield, England: Air Pilot Publishing Ltd.
- FAA Order 7400.9, Subpart E