by Todd Cox


To introduce basic airspace classifications that will assist pilots while flying online VATSIM.

When we discuss airspace, there are many rules and procedures that are involved, for both the U.S. and ICAO nations. This lesson is designed to present key concepts regarding airspace within ICAO. It is presented in the very basic of terms, as the scope of airspace in general is very technical and can cause confusion. United States airspace concepts are presented in the United States Airspace Classification lesson.

ICAO History Lesson
The Convention on International Civil Aviation∞ (also known as Chicago Convention), was signed on 7 December 1944 by 52 States. Pending ratification of the Convention by 26 States, the Provisional International Civil Aviation Organization (PICAO) was established. It functioned from 6 June 1945 until 4 April 1947. By 5 March 1947 the 26th ratification was received. ICAO came into being on 4 April 1947. The Convention on International Civil Aviation set forth the purpose of ICAO:

"WHEREAS the future development of international civil aviation can greatly help to create and preserve friendship and understanding among the nations and peoples of the world, yet its abuse can become a threat to the general security; and 

WHEREAS it is desirable to avoid friction and to promote that co-operation between nations and peoples upon which the peace of the world depends; 

THEREFORE, the undersigned governments having agreed on certain principles and arrangements in order that international civil aviation may be developed in a safe and orderly manner and that international air transport services may be established on the basis of equality of opportunity and operated soundly and economically; 

Have accordingly concluded this Convention to that end." 

Guiding Documents
The guiding documents under ICAO are called International Standards and Recommended Practices, known as ICAO Annexes. There are 18 Annexes and three Procedures for Air Navigation Services (PANS). The two annexes that are referenced in this lesson are ICAO Annex Two (Rules of the Air) and ICAO Annex 11 (ATC Services).

One item that is required by ICAO is that every country publish manuals describing its ATC system and any differences from ICAO standards. The two publications used the the U.S. are the International Flight Information Manual∞ and the Aeronautical Information Publication∞. For example, in the U.S., we pronounce decimal points as 'points', whereas ICAO recommends that it be pronounced "decimal." With this in mind, it is important to understand that ICAO rules are both similiar and different.

ICAO Airspace 101
Current ICAO airspace designations where adopted in 1990, with the U.S. adopting the same classifications, though used differently in 1993. In case you didn't know, the U.S. had 20 different types of airspace designations prior to 1993. Basically under ICAO, there is controlled airspace and uncontrolled airspace.

Controlled Airspace
Controlled Airspace is defined as airspace of defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification. Under ICAO, controlled airspace is defined as:

  • Class A:
    IFR flights only are permitted, all flights are provided with air traffic control service and are separated from each other.
  • Class B:
    IFR and VFR flights are permitted, all flights are provided with air traffic control service and are separated from each other.
  • Class C:
    IFR and VFR flights are permitted, all flights are provided with air traffic control service and IFR flights are separated from other IFR flights and from VFR flights. VFR flights are separated from IFR flights and receive traffic information in respect of other VFR flights.
  • Class D:
    IFR and VFR flights are permitted and all flights are provided with air traffic control service, IFR flights are separated from other IFR flights and receive traffic information in respect of VFR flights, VFR flights receive traffic information in respect of all other flights.
  • Class E:
    IFR and VFR flights are permitted, IFR flights are provided with air traffic control service and are separated from other IFR flights. All flights receive traffic information as far as is practical. Class E shall not be used for control zones.

Uncontrolled Airspace
Generally under ICAO, uncontrolled airspace is as follows:

  • Class F:
    IFR and VFR flights are permitted, all participating IFR flights receive an air traffic advisory service and all flights receive flight information service if requested.
  • Class G:
    IFR and VFR flights are permitted and receive flight information service if requested.

Differences in Airspace Structure
So far, you may be scratching your head as to what is so different from ICAO vs U.S. Airspace. There are many differences, from weather minimums to how the airspace is used. The biggest key is before you fly into airspace areas under ICAO, as well as the U.S., you should take a look at the particular region or ATC areas procedures before flying into those areas. 

As far as examples, Some ICAO countries authorise VFR flights above FL 195, either by establishing Class B or C airspace, or by allowing VFR flights in Class A in accordance with specific conditions and/or with special ATC instructions. Some countries relieve IFR flights from mandatory requirements for continuous two-way radio communication in Classes F and G. Other countries do not permit IFR flights in Class G. One country requires ATC clearances for IFR flights to operate in Class F airspace. SO as you can see, this can be very confusing if a little research is not done first.

Canadian Example
Transport Canada is the Canadian equivalent of the FAA in the U.S. The following link refers to Canadian Airspace Requirements and Procedures. In Canada, they use Class F airspace. Class F airspace is airspace of defined dimensions within which activities must be confined because of their nature and (or) within which limitations may be imposed upon aircraft operations that are not a part of those activities. Does this statement sound familiar? It is the same statement used in the U.S. to define Special Use Airspace.

Class F airspace in Canada may be classified as Class F advisory, or as Class F restricted, and can be controlled airspace, uncontrolled airspace, or a combination of both. An advisory area, for example, may have its base in uncontrolled airspace and its CAP in controlled airspace. The significance, in this instance, is that the weather minima would be different in the controlled and uncontrolled portions. When areas of Class F airspace are inactive, they will assume the rules of the appropriate surrounding airspace.

U.S. Differences
There are many differences between ICAO and U.S. procedures. Class B in the U.S. is more restrictive than in ICAO countries. Class C in the U.S. terminates at 4000 feet, whereas ICAO Class C can go as high as FL660. There are many other differences, especially with regards to procedures, but those will be covered in later lessons.

International Airspace
Since ICAO is a regulatory body and not a direct ATC service, international ATC is delegated to those member nations who accept responsibility for providing ATC services. As such, Annex 2, Rules of the Air, is the guiding document when flying in international airspace. Most domestic and foreign governments require their aircraft operators to abide by this annex. ICAO has divided the airspace in the world into Flight Information Regioins, or FIRS. These regions indentify which contry controls the airspace and determines which procedures are to be used. Normally, one major ATC facility isidentified with each FIR. These facilites are called Area Control Centers (ACC). They are equivalent to ARTCC's in the U.S.

For all pilots, regardless of where you are flying to, is is important to research the airspace and procedures where you will be flying. Not all airspace is the same as to the services that are provided or their weather minimums. Even right now, Eurocontrol, which is the equavalent to the FAA, has been working on implementing a strategy that will reduce the number of airspace classifications from seven to three by 2010, with a further reduction to two by 2015. The airspace names would become N, K, and U for iNtended, Known, and Unknown. If you would like to read about this strategy, you can go to the Eurocontrol Website.