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.

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.
• Instruction: A direction given by ATC that an aircraft is obliged to follow.
• Clearance: Permission for an aircraft to proceed with an action at the pilot's discretion.
• Safety alert: A warning given by ATC that a collision may be imminent, usually based on radar observation.

The classifications adopted by ICAO are:

• Class A: All operations must be conducted under Instrument Flight Rules (IFR) and are subject to ATC clearances and instructions. ATC separation is provided to all aircraft.
• Class B: Operations may be conducted under IFR, Special Visual Flight Rules (SVFR), or Visual Flight Rules (VFR). However, all aircraft are subject to ATC clearances and instructions. ATC separation is provided to all aircraft.
• Class C: Operations may be conducted under IFR, SVFR, or VFR; however, all aircraft are subject to ATC clearances and instructions. ATC separation is provided to all aircraft operating under IFR or SVFR and, as necessary, to any aircraft operating under VFR when any aircraft operating under IFR is involved. All VFR operations will be provided with safety alerts and, upon request, conflict resolution instructions.
• Class D: Operations may be conducted under IFR, SVFR, or VFR; however, all aircraft are subject to ATC clearances and instructions. ATC separation is provided to aircraft operating under IFR or SVFR only. All traffic will receive safety alerts and, on pilot request, conflict resolution instructions.
• Class E: Operations may be conducted under IFR, SVFR, or VFR. ATC separation is provided only to aircraft operating under IFR and SVFR within a surface area. As far as practical, ATC may provide safety alerts to aircraft operating under VFR.
• Class F: Operations may be conducted under IFR or VFR. ATC separation will be provided, so far as practical, to aircraft operating under IFR.
• Class G: Operations may be conducted under IFR or VFR. ATC separation is not provided.

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.

Airspace Classes in the U.S. and Canada

The U.S. adopted 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. Also, Class F airspace does not exist in the U.S. system, as it had no corresponding older class. Canadian airspace design is similar.

Use of classes

With some exceptions in Hawaii and Alaska, Class A airspace is applied to all space between 18,000 feet and Flight Level 600. Above FL600, the space 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.

Other controlled airspace is designated as Class E.

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.

The U.S. and Canada do not use ICAO Class F, since they do not have the concept of "so far as practical" ATC for IFR traffic. In Canada, the term Class F is used for Special Use Airspace, which occasionally causes confusion in discussions.

Airspace classes and VFR

Each national aviation authority determines how it uses the ICAO classifications in its airspace design. Authorities use the 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.

Airspace classes in other English-speaking countries

In the U.K., Class A is used around major airports; the area around Heathrow, for example, is off limits to VFR flights. Class B is used above Flight Level 245, and there are no instances of Class C airspace. Smaller airports have Class D areas, and other segments are classified as E, F, and G. In addition, the U.K. allows IFR flight in uncontrolled airspace without the requirement to carry a radio.

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

• Airspace in the U.S., from the FAA's Aeronautical Information Manual
  • Special Use Airspace in the U.S.
• UK Visual Flight Rules and Airspace Classification/Limitation