IBAC Bulletin B01-1

Subject: Use of lateral offsets and the effect on airspace safety.

The following ICAO State Letter and accompanying Guidance Material was prepared and distributed for the guidance of States and ICAO Regional Planning Groups.

Although this material was not prepared for the attention of flight crews nor has any request been made for distribution to flight crews, IBAC considers that it would be appropriate and in the interests of safety for business aviation flight crews to take note of this material.

 Text of ICAO State Letter as follows:  

Ref.: AN 13/11.6-00/96                        3 November 2000

This a circular State letter (to all Contracting States). Sir/Madam,  

1 .                     I have the honour to invite your attention to the attached guidelines on the use of lateral offsets and the effect on airspace safety, which were prepared by the Review of the General Concept of Separation Panel (RGCSP) and approved by the Air Navigation Commission for circulation to States and international organizations.  The material relates to aircraft with highly accurate navigation systems, such as the global navigation satellite system (GNSS), when pilots apply lateral offsets as a safety measure to reduce a perceived increase in the risk of collision in the event of a loss of vertical separation.  

2 .         At present, the application of lateral offsets is unregulated and, in some instances, pilots have applied lateral offsets at their discretion without first obtaining approval from the appropriate air traffic control (ATC) unit.  This practice does not comply with Annex 2 — Rules of the Air, Chapter 3, paragraph 3.6.2.1.1, which states: “Unless otherwise authorized or directed by the appropriate air traffic control unit, controlled flights shall, in so far as 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.”  

3 .         The RGCSP recognized that the magnitude and direction of offset, in some cases, could have an adverse effect on safety.  In this regard, the RGCSP carried out a technical analysis of ATS route configurations to quantify the effect on safety of lateral offsets.  In the time available, only routes in oceanic or remote airspace in a procedural ATC environment were examined, using existing collision risk models.  The analyses showed that there was, potentially, a significant increase in the risk of collision on some route configurations.  

4 .         In view of the safety implications of pilots applying lateral offsets in an unregulated and random manner, the guidelines are intended to assist States and planning and implementation regional groups (PIRGs) to identify routes and airspace where application of lateral offsets could be authorized.  It is considered that the material would assist States to regulate the use of lateral offsets and thereby enhance existing safety levels.  

5 .         States are urged to keep ICAO informed of any safety-related issues associated with the application of lateral offsets in accordance with these guidelines.  

            Accept, Sir/Madam, the assurances of my highest consideration.

                       R.C. Costa Pereira

                        Secretary General

 Enclosure:     Guidelines on the application of lateral offsets  

ATTACHMENT to State letter AN 13/11.6-00/96  

GUIDELINES ON THE USE OF

LATERAL OFFSETS AND THE EFFECT ON AIRSPACE SAFETY

1 .            Introduction  

1 . 1                     These guidelines are based on studies carried out by the ICAO Review of the General Concept of Separation Panel (RGCSP) to address airspace safety issues associated with pilots applying lateral offsets when navigating by the global navigation satellite system (GNSS).  Information received by ICAO indicates that, when navigating by GNSS,  there is a widespread practice among pilots to apply a lateral offset as a safety measure to reduce a perceived increase in the risk of collision due to a loss of vertical separation.  Furthermore, pilots are using various offset values and apply them in different directions, in some cases without obtaining approval from the appropriate air traffic control (ATC) unit.  The impact of the use of lateral offsets on overall airspace safety had not previously been evaluated from a safety perspective, and  the RGCSP carried out a technical analysis of safety-related issues.  These guidelines are based on the results of this analysis and are provided to assist States and regional planning groups to consider air traffic services (ATS) routes and airspace where the use of lateral offsets could be authorized to obtain a safety benefit, thereby enhancing existing levels of safety.  Information is provided for pilots and operators on operational issues concerning the effect of lateral offsets on airspace safety and how a safety benefit could be obtained.  

1 . 2               The RGCSP studies only took into account the effects of lateral offsets on the safety of ATS routes in oceanic and remote area airspace, i.e. where radar service was not provided and controller-pilot communication was by  high frequency (HF) radio operated by a third party operator.  It was also assumed that flight levels would be allocated in accordance with the Tables of Cruising Levels in Annex 2 — Rules of the Air, Appendix 3.  

1 . 3               In regard to the provisions in Annex 2 requiring aircraft to operate on a route centre line, further detailed studies are required on issues related to precision navigation and the risk of collision in the event of a loss of vertical separation for all aircraft operating environments, including  high density terminal airspace where radar service is provided, and route systems based on RNP types less than RNP 10.  In this regard, the tenth meeting of the RGCSP (May 2000) included an item in its work programme to develop requirements for the reduction of separation minima in terminal airspace based on RNP 1 and safety issues associated with precision navigation would be examined.  

2 .       Background                     

2 . 1               In accordance with Annex 2, pilots intending to deviate from the centre line of an ATS route are required to obtain authorization from the appropriate ATC unit.  Annex 2, Chapter 3, paragraph 3.6.2.1.1, states: “Unless otherwise authorized or directed by the appropriate air traffic control unit, controlled flights shall, in so far as 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.”  It should be noted that ICAO separation minima, including lateral route spacing, are based on the assumption that aircraft operate on the centre line of a route; any unauthorized deviation from this requirement could compromise safety.  In view of the potential reduction to the risk of collision due to a loss of vertical separation, which may be achieved by the application of lateral offsets, ATS authorities may wish to consider authorizing the use of lateral offsets in line with these guidelines.  The feasibility of allowing lateral offsets would depend on specific local or regional circumstances such as the configuration of route systems and the effect, if any, of any adjacent airspace.  

3 .       Aircraft navigation performance and airspace safety  

3 . 1                     Aircraft operating navigation systems that use GNSS in the navigation solution achieve significantly better navigation accuracy than those without GNSS, e.g. inertial navigation systems (INS).  A recent study of aircraft navigation performance accuracy in the North Pacific area showed that aircraft equipped with the FANS-1 system, which uses the GNSS to obtain the navigation solution, had a standard deviation of cross-track (lateral) deviations of approximately 0.2 km (0.11 NM) whereas aircraft navigating by other means had a standard deviation of 2.2 km (1.17 NM).  When an operational error results in a loss of vertical separation between aircraft on the same route, a collision may be avoided by virtue of the random lateral deviation.  By reducing the magnitude of lateral deviations from the route centre line, the use of GNSS increases the probability of a collision.  By using offsets to provide lateral spacing between aircraft, the effect of this reduction in random lateral spacing would be mitigated, thereby reducing the risk of collision.  

3 . 2               In the development of separation minima, aircraft navigation accuracy is taken into account.  Therefore, intentional unauthorized deviation by pilots from a route centre line undermines the principles on which airspace and route systems are designed.  In cases where safety analyses have been carried out for route systems, and a minimum safety level has been established, such deviations violate the assumptions on which the analyses were based and may have an adverse effect on the system’s actual safety level.  However, in some cases a lateral offset could achieve a safety benefit and these guidelines provide information on how this could be obtained.  

4 .       The effects on safety of lateral offsets in oceanic and remote area airspace  

4 . 1                     Lateral offsets should not be applied by aircraft which do not use GNSS in the navigation solution, because there would be no reduction in the vertical risk, and no safety benefit would be achieved.  Moreover, in a parallel route system the lateral collision risk may increase significantly.  

Application of lateral offsets on bi-directional single routes  

4 . 2               The application of lateral offsets on bi-directional single routes by aircraft using navigation systems which use GNSS does not adversely affect safety.  In fact, under these conditions, the use of offsets substantially reduces the risk caused by a loss of vertical separation, e.g. due to operational errors.  

Application of lateral offsets on parallel route systems  

4 . 3               In parallel route systems the application of lateral offsets does not adversely affect lateral safety under the following circumstances:  

                        b) the magnitude of the lateral offset does not exceed 1.9 km (1 NM); and  

                        c) the offsets are applied only by aircraft using navigation systems which use GNSS.  

4 . 4                     Offsets applied in parallel route systems under circumstances which differ from those described above could adversely affect the lateral collision risk and are not recommended.  

Application of lateral offsets at track intersections

4.5              Provided that lateral offsets of no more than 1.9 km (1 NM) are applied only by aircraft with navigation systems which use GNSS, the collision risk modelling has shown that the application of lateral offsets on intersecting tracks does not adversely affect system safety at the intersection point.  

4.6        In the case of aircraft operating in the same direction, a safety benefit would only be attained if aircraft in lateral overlap apply a staggered offset.  Procedures would need to be developed for application of such offsets worldwide and further studies are required to provide appropriate procedures.  

1 .          Direction of lateral offset  

5.1                    The offset should be applied to the right of the centre line relative to the direction of flight.   

6.      Magnitude of lateral offset  

6.1        The analysis carried out by the RGCSP has shown that an offset as small as 0.37 km (0.2 NM) significantly reduces vertical risk by reducing the probability that aircraft are in lateral overlap when operating at adjacent flight levels on the same route.  An offset of 1.9 km (1 NM) reduces the probability of lateral overlap by approximately two orders of magnitude as compared to the case of no offset. (The reduction depends not only on the magnitude of the offset, but also on the proportion of aircraft navigating by GNSS).  However, to increase the magnitude of the offset to a distance greater than 1.9 km (1 NM) in a parallel route system yields negligible further reduction in vertical risk, but may adversely affect horizontal risk by decreasing the effective separation between adjacent tracks.  

7.   Other considerations on the use of lateral offsets  

7.1        When planning for the use of lateral offsets States and regional planning groups should take into account the operational consequences of applying lateral offsets and make suitable arrangements for implementation including consideration of the following:  

            b) offsets applied during en-route operations at cruising levels should not affect obstacle clearance criteria.  In cases where this may be an issue, appropriate restrictions on the use of offsets should be taken; and  

            c) these guidelines are not applicable to the use of tactical offsets by ATC.  

8.            Summary of the use of lateral offsets     

8.1        When considering the use of lateral offsets to obtain a safety benefit, the following conditions should be taken into account:  

            a) offsets may be applied in oceanic or remote area airspace only;  

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

            c) the magnitude of the offset is not more than 1.9 km (1 NM) from the route centre line;  

            d) the offset should be carried out to the right of the centre line relative to the direction of flight;  

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

            f) aircraft to be assigned flight levels in accordance with the Tables of Cruising Levels in Annex 2, Appendix 3; and  

            g) offsets may be applied only when authorized by the appropriate air traffic control unit and, on a regional basis, when coordinated by the regional planning group.

   

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