Airspace Classification

Airspace Classification

35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996

WP No. 83

Airspace Classification


The ICAO airspace structure , offering seven different airspace classes (A to G), has been accepted in 1991.

The IFATCA policy on VFR Operations and Classification of Airspace, carried at the 20th. IFATCA Conference in Cairo, Egypt , has been deleted due to the profound changes that occurred in 1991. An update of the IFATCA policy regarding airspace classification is therefore urgently required.

Many states had serious difficulties in implementing the new ICAO airspace classification on a national basis. IFATCA requires a clear policy on this matter permitting its representatives to make proposals on how to overcome these problems.

The seven ICAO airspace classes, cater for all possible requirements individual member states have, to classify and regulate their national airspace. As ICAO didn’t give clear indication on which criteria the ATS authorities have to select those airspace classes that are most suitable to regulate their airspace, many different solutions appeared. Procedures much too complicated, often over- or under-classifying the airspace or lacking of harmonisation with neighbouring countries, was the result. This development is a clear step back in safety, and not at all what was initially intended by ICAO, when working out the new airspace classification model.

Two questions describe the problem; “ How much do we have to regulate and restrict the airspace ?” and “ How far can we let the airspace unrestricted without taking too many risks?”. In other words , restrict and regulate the airspace where this is necessary, let it free and accessible for all airspace users wherever this is possible. It has to be recognised that this is not only a purely operational question, as considerations of political nature e.g. the national airspace policy, have also some impact on the airspace classification. IFATCA is limiting its considerations on operational matters only.

First problem : over-classification

A European State selected airspace Class A (IFR only operations, VFR flight prohibited) to regulate the airspace over its nations capital, where two of the busiest airports of the continent are within close proximity. It is reported in almost every annual report of the National Air Safety Commission, that there are every year 6 or 7 close encounters (near misses) between IFR and VFR flights in that same portion of restricted airspace. To publish an “IFR only airspace”, where VFR flights frequently have to take place is without doubt an over-classification.

A different type of over-classification appeared as many states decided to publish much too restrictive airspace classes, e.g. class B or C, in airspace’s where IFR traffic is rather sparse so that other airspace users could be accommodated without too much difficulties. But the ATCO’s serving that restricted airspace are often unable to clear VFR flights in due to the high workload involved. This is especially true when insufficient manpower or inadequate technical equipment make it difficult to perform efficiently. This denial of clearance for no apparent reason is often very badly felt by the recreational airspace users and leads to unnecessary tensions between ATCO’s and pilots.

Second problem : under-classification

In Europe and North America there are many major and medium sized international airports that fail to protect IFR flights in an effective way from uncontrolled VFR flights. Public transport aircraft on descent to these airports are encountering VFR traffic until shortly before becoming established on the ILS localiser. It is still very possible to encounter unknown VFR traffic. These IFR flights have to see and avoid small VFR aircraft , which are operating legally in this area without radio communication and unknown to ATC. The French National Air Safety commission the CNSCA , investigating the near misses in French airspace, came to the conclusion that the workload during approach and departure at busy international and regional airports in current generation jets, with two man crew only, is definitely too high to permit the concept “ see and avoid” to be applicable. Unknown VFR traffic, very small and difficult to spot, flying at very low speeds, can hardly be seen and avoided without the application of positive separation, or at least traffic information provided by ATC.


This problem is very acute in parts of the world where the borders of independent States are situated close to international airports or busy airways and where the appropriate ATS authorities failed in the co-ordination and harmonisation of their airspace classifications prior to its publication. The outcome was a regional airspace structure too complicated, changing frequently and without any concept once a border is crossed. In parts of Central Europe, a VFR flight flying a straight track , in level flight, crosses within 100 nautical miles four different airspace classes, changing 10 times from class G to D, D to C, C to E etc. Involving four different ATC units responsible for the issuance of ATC clearances and traffic information for this flight. It is generally recognised that normally the simpler and easier the procedures are, the safer they are. Under the current conditions it is certainly an almost impossible task, even for a very experienced VFR pilot, to know exactly in which airspace class he is actually flying in, which ATC unit is responsible to issue the required ATC clearance, and on which frequency this clearance has to be requested and obtained.

The same is valid for instrument pilots following a SID a STAR and associated approach procedures , or on an airway in this un-harmonised area. Crews are not aware in which airspace class they are currently flying, as the IFR charts often do not inform them of the airspace classification applicable. For this reason these pilots do ignore, that in the vicinity of large international airports, especially those situated in border regions, the protection of commercial IFR flights, due to a lack of harmonisation. Many crews are then more than surprised to when ATC fails to warn them of unexpected VFR traffic that crosses their flight path.


Although the introduction of seven airspace classes with standardised world-wide airspace categories, defining clearly the degree of service provided by ATC was a good idea, meant states have had difficulties with the implementation , resulting in frequent problems being encountered. This has been compounded because the initial classifications have not yet been reviewed by ICAO.

Over-classification has the negative effect of putting pressure on the ATCO’s serving a restricted airspace as many clearance requests from smaller airspace users (e.g. VFR flights) have to be refused due to the high workload involved. In airspace of class B, C or D, an IFR separation or traffic information must be guaranteed without any exceptions, even to VFR flights. The ATS authorities that are responsible to determine the priorities and procedures applicable on a national basis, often lack of interest and courage in defining clearly, the priorities of giving access to restricted airspace due to political pressure from the VFR lobby. To avoid overclassification, it must be ascertained that the degree of regulation of airspace is consistent with the traffic flying in it. In case a more restrictive airspace type is required, ATCOs serving that airspace should have enough manpower and adequate technical equipment at their disposal to manage it in a safe and expeditious manner for the benefit of all airspace users. It is evident that all flights taking advantage of such a service provided by ATC must help to cover its expenses.

Under-classification has the detrimental effect in that a large number of VFR flights have unrestricted access to the same portion of airspace where public transport aircraft under IFR exist. Especially for landing and departure these two different airspace user categories are obliged to share the same airspace without any traffic information or separation provided by ATC. Dangerous situations, are more likely to occur more frequently, due to high workload in the cockpit, especially when in vertical transition during landing and departure, together with the great speed differences and the lack of awareness of the IFR crews about the current airspace classification applicable, make the concept “see and avoid” unable to offer the level of safety required.

As safety is of high priority for flights of public transport aircraft, a concept of collision avoidance using “see and avoid” is not able to meet this requirement Only ICAO airspace classes A,B,C or D, where all flights are known to ATC, are able to offer this kind of protection to the flying public. To enter and operate in these four classes , all flights, being VFR or IFR, require an ATC clearance. IFR separations, or traffic information, provided by ATC according to the flight rules and the airspace type, is the only way to offer safe and satisfactory operating conditions to all flights.

In all other airspace’s where no public transport flights under IFR take place, an airspace classification of class E, F and G is acceptable. In a traffic mix where IFR and VFR flights use the concept “see and avoid” for collision avoidance, the provision of a flight information service (FIS) delivering traffic information and collision avoidance advice to all traffic in contact with ATC is certainly beneficial for the overall safety. The risk to encounter unknown VFR traffic cannot be excluded though.

Many states missed the opportunity to harmonise and co-ordinate with their neighbours the new airspace classification prior to implementation. This omittance created many unsatisfactory situations where large international or regional airports situated close to a border lack sufficient restricted airspace to permit a safe traffic handling. ATCO’s serving these airports know all to well how frequent they have to cope with unknown traffic that missed to call, or is allowed to fly legally close to the approach axis or departure tracks. This situation is not only hindering the work of ATC, but is also a clear danger for all airspace users flying in such dense traffic conditions.

Harmonisation across borders can be achieved by signing letters of agreement (LOA) regulating the standard working procedures, the exchange of flight plan data and the transfer of communication in the area. In co-ordinating the airspace classification on a regional basis, creating contiguous trans-border buffer zones and installing direct speech facilities controller to controller, a high degree of harmonisation can be reached. Another good solution to solve the harmonisation issue is the delegation of airspace to the foreign ATC unit working on the other side of the border.

It is recommended that :

ATS Authorities are urged to co-ordinate and harmonise with all neighbour states their national airspace classification to permit safe and efficient operating conditions to all airspace users and air traffic controllers.
Airspace classification should be appropriate for the traffic operating in the airspace, to avoid over and under classification. As traffic situations change , the classification may have to change accordingly.

Last Update: September 28, 2020  

December 21, 2019   305   Jean-Francois Lepage    1996    

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