Review Provisions of ATS with Particular Reference to Airspace Classification

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Review Provisions of ATS with Particular Reference to Airspace Classification

36TH ANNUAL CONFERENCE, Taipei, Taiwan, 17-21 March 1997

WP No. 85

Review Provisions of ATS with Particular Reference to Airspace Classification


Following the discussion and the acceptance of policy at the 35th IFATCA Conference in Tunis, it was decided to continue with this work item on the list of the tasks of SC1 for the season 96/97.

This paper is proposing to the Federation new policy regarding the distinction between controlled and uncontrolled airspace types that is different from the actual ICAO publications. Furthermore, this working paper points out some additional inconsistencies and problems regarding ICAO airspace classification that require clarification or correction.

As there are no VFR flights operating in airspace of Class A no VMC minima is defined by ICAO for this airspace class. Some official ICAO procedures such as « clearances to fly maintaining own separation while in VMC », as described in document 4444, PANS-RAC, in paragraph 13 on page 3-19, require that such a minima is defined. It is known that many states, e.g. the UK, do forbid this procedure when radar separation can be applied. However, many other states continue to permit VMC separations so that there is a need to clarify the conditions of use on an international level.

ICAO procedures regarding radio communication failures continue to differ according to the flight conditions experienced, either VMC or IMC. Therefore the lack of a VMC minima in airspace of Class A is opening the door for uncertainties. Radio failure procedures differ significantly according to the meteorological conditions encountered in flight. It is very difficult for pilots to operate their aircraft according to the rules if there is no VMC minima defined in an airspace class. Question is if a flight in Class A airspace has to follow the provisions for IMC even if the flight visibility is 100 km or more with no clouds in the sky?

In paragraph 9 on page 2-11 of document 4444, under point 9.2, where ICAO describes the procedures for a change of a flight from IFR to VFR, there is written « no reply other than the acknowledgement “IFR FLIGHT CANCELLED AT….(time)” should normally be made by an air traffic services unit ». This procedure must be amended with the restriction that this reply is applicable for all airspace classes, except for Class A where VFR flights aren’t permitted.

During the discussions held in Tunis last year it appeared that the lack of knowledge of the airspace classification applicable and the wrong feeling of being protected by ATC once radar service is provided to IFR flights came out as a major deficiency. This serious problem continuous to be of actuality, particularly for single-engine IFR aircraft that cruise at low altitudes on long distance international flights operated in foreign airspace. Encountering unknown VFR traffic for these low level IFR flights is a real danger. In Europe where many small independent states with different and unharmonised airspace classifications are situated close together, a serious flight preparation regarding the airspace classification applicable and the degree of protection offered would require many hours of planning and cost an enormous amount of money as all national AIP’s have to be consulted. Not surprisingly, many pilots neglect this task for very obvious reasons.

Many states were unable to find in the menu of the 7 airspace classes offered by ICAO the type of airspace class that was able to meet in full their local airspace classification requirements. These countries needed easily applicable airspace handling procedures that were able to accommodate in the most efficient manner a multitude of different kinds of airspace users in the same airspace without creating delay or heavy restrictions. From the beginning it became evident that the rigid ICAO set of seven airspace class types couldn’t meet these needs. This is the reason why many airspace classifications with « D-plus airspace » or « C minus airspace » appeared everywhere, but especially in high density TMA’s or CTR’s where a mix of IFR and VFR traffic is a reality. These deviations are detrimental to safety as foreign users of the airspace expect to get the service and protection according to ICAO standards. They may not know the particular national procedures that contradict the ICAO regulations.

Another problem which is complicating the application of the airspace classification is the fact that the airspace class is governing the entry requirements for VFR flights. It often happens in busy TMA’s where ATC is radar vectoring IFR flights to final approach that pilots wrongly believe they benefit from full radar service, including IFR separations and traffic information, regarding all traffic in the airspace. This is for instance in class E airspace not at all the case. It can well be possible that IFR flights that are lined up in sequence on ILS have to give way to an unknown VFR flights that cross the localizer left to right without radio contact. See and avoid is applicable without restrictions. Is this reasonable?

There are states where IFR flights receive in Class C airspace , traffic information only regarding VFR flights in the same airspace. This is in clear contradiction to ICAO procedures. In another state, where some TMA’s (or better the TCA’s) are classified Class B airspace , VFR flights are not separated by ATC with a standard IFR separation from other VFR flights. Only as far as practical traffic information, is provided by ATC. In this same country VFR-flights do not need to obtain an ATC clearance to enter Class C airspace . The only entry requirement for this airspace class is two-way radio contact. In practical life this means that an incoming VFR flight is only required to make sure that the ATC unit serving the airspace has received and acknowledged the callsign correctly in order to enter. Even the correct callsign and a standby from ATC is sufficient to enter and continue the flight. These deviations from the standard rules set by ICAO are for the least dangerous as they are undermining the goal that ICAO had when the new airspace classification was implemented some five years ago: harmonisation and standardisation.

There are many examples of class E, F or G airspace that is immediately adjacent to airspace classes with a higher degree of regulation. Whereas the case of vertical separation between airspace classes meeting at a certain level is solved by ICAO saying that the lower, less restrictive airspace class is applicable when two different airspace classes meet in the vertical plane, the case of airspace classes meeting in the horizontal plane is not yet solved by ICAO. The need for some kind of buffer zone is clearly felt as for instance an IFR flight in class C airspace being vectored close to the airspace border of a 10 N.M. wide airway corridor is expecting a full IFR separation to all other IFR and VFR flights (3 or 5 N.M.) according to the rules for Class C airspace. This cannot be guaranteed as there may well be a VFR flight operating just at the border of the less restrictive airspace putting this aircraft within the required IFR separation minima of the above-mentioned IFR flight.

The fact that ICAO declares airspace of class E as a controlled airspace, but allowing that IFR flights fly in there without ATC-clearance and without two-way radio contact in this same airspace causes problems. The only guaranteed spacing available in Class E airspace remains therefore the old « see and avoid ». Even if ATC does separate all IFR flights and provides traffic information in relation to all known VFR flights, this service does only part of the task of collision avoidance. Particular attention to airspace observation must be given by all flights, including those that are considered to be controlled flights.

According to ICAO a controlled airspace is an 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. There is no doubt that air traffic control service cannot be provided in Class E airspace between IFR and all VFR flights as the latter need neither to maintain two-way communication with ATC, nor require an ATC clearance to operate in this airspace. Furthermore, according to ICAO, a controlled flight is any flight which is subject to an air traffic control clearance.

A VFR-flight that doesn’t call ATC and so unknown cannot be considered as a controlled flight. Saying this, and in order to avoid the ambiguity and confusion created by the ICAO definition of a controlled airspace, it is clear that airspace of class E cannot be considered as controlled airspace type. Control exercised by ATC in this airspace class is only partial and does not cover all airspace users operating in the airspace. The very dangerous mix of controlled and uncontrolled traffic operating in the very same airspace has to be avoided.


There are difficulties of application of several official ICAO procedures as they did not define a VMC minima for class A airspace. For instance « clearances to fly maintaining own separation while in VMC » and also the ICAO radio communication failure procedures require a VMC definition for class A airspace.

The procedures regulating the cancellation of an IFR flight in ICAO document 4444 PANS-RAC lack to mention that the procedure is not applicable in airspace of class A, as VFR flights are prohibited in this class of airspace. This restriction should be added to clarify the situation.

The goal of ICAO was to harmonise and standardise with a menu of 7 different airspace classes, from class A to G, that is applicable on a worldwide basis. Many states used these ICAO airspace class designators but with conditions and procedures that deviate very significantly from the proposed ICAO standard set. There are states that have now « B plus airspace », or « C minus airspace » with major differences to ICAO rules. This application is detrimental to safety and is undermining the worldwide effort of standardisation. This very unsatisfactory situation should be addressed by ICAO and a solution found to overcome it.

All airspace classes that can be considered as controlled airspace must be of an airspace type where all flights must be known to ATC. Therefore two-way radio communication and an ATC clearance prior to entry is the absolute minimum for all flights operating in controlled airspace. Only the fulfilment of these requirements ascertains that all traffic is known to ATC so that collisions can be avoided by either IFR separations or traffic information issued and provided by ATC. Airspace of class E where VFR flights can enter without establishing two-way radio contact and without ATC clearance has therefore to be classified as an uncontrolled airspace type. It is not acceptable to have uncontrolled flights in a controlled airspace type thereby compromising the service provided by ATC.

It is recommended that:

With regard to the ICAO classification of airspace, the definition of controlled airspace is the generic term which covers airspaces of class A, B, C and D. Classes E, F and G being uncontrolled.

Class E is a special case of uncontrolled airspace in which a limited separation service will be provided between defined aircraft.

Last Update: September 28, 2020  

March 4, 2020   746   Jean-Francois Lepage    1997    

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