Review of Lateral and Longitudinal Separation

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Review of Lateral and Longitudinal Separation

25TH ANNUAL CONFERENCE, San José, Costa Rica, 21-25 April 1986

WP No. 57

Review of Lateral and Longitudinal Separation


At IFATCA 83 , Committee B discussed WP 57/83, presented by New Zealand . SC 1 produced WP 61/84 at Estoril and New Zealand an additional paper WP 61/84. This last paper introduced longitudinal separation into the previous review on lateral separations. At IFATCA 85 SC 1 presented WP 51/85 and was subsequently tasked with the production of a questionnaire for distribution to MA’s. The results would form the basis of an SC 1 brief to the Executive Board in their discussions with ICAO.

The questionnaire was distributed via the secretariat in early 1985 following conference. Replies were to be returned by July 31st. 1985. 16 MA’s replied in addition to information derived from a question sheet distributed at IFATCA 85. In total 18 member associations have provided input on this subject since 1983.

Previous working papers have been accepted as guidance material or for information. These previous papers for an essential part of the background to the Executive Board brief.

Discussion will be on the basis of a question by question report received from MA’s plus where appropriate additional comments. It should be noted that some questions were not answered so for some questions the replies were less in number than the total number of replies.

Question 1: Are your separation standards written down in:

a) ICAO Doc 4444 PANS/RAC;
b) National Air Traffic Service Documents;
c) UK;
d) USA;
e) Other.


9 MA’s used their own national documents; 2 used PANS/RAC; and 7 used national documents in conjunction with PANS/RAC.

Question 2: If you were b, c, d, or e in question 1, are the separation standards:

a) The same as ICAO 4444;
b) Use greater separations that ICAO 4444;
c) Use smaller separations than ICAO 4444.


No clear trend appeared in the answers, as some used PANS/RAC, some used separations which were greater or smaller than PANS/RAC, and some nations varied between exceeding and reducing the PANS/RAC separations. There was no obvious correlation between observance of separation standards and documentation as discussed in 2.2 (Q1) except that most nations which issued their own documents tended to increase lateral separations.

Question 3: ICAO Doc 4444 talks of increases in separation standards to allow for error in track keeping, time accuracy etc. Do your separation standards have an equivalent paragraph to PANS RAC page 3-4 para 7?


About half of the MA’s who replied were able to identify that their standards contained an allowance for errors in track keeping or time accuracy.

Question 4: Do your separation standard documents give guidance on methods of application by the controller?


10 MA’s received guidance on applying separation standards, 1 received guidance under some circumstances; but 6 members considered their documents lacked any guidance on application techniques.

Question 5: Do your separation standard documents give guidance on phraseology to be used when applying separation standards?


Guidance on phraseology to be used when applying separations was given to 50% of the MA’s though one had to seek the information in a separate document.

Question 6: Do your separation standard documents allow for aircraft routing off track to avoid weather etc.?


Only 7 members had documents which allowed for aircraft routing off track. 10 MA’s were unaware of any allowance. Only 2 MA’s required aircraft to request rather than inform ATC of any deviation in track. This item is of particular concern to operations on long ill-defined routes such as oceanic, desert or remote mountainous or arctic regions.

Question 7: Do your separation standard documents define the phrase:

a) crossing track
b) diverging track
c) converging track


50% of MA’s had a definition of crossing track, but only 6 defined diverging track and 5 converging track. 11 MA’s had no definition for the last two situations. Where crossing track was defined the definition fell into two broad categories based on differences in angular difference in track and one or two individual cases. The UK adopts a different approach incorporating the information within the separation standard. This is published in diagrammatic form and makes allowance for speed difference creating a phased lateral separation based on angular difference and time.

Question 8: In ICAO Doc 4444 PANS RAC Page 3-5, are listed several lateral separation minima. If your separation standards are the same or less than ICAO 4444 para, do you use the separation for :

a) diverging tracks only
b) diverging and converging tracks only
c) diverging, converging tracks, and tracks with one aircraft inbound to and one aircraft outbound from the fix.


6 MA’s only used the minima in PANS/RAC 3-5, for diverging tracks only. 12 used the separations for diverging and converging tracks and even for separating tracks with one aircraft inbound to and one outbound from the beacon. Many states have increased these separations in various ways or seek to achieve track separation by differing methods which effectively increase the minima. Others apply these rules to VOR’s and not NDB’s.

Question 9: Do your documents on applying lateral separations require you to establish that aircraft are on track before applying the lateral separation?


13 MA’s were required to establish that aircraft were established on track prior to breaking vertical separation. Only 4 MA’s did not. In some cases the requirement was for some but not all separation standards and at least one member requested the information as a matter of common sense though not required to.

Question 10: In your judgement or to your knowledge do you feel that your separation standards are adequate?


14 MA’s felt that in their knowledge or judgement their minima were satisfactory. 3 members were not satisfied with their minima.

Question 11: In your experience, does your Air Traffic Service have operational difficulties when applying separation standards, as written in your documents?


4 members considered they had operational difficulties in applying separations, but 13 members did not.

To conclude

Interpretations have been made on the basis that where MA’s have frequent experiences of dissent from PANS/RAC, then those procedures have some relevance in determining separation.

No attempt to quote values for separation has been made, only an attempt to establish principles for operating and applying separation minima.

Some care should be taken in interpreting the answers to 2.11 and 2.12 as both are dependent on the traffic loading on a particular ATC service. Even a difficult to apply procedure, or one which is considered dangerous can be accommodated in an environment of lost traffic, lots of airspace, the opportunity to monitor traffic without distraction or increase minima as required. Similarly such conditions may hide inadequacies in an ATC system until traffic growth makes them apparent.

The Executive Board are to note the frequent use of the ICAO separations in PANS/RAC 3-4 , in circumstances which on the evidence of the questionnaire meet with widespread distrust. Many states and MA’s have increased the minima or limited the potential applications or even found alternative methods of achieving this separation. In particular, the use of this separation between traffic which is not diverging from a beacon, or is diverging from an NDB rather than a VOR is treated with suspicion or is increased or is not used.

The attention of the Executive Board is drawn to PANS/RAC in particular to page 5 para 4 & 5 which give guidance on reductions in separations. PANS/RAC procedures for reducing separations are on page 3-12 para 9, and 4-1 para 1.

It is recommended that:

Except in accordance with PANS/RAC 3-12 and 4-1, separation standards shall not be reduced below ICAO minima.

Published separation standards are to state that they contain an element of separation to allow for track keeping errors or time keeping errors. This element should be published within the source documents on ATC separation standards.

Publications promulgating separation standards are required to include guidance material on practical methods of application and the associated phraseology.

ICAO should define procedures in order that ATC is always made aware of a known divergence from track.

ICAO should define the terms “Crossing Track”, “Diverging Track” and Converging track” and the definition will require a stated angular value between tracks.

ATC are required to establish that all aircraft about to be separated by methods involving track difference are established on those appropriate tracks before applying the separation.

Last Update: September 20, 2020  

December 1, 2019   777   Jean-Francois Lepage    1986    

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