The Use of Lateral Offsets

The Use of Lateral Offsets

40TH ANNUAL CONFERENCE, Geneva, Switzerland, 19-23 March 2001

WP No. 88

The Use of Lateral Offsets

Introduction

The subject of Lateral Offsets has been an agenda item within SC1 for the past 3 years. Policy in relation to the North Atlantic has been developed which recommended “the mandatory use of 2 nm offset in which flights an odd levels (310,330 etc) would fly offset to the left and flights at even levels would fly offset to the right (320,340 etc).” However last year, RGCSP considered the issue of lateral offsets in a wider sense and produced draft guidance on the use of lateral offsets. At the November meeting of SC1, it was agreed to produce an information paper in order to bring the subject to the attention of Annual Conference. As the timescale for preparation has been extremely limited, the purpose of this paper is to highlight the salient points in the ICAO draft guidance with a view to expanding on the subject in the year 2001. Also, it should be noted that any addition or amendment applied to the draft guidance (03 August 2000) has not been taken into account in this paper. Additionally, an annex has been attached which underlines existing IFATCA policy for the lateral offsets in the North Atlantic and emphasises the safety benefits if adopted by States and regional authorities within the NAT.

Discussion

Annex 2 Chap 3 para 3.6.2.1.1 states: “unless authorized or directed by the appropriate air traffic control unit, controlled flights shall, in so as is practicable: (a) when on an established ATS route, operate along the defined centre line of that route; or (b) when on any other route, operate directly between the navigation facilities and/or points defining that route”. ICAO separation minima, including lateral spacing, are based on aircraft operating on the centre line of a route andanyunauthorizeddeviationfromthisrequirementcouldcompromisesafety. However,ifan appropriate ATC unit approved a deviation then the minima would not be compromised.

What is the problem? The increasing availability and use of GNSS for navigational purposes allows pilots to fly their aircraft much more accurately and therefore adhere much closer to the published routes than was possible before. ICAO quote studies of aircraft navigation performance accuracy in the North Pacific that showed FANS-1 aircraft using GPS were achieving a standard deviation of cross track (lateral) deviations of approximately 0.2km (0.11nm) as compared to non GPS aircraft having a standard deviation of 2.2km (1.17nm). It can be seen that, when an operational error occurs resulting in a loss of vertical separation, there is an element of random lateral or longitudinal separation in the case of the non GPS aircraft that may avoid a collision. In the same circumstances for the GPS equipped aircraft, the element of randomness is decreased by nearly 90% thereby increasing the risk of collision. One of the ways to mitigate this risk is to use a lateral offset.

ICAO had received information that it was widespread practice used by pilots when navigating by GNSS to apply a lateral offset to reduce the perceived risk of collision in the circumstances of a loss of planned vertical separation. Furthermore, pilots were using different values for offsets and also in some cases, without the approval of the ATC unit. In the development of separation minima, aircraft navigation accuracy is taken into account therefore unauthorized deviation from a route centre line undermines the principle on which airspace and route systems are designed. In cases where safety analyses were carried out for route systems and a minimum safety level established, such deviations violate assumptions on which the analyses were based and may have an adverse effect on the system’s actual safety level. The following is the guidance provided by ICAO for the use of lateral offset to achieve a safety benefit:

a) offsets may only be applied in oceanic or remote area airspace, i.e. in airspaces that do not have a ground navaid infrastructure to support highly accurate navigation or radar service;

b) in a parallel route system, an offset is only to be used when the route spacing is 93km (50nm) or more;

c) the magnitude of the offset is not more than 1.9km (1.0nm) from the route centre line;

d) the direction of the offset is applied to the right of the centre line relative to the direction of flight;

e) on uni-directional flights, the offset is applied subject to the pilots of proximate pairs of aircraft co-ordinating with each other as to which aircraft applies the offset;

f) the offset is only applied by aircraft using navigation systems which use GNSS to obtain the navigation solution;

g) aircraft to be assigned to flight levels in accordance with the Tables of Cruising Levels contained in Annex 2 Appendix 3: and

h) offsets may only be applied when authorized by States or regional authorities.

ICAO acknowledges that, in developing the above guidelines, only the en-route operating environment in oceanic or remote area airspace had been taken into account and that detailed analysis of the effects of lateral offset would need to be carried out for other airspace environments. In this regard, RGCSP have included an item on its work programme to develop requirements for the reduction of separation minima in terminal airspace based on RNP 1; and safety issues associated with precise navigation would be examined.

It should be noted that IFATCA is in general agreement with the above guidance except in the following paras:

c) IFATCA believes the minimum should be 2nm which is being used regularly in the NAT on adjacent tracks with 60 nm lateral separation and

e) IFATCA totally opposes the principle of pilots co-ordinating with each other to decide who will offset – this would lead to a confusing and potentially unsafe situation.

It is not within the remit of this paper to comment on the IFALPA position but suffice to report that the pilots believe that the safety benefits of lateral offsets can be realised in areas other than “oceanic/remote airspace with single routes or route spacings of 50nm or greater”. A further concern expressed relates to the risk of gross (vertical) navigation errors that have occurred and will continue to do so.

Two incidents illustrate the IFALPA concerns:

On 28 June 1999, two Boeing B747 had a serious airprox in Chinese airspace caused by a faulty input in the TCAS. The IFALPA view of this incident is that it would have been a collision if both aircraft had been equipped with GNSS sensors.

On 2 Oct 2000 in North Atlantic airspace, an A330, 1000ft above an A340 were in clear air with the A330 overtaking with the A340 slightly to its right and virtually abeam it when the incident occurred. Both aircraft experienced turbulence and the A340 was observed to climb rapidly through the A330’s level (uncommanded autopilot disconnect followed by rapid change in pitch angle). The main thrust of the AAIB Report is that a risk of collision occurred because one aeroplane unexpectedly left its cleared level and – with a high rate of climb – crossed the level occupied by another aeroplane with little lateral separation.

IFATCA has noted the concerns of IFALPA and will wish to continue a dialogue on the various issues.

Conclusion

IFATCA has existing policy on lateral offsets for the North Atlantic. This policy was developed as a result of safety concerns, particularly wake turbulence and false TCAS alerts, arising from the implementation of RVSM and the accuracy of the aircraft navigation. Lateral offsets up to 2 nms are used regularly in the NAT but it is an ad hoc system.

ICAO has produced draft guidance for the use of lateral offsets, which is restricted to GNSS equipped aircraft operating within oceanic/remote areas. Whilst there is a large area of agreement between the IFATCA position and that of the ICAO guidance, IFATCA differs in that it recommends a mandatory 2nm lateral offset in the NAT to be used by RVSM certified aircraft. Also, it disagrees with the ICAO guidance that pilots of proximate pairs should co-ordinate an offset on uni-directional flights.

IFATCA supports the view that the use of lateral offsets must be considered in the light of safety benefits to be gained; that current separation minima take account of navigational accuracy and that any deviation has the potential to erode the safety margins. Nonetheless, it is accepted that the risks associated with either gross (vertical) navigational error or loss of planned vertical separation are increased with the use of highly accurate navigational systems. It is proposed that the IFATCA policy be extended beyond the NAT to include other airspace environments.

References

ICAO Annex 2 – Rules of the Air.

ICAO Guidelines on the use of Lateral Offsets and the effect on Aircraft Safety (Draft 03 Aug 00).

ICAO Doc 9689 – Manual on Airspace Planning Methodology for the Determination of Separation Minima.

Last Update: September 29, 2020  

March 12, 2020   282   Jean-Francois Lepage    2001    

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