Airspace Class - ICAO Definitions

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). 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:

• 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.
• Flight Rules: Aircraft can operate under Visual flight rules (VFR) or Instrument Flight Rules (IFR). There is also an intermediate form, Special visual flight rules (SVFR).

ICAO adopted classifications

Note: These are the ICAO definitions. Country specific adaptations (such as "two-way communications" instead of "clearance" for Class C in the US) can be discussed in the sections below, please do not place them here.

• Class A: All operations must be conducted under IFR or SVFR. All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
• Class B: Operations may be conducted under IFR, SVFR, or 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 (country specific variations notwithstanding). Aircraft operating under IFR and SVFR are separated from each other and from flights operating under VFR, but VFR flights are not separated from each other. 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 flights are subject to ATC clearance (country specific variations notwithstanding). 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.

The table below provides an overview of the above classes, and the specifications for each.

As of 2004, ICAO is considering a proposal to reduce the number of airspace classifications to three (N, K and U), which roughly correspond to the current classes C, E and G.