Off–Set Tracking in the North Atlantic (NAT)

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Off–Set Tracking in the North Atlantic (NAT)

38TH ANNUAL CONFERENCE, Santiago, Chile, 15-19 March 1999

WP No. 89

Off–Set Tracking in the North Atlantic (NAT)


Due to the continued reduction in the North Atlantic (NAT) separations, the importance of utilising the airspace between the NAT tracks has now become paramount.

Accuracy of aircraft navigational systems basically means that during periods of intense traffic it is possible to have nine aircraft, one on top of another, with one thousand feet between each during Reduced Vertical Separation Minima (RVSM) 2. When RVSM 3 is introduced the possibility is there for thirteen profiles to be used. This ever increasing density on the NAT tracks will lead to an increase in problems , for pilots and controllers unless something positive is done to alleviate the situation.

Wake Turbulence

Although it is recognised that wake turbulence is not a major problem, there are numerous reports of wake turbulence being experienced on NAT tracks since the introduction of RVSM.

It has been reported that wake turbulence extends below the aircraft for 1700’, therefore, if an aircraft is only 1000’ below, it is extremely possible that wake turbulence will be experienced, especially if the aircraft above is a B747 – 400. If the aircraft below is flying at the same mach number as the aircraft above and along the same track, there exists the possibility of wake turbulence for the whole of the flight within the NAT area.

The effect this has on passenger comfort and aircrew workload can be significant and when not identified quickly enough, can cause extreme problems, Emergency Descent, TCAS RA’s etc.

To alleviate this problem, a NOTAM was issued to permit aircrew, if they considered it was required, to fly off-set of the cleared track by a maximum of two miles without having to obtain a re-clearance from the appropriate ATC agency. The only requirement is to notify ATC of the fact.

This action results in the problem associated with wake turbulence apparently being resolved.

The issuing of the NOTAM has addressed the problem of wake turbulence but not in a structured and controlled way.


It is a fact that the current TCAS II logic with software version 6.04A does not recognise 1000’ vertical separation above FL 290 as safe and will therefore in certain circumstances either issue a traffic Advisory or a Resolution Advisory.

Appropriate instructions and training have been given to aircrew as part of their RVSM approval, therefore the problem has been identified and mitigated through training.

Flying random off-set and training regarding the interpretation of TCAS alerts resolves the problems, however what IFATCA believes is that there is a requirement for a more permanent solution.


The area between tracks in the NAT system is sixty miles laterally. This area could be utilised to provide a structured off-set by without reducing separation standards but resolving the two previously identified problems.

The proposal therefore is that aircraft will fly the NAT track system off-set from the centre line dependent on the level flown.

This off-set would be determined through the appropriate safety analysis. It is assumed that the approved random 2 nm off-set currently used meets the safety requirement, then it is proposed to use that as the off-set standard in the NAT.

In order to regulate the off-set it is proposed to have aircraft fly an off-set appropriate to the level flown:

  • Aircraft flying at even RVSM levels 320, 340 , 360, 380, 400 would fly off-set to the right of track;
  • Aircraft flying at odd RVSM levels 310, 330, 350, 370, 390 would fly off-set to the left of track.

The most common manoeuvre when established on tracks is to change level. In the off-set scenario this would be achieved by the aircraft climbing in the current off-set mode and adjusting when level at the new level should that be necessary in the case of climbing from FL 310 to FL 340 or remaining in the same off-set position when climbing from FL310–FL330.

Modern aircraft avionics and navigation capability make this a simple enough task which could be introduced with appropriate pilot training.


When off-set is in operation there exists two thousand feet between aircraft negating the effect of wake turbulence.

Removing the need to embargo use of opposite direction levels e.g. FL 370 eastbound against FL350 and FL360 westbound.

Reduction in the nuisance TA’s and RA’s.

When entering or exiting the OAC areas from a radar environment there would be:

(i)  Exiting – easier identification;

(ii)  Entering – availability of later climbs to OAC level;

(iii)  Exiting – availability of change of level without the extensive use of radar headings.

Improvement in passenger comfort e.g. seat belt signs on/off, meals service, etc. with a reduction in pilot workload.


The reduction in non radar separations in the case the introduction of RVSM in the North Atlantic as a trial has highlighted two problems TCAS and Wake Turbulence.

The effect of nuisance TAs/RAs may be resolved when software version 7 is operational, but until that time, flying a off-set in NAT airspace would alleviate the problem now.

The effects of in-trail wake turbulence will only be resolved by flying off-set as there is no technical solution available.

IFATCA in co-ordination with IFALPA should take a consensus approach on this issue and make relevant proposals to North Atlantic System Planning Group (NATSPG) for their consideration.

It is recommended that:

Subject to the appropriate Collision Risk Modelling, an off-set procedure is to be adopted in the NAT region as detailed in appendix A that prior to aircraft establishing on tracks that they establish to the following off-set procedure:

  • Off-set left when flying at odd levels i.e. 310, 330,350, 370 390;
  • Off-set right when flying at even levels i.e. 320,340,360,380, 400;
  • That the off-set shall be 2nm.

Last Update: September 28, 2020  

March 10, 2020   257   Jean-Francois Lepage    1999    

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