Vertical Spacing Between Controlled and Uncontrolled Aircraft at or near Airspace Boundaries

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Vertical Spacing Between Controlled and Uncontrolled Aircraft at or near Airspace Boundaries

58TH ANNUAL CONFERENCE, Conchal, Costa Rica, 20-24 May 2019

Agenda Item: B.5.2 – WP No. 87

Vertical Spacing Between Controlled and Uncontrolled Aircraft at or near Airspace Boundaries

Presented by TOC


Vertical spacing between controlled and When handling controlled traffic with minimal distance from uncontrolled traffic, controllers may experience difficulties associated with the spacing with controlled traffic and traffic on which they have no influence, especially across airspace boundaries. These issues may include when and how a controller should intervene to preserve safety.


1.1.  As worldwide commercial traffic increases almost twofold every fifteen years the skies are growing ever more congested. This problem is amplified by the increase in larger aircraft taking to the skies as they are known to cause more severe wake turbulence that may affect other traffic.

1.2.  Aircraft that are operating in uncontrolled airspace in the vicinity of controlled airspace without communicating with ATS units forgo the services that could be provided such as traffic information. Complicating matters further are the unknown intentions of the uncontrolled aircraft. This essentially leaves the controller with one hand tied and only the controlled aircraft to communicate with. Where controlled and uncontrolled airspaces meet, prescriptions for the different airspaces may be contradictory. In this case, controllers may find themselves with only 500 feet spacing, or even less, while their handbook states 1000 feet must be achieved to ensure separation. Similar issues may also present between two adjacent airspaces which utilize different separation minima, but this matter was deemed to be outside of the scope of this paper.

1.3.  A point of concern is the duty of care associated with a controller’s job, which is to say that ATCOs are always expected to provide the optimum level of safety possible. This could cause problems if ATCOs are expected to do something but lack the authority and the tools to do so, or controllers think that maybe they should do something, but their regulations are unclear.


Separation as defined by regulators

2.1. ICAO Document 9689 defines separation as:

Separation is the generic term used to describe action on the part of air traffic services (ATS) to keep aircraft operating in the same general area at such distances from each other that the risk of collision is maintained below an acceptable safe level.

(ICAO DOC 9689, edition 1998 (2002 update), Chapter 1.1)

2.1.1. ICAO Document 9689 also states:

The determination of vertical separation or time and distance-based longitudinal separation minima should be based on the quality of information available to ATC and the pilot.

(ICAO DOC 9689, edition 1998 (2002 updates), Chapter 1.13)

2.2. Types of airspace and rules as laid out in ICAO Annex 11:

2.6 Classification of airspaces

2.6.1 ATS airspaces shall be classified and designated in accordance with the following:

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.

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. Note.— Where air traffic advisory service is implemented, this is considered normally as a temporary measure only until such time as it can be replaced by air traffic control. (See also the PANS-ATM (Doc 4444), Chapter 9.)

Class G. IFR and VFR flights are permitted and receive flight information service if requested.

2.6.2 States shall select those airspace classes appropriate to their needs.

2.6.3 The requirements for flights within each class of airspace shall be as shown in the table in Appendix 4.

Note.— Where the ATS airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B airspace is therefore considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc.

(ICAO Annex 11,14th edition, Chapter 2.6)

2.2.1  There are instances where vertically adjoining airspaces may apply different standards for reasons including, but not limited to airspace design and equipment capabilities. Where these differently classed airspaces share a border then the note in Annex 11 2.6 is pertinent which states the rules of the less restrictive airspace apply on the border.

2.2.2  Airspace should be designed to comply with provisions laid out within Annex 11 and those designs should consider the needs of local ATS users and the capabilities of the providers.

2.3 PANS 4444 details what is expected of controllers when working in controlled airspace. Vertical or horizontal separation shall be provided:

a) between all flights in Class A and B airspaces;
b) between IFR flights in Class C, D and E airspaces;
c) between IFR flights and VFR flights in Class C airspace;
d) between IFR flights and special VFR flights; and
e) between special VFR flights, when so prescribed by the appropriate ATS authority;

except, for the cases under b) above in airspace Classes D and E, during the hours of daylight when flights have been cleared to climb or descend subject to maintaining own separation and remaining in visual meteorological conditions. Conditions applicable to the use of this procedure are contained in Section 5.9. No clearance shall be given to execute any manoeuvre that would reduce the spacing between two aircraft to less than the separation minimum applicable in the circumstances. Larger separations than the specified minima should be applied whenever exceptional circumstances such as unlawful interference or navigational difficulties call for extra precautions. This should be done with due regard to all relevant factors so as to avoid impeding the flow of air traffic by the application of excessive separations. 5-2 Air Traffic Management (PANS-ATM) 10/11//16 Where the type of separation or minimum used to separate two aircraft cannot be maintained, another type of separation or another minimum shall be established prior to the time when the current separation minimum would be infringed.

Note.— Unlawful interference with an aircraft constitutes a case of exceptional circumstances which might require the application of separations larger than the specified minima, between the aircraft being subjected to unlawful interference and other aircraft.

(ICAO DOC4444 PANS-ATM, 16th edition, Chapter 5.2.1)

2.3.1 PANS ATM includes a statement on regional air navigation agreements, also known as SUPPs, which illustrate their importance in maintaining an overall view of how adjoining airspaces interact. Such agreements could help facilitate clear and robust procedures on how to accommodate traffic along their borders.

2.1.2 The requirements in respect of services, systems and procedures applicable to airspaces and aerodromes should be established on the basis of a regional air navigation agreement in order to facilitate the harmonization of ATS in adjacent airspaces

(ICAO DOC4444 PANS-ATM, 16th edition, Chapter 2.1.2)

2.4  Though worldwide airspace is (generally) compliant with the ICAO standards the specifics of how airspace is laid out and utilized may vary widely between ANSPs.

2.5  The IFATCA TPM states that:

MA’s shall urge ATS Authorities to coordinate and harmonise with all neighbouring states their national airspace classification, in accordance with ICAO Annex 11 Appendix 4, 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. Local operational controllers should be involved in the airspace classification process.

(IFATCA TPM, 2018 edition, ATS 3.3)

This policy was adopted at the 2016 conference and remains pertinent. It operates from the principle that problems that are inherent to the airspace one is working in should be solved at the design stage (or by a return to that stage) rather than cause problems for airspace users and the controllers guiding them.

Spacing between aircraft in adjoining airspace classes

2.6 As mentioned in the introduction, where different airspace classifications adjoin, 500 feet spacing or less might exist between two aircraft, without violating minimum separation. On the border of the two, the least restrictive set of rules applies, which can leave separation minima ambiguous.

2.6.1  It can be argued that 500 feet spacing or less between, for example, traffic in class A and traffic in class G is a violation of the separation standard for the IFR traffic. The ambiguity mentioned above arises from several factors. One example is that any altitude information derived from the radar returns of the VFR traffic could be unverified, and in that case cannot be considered trustworthy enough to be used for separation. It could also be procedural ambiguity when 1000 feet vertical separation is prescribed but the airspace does not cater for this in all situations and while ICAO clearly states that the less restrictive airspace rules apply at the common boundary, determining what rules apply when aircraft are operating close to the common boundary requires a closer reading of the provisions.

2.6.2  Consider this hypothetical scenario: PANS-ATM prescribes separation of IFR traffic from VFR traffic in class C. The lower level of our hypothetical class C airspace is FL55, below is class E airspace. An IFR aircraft is assigned FL60. A VFR aircraft is cruising at the boundary, at FL55. PANS-ATM says the less restrictive rules apply on the boundary. Therefore, the VFR is in class E and separation is not required, as it does not find itself in the same airspace class as the IFR traffic.

2.6.3  The lack of altitude verification makes it possible that the VFR traffic might in fact be even closer than the buffer spacing provided between controlled and uncontrolled traffic, creating a situation where an ATCO cannot be presumed to have enough information to resolve a conflict that may or may not exist.

2.6.4  As was pointed out in our discussions, there are certain situations where no buffer zone is created at all. This can happen if the boundary between airspace classes is placed at an IFR level and no further mitigating measures are in place. Even in this case, the design is technically compliant with ICAO requirements as a buffer zone is not mandated.

Wake turbulence

2.7 The increase of aircraft dimensions exacerbates issues with wake turbulence which come into play where airspaces adjoin.

2.7.1 Wake turbulence is a problem in all airspaces and phases of flight. The effects of the vortices are more severe when aircraft are spaced at 500 feet rather than 1000. As most uncontrolled airspace is at lower altitudes, overflying heavier traffic can be expected to be flying at lower speed, which further increases the severity of the vortices.

2.7.2  A relevant incident occurred when a Challenger passed 1000 feet below an Airbus A380 over the Arabian Sea and encountered its wake vortex. This encounter led to the pilots losing control of the Challenger, which went into a spin and suffered major structural damage. Luckily the crew regained control and landed the aircraft safely, but the airframe has since been decommissioned due to the damage sustained during the incident.

2.7.3  Strictly speaking this incident falls outside of the scope of this working paper, but it illustrates the danger that wake turbulence vortices can pose to lighter aircraft. As most traffic in uncontrolled airspaces are likely to be smaller general aviation aircraft, the risk is compounded accordingly. It stands to reason that a similar incident may occur at lower altitude, with lighter aircraft and less spacing. Those encounters might be more severe and critical.

Other influencing factors

2.8 The equipment that ATCOs use to provide air traffic services might be of influence as well. For

example, traffic in uncontrolled airspaces may be filtered so that controllers are not aware that they are overflying uncontrolled traffic with the flights under their supervision.

2.8.1  Another possibility is that the working position rounds mode C altitude numbers in such a way that spacing is less (or more) than the controller believes it to be.

2.8.2  Both of these factors are obviously outside the ATCO’s control, but it must be stressed that the expectation that they are aware of these may still exist.

2.9  When aircraft are operating with minimal spacing, the risk of TCAS alerts increases significantly. While TCAS in and of itself is a valuable safety net that has contributed greatly to aviation safety in general, it is not suited in this situation, as the slightest deviation in flight path might trigger a traffic advisory (TA) or even a resolution advisory (RA) in some cases. This is a common occurrence in airspaces where 500 feet spacing or less exists between controlled and uncontrolled traffic, leading to stressful situations for the ATCOs and crews involved.

Directions for solutions to the issues

2.10  It has been suggested that increasing the buffer zone from a minimum of 500 feet to 1000 feetmight be an adequate solution to the problem. While this might be an applicable solution in some airspaces, others which are more congested would require a radical redesign if such a direction were chosen.

2.11  The current ICAO ruleset covers these issues. If airspace design is compliant with the provisions laid out in Annex 11, ATCOs should assume no responsibility or liability for any incidents that occur in cases where they have followed procedure as it is laid out by their ANSPs. It should not be the ATCO’s responsibility to determine whether procedures are compliant or not while working, but rather design and implementation of airspace and its contingent procedures should be checked for compliance before operational use.

2.12  The above does not offer a solution for the TCAS related aspects of this set of problems, and unfortunately there does not seem to be one which would satisfy all stakeholders. All directions for solution have a decidedly negative effect on one or more.

2.13  There is currently an investigation underway at NLR (Dutch Airspace Laboratory) to investigate the impact of wake vortices with 500 feet spacing at low altitudes. This may offer some more insight into the severity of the issue. At the time of writing it is unknown when this report will be published.


3.1  Spacing of 500 feet or less between controlled and uncontrolled aircraft is common practice in many parts of the world. The contradiction of the ATCOs responsibility on one hand, and their inability to remedy the situation on the other, makes for an ambiguous and undesirable situation.

3.2  Airspace classifications are standardized by ICAO, but layout and implementation vary worldwide. There is no agreed standard for a buffer zone between controlled and uncontrolled traffic.

3.3  It is the opinion of TOC that the current ICAO ruleset offers tools to design airspace that is to the satisfaction of most airspace users, but probably not all, all the time. If ATS users of any kind experience an issue pertaining to the spacing of controlled and uncontrolled traffic, new procedures or airspace may offer a solution for them. IFATCA’s policy on airspace classifications supports this approach. However, it must be noted that configurations where less than 1000 feet spacing is provided are fully compliant with all current and pertinent ICAO regulations.

3.4  It should not be the ATCO’s responsibility to determine whether procedures are compliant or not while working, but rather design and implementation of airspace and its contingent procedures should be checked for compliance before operational use.

3.5  Regarding the above, TOC chooses not to recommend any policy at this time, as airspace classification and procedural spacing should be tailored to local user needs.


4.1  It is recommended that this paper be accepted as information material.


ICAO, 2016. Annex 11 14th Edition.

ICAO, 2016. Document 4444 PANS-ATM 16th Edition.

ICAO, 2002. Document 9689 1998 Edition with 2002 updates.

IFATCA, 2018. Technical and Professional Manual.

FAA, 2014. Advisory Circular 90-23G. Available at:

Last Update: October 2, 2020  

November 3, 2019   439   Jean-Francois Lepage    2019    

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