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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. Individual nations also designate ''Special Use Airspace'', which places further rules on air navigation for reasons of national security or safety.

Contents

ICAO definitions

Use of airspace classes

United States

Canada

Germany

Lithuania

United Kingdom

Australia

Airspace classes and VFR

Special-use Airspace

External links

ICAO definitions

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). 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.
The classifications adopted by ICAO are:

★ Class A: All operations must be conducted under Instrument Flight Rules (IFR) or Special visual flight rules (SVFR) and are subject to ATC clearance. All flights are separated from each other by ATC.

★ Class B: Operations may be conducted under IFR, SVFR, or Visual flight rules (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. IFR aircraft are subject to ATC clearance. VFR aircraft require radio contact prior to entering airspace (Not to be confused with ATC clearance). Aircraft operating under IFR and SVFR are separated from each other and from flights operating under VFR. 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. IFR aircraft are subject to ATC clearance. VFR aircraft require radio contact prior to entering airspace (Not to be confused with ATC clearance). 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.
Classes A-E are referred to as controlled airspace. Classes F and G are uncontrolled airspace.
As of 2004, ICAO is considering a proposal to reduce the number of airspace classifications to three, which roughly correspond to the current classes C, E and G.

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.

United States

Main articles: 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 wholesale. 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 and Flight Level 600 (approximately 60,000 ft). Above FL600, the airspace reverts to Class E (Reference Order 7400.9P, Subpart E). The transition altitude (see ''Flight level'') is also consistently . 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 a funnel shape that is designed to contain arriving and departing commercial air traffic operating under IFR, up to above MSL (12,000 feet above Denver, Colorado). Class C airspace is used around airports and military air bases 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 (above ground level) or 1,200' AGL. Class E airspace exists above Class G surface areas from 14,500' MSL (mean sea level) to 18,000 MSL. Federal airways from 1,200 AGL to 18,000 MSL within 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 airspace (Uncontrolled) is mostly used for a small layer of airspace near the ground, but there are larger areas of Class G airspace in remote regions.

Canada

Canada generally follows the United States in application of airspace with some differences. For example, Canadian class "C" airspace is procedurally equivalent to United States class "B" airspace. Additionally, the term "Class F" is used for Special Use Airspace, this includes Advisory airspace and Restricted airspace.

Germany

In Germany, Classes A and B are generally not used at all. Class C is used for all Airspace above Flight Level (FL) 100 (or FL 130 near the Alps). Airspace is divided into ''lower airspace'' below FL 285 and ''upper airspace'' above FL 285.

★ ''Class A'' is not used.

★ ''Class B'' is not used.

★ ''Class C'' is used for controlled zones above and around airports and all airspace above FL 100 (or FL 130 near the Alps.)

★ ''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 . AGL (around airports . or 1700 ft. AGL) and FL 100.

★ ''Class F'' is used for IFR-Flight in uncontrolled airspace.

★ ''Class G'' is used below . AGL (around airports below . AGL, then rises via a step at . to . AGL)

Lithuania

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).
'Source': Airfield Guide Lithuania, 29 SEP 2005, ENR 1.1-1

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 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, Cotswold and Worthing.
'Class C'
All UK airspace between FL 195 and FL 660.
N.b: The Upper Flight Information Region (UIR) boundary begins at FL 245
'Class D'

★ The CTRs and CTAs around the larger airfields except London Heathrow, such as London Gatwick, Glasgow, Birmingham and Newcastle upon Tyne.

★ A few airways in less busy areas allowing mid-level military VFR flights.
'Class E'
Parts of the Belfast and Scottish TMAs and a small part of the Durham Tees Valley CTR.
'Class F'
"Advisory Routes" (ADRs): 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 IFR flight in Class G airspace is relatively common and ATC units may provide an "as far as is practical" form of separation between some such flights.
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 of between 1.5 nm and 2.5 nm from the surface to AAL set up around an airport, where aircraft must obey the instructions of the tower controller (if present), must make radio contact with the Information Officer or Air/Ground radio unit on the airport 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 AAL set up around military air bases in class G airspace. Military aircraft treat these as if they are controlled airspace; civilian traffic is advised but not obliged to do the same.

Australia

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 . 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 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.
In addition, Australia has 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 provides procedural clearances for all aircraft inside the zone. Additionally, any aircraft operating within 5 nm 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 VMC, IFR aircraft are not provided with full IFR services. During IMC, or marginal VMC, VFR operations are restricted in order to facilitate full IFR service for IFR aircraft.

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. An aircraft operating under VFR may not be 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 does not apply above , so the visibility requirements are higher.
On the other hand, in Class B airspace, separation is provided by ATC to all flights. Now the VFR flight only needs to see where it is going, so visibility requirements are reduced and there is no designated minimum distance from clouds.
Similar considerations determine whether a VFR flight must use a two-way radio and/or a transponder.

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.

External links

★ 3D representations of US Airspace and Sectional Charts of the United States via Google Earth

★ Airspace in the U.S., from the FAA's Aeronautical Information Manual

★ UK Manual of Air Traffic Services, Part 1

★ US Airspace Aeronautical Charts