Airspace classes
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.
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. All aircraft are subject to 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. All aircraft are subject to 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 other VFR 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. The airspace classes are pronounced using
radio phonetics, so that
class B, for example, is pronounced "class bravo", and
class C is pronounced "class charlie".
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.
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 exisited before the ICAO standardisation.
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 exception is some Terminal Radar Surface 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 and
Flight Level 600. Above FL600, the airspace reverts to Class E. The transition altitude (see
Flight level) is also consistently 18,000 feet.
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 10,000 feet 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.
The U.S. does not use ICAO Class F.
Class G 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 broadly follows the US in application of airspace. It also does not use class F, instead, the term
Class F is used for Special Use Airspace, which occasionally causes confusion in discussions.
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 aispace 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 nessecary.
Class E is used for airspace between usually 2500 ft. AGL (around airports 1000 ft. or 1700 ft. AGL) and FL 100.
Class F is used for
IFR-Flight in uncontrolled airspace.
Class G is used below 2500 ft. AGL (around airports below 1000 ft. AGL, then rises via a step at 1700 ft. to 2500 ft. AGL)
United Kingdom
Class A* All airways up to FL 245 with the exception of those 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 this).
* The CTAs of Daventry, Cotswold and Worthing.
Class BNot used in the UK.
Class CThe Upper Flight Information Region (UIR) between FL 245 and FL 660.
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 EThe airways not in Class A, parts of the Belfast and Scottish TMAs and a small part of the
Teesside 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 GAll remaining airspace, comprising by far the largest part of the airspace below FL 245. 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 1.5nm and 2.5nm from the surface to 2,000ft 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 3,000ft 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 FL180 along the populated coastal areas, and above FL245 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 FL180 over these airports. It also overlies 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 4500 feet. Above this, Class C airspace is used, although generally only in an a sector, and not 360° around the airport.
Class E is used along the populated coastal areas, from 8500 feet 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 5nm 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.
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 10,000 feet, 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.
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.
*
Airspace in the U.S., from the FAA's Aeronautical Information Manual*
UK Manual of Air Traffic Services, Part 1*
US Airspace Aeronautical Charts
