Airspace classes

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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

1 ICAO definitions
2 Use of airspace classes

2.1 United States
2.2 Canada
2.3 Germany
2.4 United Kingdom
2.5 Australia

3 Airspace classes and VFR
4 Special-use Airspace
5 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. 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.

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 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

In the U.K., Class A is used for airways and for busy Terminal Maneouvering Areas (TMAs). Class A is also used for the London Control Zone around Heathrow, making this airport off-limits to VFR flights.

Class B is not used.

Class C is used between FL245 and FL660

Other larger airports have Class D Control Zones and Control Areas; there are also a few class D airways in less busy areas to allow mid-level military VFR flights.

Class E is used in less-busy TMAs.

Class F is used on "Advisory Routes" - these resemble airways but are subject much less traffic hence are uncontrolled airspace.

The remainder and by far the largest part of the airspace below FL245 is Class G. 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.

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.

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 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.

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

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