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 (SUA) with further rules for reasons of national security or safety.
|Air traffic control
|A service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of air traffic, and provide information and other support for pilots.
|Air Traffic Service
|A generic term meaning variously, flight information service, alerting service, air traffic advisory service, air traffic control service (area control service, approach control service or aerodrome control service).
|Aerodrome traffic zone
|Circular zones around an airport with a radius based on the length of the runway
|A controlled airspace which extends from a lower limit to an upper limit.
|A controlled airspace extending upwards from the surface to an upper limit.
|FLxxx (or FL xxx)
|Defined as a vertical altitude at standard atmospheric pressure, nominally expressed in hundreds of feet. Flight levels are usually designated in writing as FLxxx, where xxx is a two or three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 (29,000 ft; 8,850 m) would be pronounced as "flight level two nine(r) zero." The phrase "flight level" makes it clear that this refers to the standardized pressure altitude.
|International Civil Aviation Organization
|A specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth
|Instrument flight rules
|A set of regulations that concern flying by reference to instruments in the flight deck, and where navigation is accomplished by reference to electronic signals.
|Military aerodrome traffic zone
|A version of ATZ for military air bases.
|Special visual flight rules
|A set of aviation regulations under which a pilot may operate an aircraft. It's a special case of operating under visual flight rules (VFR) where a VFR flight is cleared by air traffic control to operate within a control zone in meteorological conditions that are poorer than visual meteorological conditions.
|Terminal control area
|A control area normally established at the confluence of ATS Routes in the vicinity of one or more major aerodromes.
|Terminal maneuvering area
|A designated area of controlled airspace surrounding a major airport where there is a high volume of traffic.
|Terminal radar service area
|A delimited airspace in which radar and air traffic control services are made available to pilots flying under instrument flight rules or (optionally) visual flight rules for the purposes of maintaining aircraft separation.
|Visual flight rules
|A set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going
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:
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.
Special Airspace: these may limit pilot operation in certain areas. These consist of Prohibited areas, Restricted areas, Warning Areas, MOAs (military operation areas), Alert areas and Controlled firing areas (CFAs), all of which can be found on the flight charts.
Classes A–E are referred to as controlled airspace. Classes F and G are uncontrolled airspace.
The table below provides an overview of the above classes, and the specifications for each.
|Provided for all flights
|Provided for all flights
|Provided for all IFR/SVFR to IFR/SVFR/VFR
|Provided for all VFR
|Provided for IFR/SVFR to other IFR/SVFR
|Provided for all IFR and VFR
|Required for IFR and SVFR
|Provided for IFR/SVFR to other IFR/SVFR
|Provided for all IFR and VFR flights where possible
|Provided for IFR/SVFR to other IFR/SVFR where possible
|Provided where possible if requested
|Provided where possible if requested
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):
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.
In 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.
Further information: Canadian airspace
There are seven airspace classes in use in Canada (letters A through G), but the letters do not always correspond with ICAO definitions.
In Estonia, airspace is divided into only classes C, D and G.
In France, Classes B and F are not used at all. Airspace is divided into lower airspace below FL195 (19,500 ft; 5,950 m) and upper airspace above FL195 (19,500 ft; 5,950 m) .
In Germany, Classes A, B, and F are not used at all. Airspace is divided into lower airspace below FL245 (24,500 ft; 7,450 m) and upper airspace above FL245 (24,500 ft; 7,450 m) .
In Iraq, the Flight Information Regions (FIR) is known as Baghdad FIR. It is classified into Class A, D, E and G airspace. Class B, C and F airspace are not used in the Baghdad FIR. Air traffic services are provided in all controlled airspace, by the controlling ATC Unit, based on an ATS Surveillance System (supplemented by procedural non-ATS Surveillance System procedures) or MRU where authorized based on Procedural (non- ATC Surveillance System) procedures and supplemented by ATC Surveillance System where possible.
In Ireland, airspace is divided into classes A, C and G only.
Classes A, C, D, G are used in Kenyan airspace, alongside unclassified military operation areas which are defined in Restricted Areas and Prohibited Areas, and are controlled by military air traffic control units.
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:
Classes A, C and G are used in Mauritius.
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 built up with class A. It starts at 1,500 ft (460 m) MSL, and ends at FL195 (19,500 ft; 5,950 m) . 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 (19,500 ft; 5,950 m) and changes into Class C. Most of the CTRs are class D, some of them are class C. Class F is the only class that cannot be found in the Dutch airspace. 
In New Zealand, Classes B, E and F are not used at all. Class A is used in Oceanic airspace above FL245 (24,500 ft; 7,450 m) whereas Classes C and D are used in domestic airspace as part of the New Zealand FIR.
In Norway, airspace is divided into classes A, C, D and G.
Russia adopted a modified version of ICAO airspace classification on November 1, 2010. The division into classes for the airspace of the Russian Federation was introduced for the first time in the history of Russia.
The airspace above the territory of the Russian Federation is divided as follows:
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.
Specific boundaries of airspaces are determined by the Order of the Ministry of Transport of the Russian Federation #199 of September 15, 2010.
In Sweden, airspace is divided into airspace class C and G only with a small E class area stretching over the Danish border into Swedish airspace.
Some airways and CTAs may have sections of Class C.
In addition the UK has a couple of special classes of airspace that do not fall within the ICAO classes:
Main article: Airspace class (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.
Authorities use the ICAO definitions to derive additional rules for Visual Flight Rules (VFR) cloud clearance, visibility, and equipment requirements.
For example, consider Class E airspace. It is possible that an aircraft operating under VFR is not in 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 (460 km/h; 290 mph) 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. In these situations the VFR pilot only needs to see where his/her own aircraft is going, so visibility requirements are less stringent, and there is no designated minimum distance from clouds.
Similar considerations determine whether a VFR aircraft must use a two-way radio and/or a transponder.
See also: 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.