47TH ANNUAL CONFERENCE, Arusha, Tanzania, 10-14 March 2008
WP No. 90
Study Strategic Lateral Offset Procedure
Presented by TOC
Strategic Lateral Offset Procedures (SLOP) are mitigating the risk of vertical collision and wake turbulence encounters between aircraft with high precision navigation capabilities. Provisions of SLOP are published by ICAO in Doc 4444 Air Traffic Management as special procedures for oceanic and remote continental airspace. Strategic Lateral Offset Procedures are pilot initiated and more or less transparent to controllers. After the mid-air collision in Brazil, some stakeholders in air transport industry highlighted the issue that SLOP have not been implemented by Air Traffic Services (ATS) authorities as widely as possible. ICAO has since started to investigate and develop SLOP to extend its applicability to other than oceanic and remote airspaces.
IFATCA has policy from 1988, which is recommended to be replaced by provisional policy taking account current ICAO provisions and development for possible extension. SLOP is also recommended to remain in TOC work program for the 2008 – 2009.
1.1. Since 1998 Strategic Lateral Offset Procedures (SLOP) have been studied and discussed regularly in IFATCA. This paper includes only the most recent developments of the subject which are not covered in previous policy papers or information material.
1.2. The topic was included in Technical and Operations Committee (TOC) work program for 2007 – 2008 in Istanbul conference after mid-air collision in Brazil had provoked discussion about wider global utilisation of SLOP.
1.3. Currently ICAO has somewhat mature and well researched procedures applicable for Oceanic and Remote airspaces, where aircraft equipped with automatic offset tracking capability could be permitted to offset 1 or 2 NM right of the centre line relative to the direction of flight.
1.4. Investigation to implement strategic lateral offsets to other airspaces has been initiated in 2007 by ICAO, which necessitates the review of relevant IFATCA Policy.
2.1.1. The current policy dates from 1998 and resides under Separations –section of Technical Policy. Specifically it is one of the policies about the use of 1000ft vertical separation above FL290.
IFATCA Policy is:
|“IFATCA endorses the investigation of the concept regarding the use of lateral offset in Oceanic RVSM airspace.”|
2.1.2. In the IFATCA Technical and Professional Manual there are three comprehensive information papers about The Use of Lateral Offsets. These documents have been working papers in development of previous IFATCA policies:
- WP 107 – Toulouse 1998
- WP 88 – Geneva 2001
- WP 92 – Cancun 2002
2.1.3. In respective Conferences Committee B discussions have been generally accepting SLOP as a proper procedure against wake turbulence and collision risk in oceanic and remote airspaces. However, there have been concerns should random SLOP be extended to ie. high density airspaces, that it could cause significant problems to the controllers’ function.
2.1.4. A questionnaire was distributed to IFATCA member associations during October – November 2007 to capture practical experiences of the procedure from those whose airspace Strategic Lateral Offset is already in use and to find out how would controllers feel if such procedure is to be newly implemented in their airspace. Replies have been used in preparation of this paper and will be guiding possible studies in future beyond Arusha Conference.
2.1.5. Main concern of IFATCA TOC is how current SLOP procedures will suit into airspace where ATS surveillance service is provided. As basically there is no requirement for pilot to inform ATC e.g. about commencing or terminating strategic lateral offset, these manoeuvres up to 2NM might cause problems being something unexpected to the controller.
2.2.1. Chapter 15 of ICAO Doc 4444 Air Traffic Management contains procedures for strategic lateral offsets. SLOP provisions are in the paragraph of Special procedures for in-flight contingencies in oceanic airspace. See Appendix A.
2.2.2. This location has been questioned to be misleading since SLOP is considered by various stakeholders to be a standard operating practise for risk mitigation, not a contingency measure. ICAO headquarters has indicated that SLOP provisions will be relocated from contingency section as soon as possible.
2.2.3. Offset procedures in Doc 4444 Air Traffic Management are based on studies made by ICAO Separation and Airspace Safety Panel (SASP). These studies are being described more detail in three ICAO state letters. IFATCA expressed acceptance of interim guidelines in state letter annexed to SASP report (WP 78) in Cancun conference 2002. Most recent State Letter is the “Revised guidelines on the use of lateral offsets” published in August 2004.
2.2.4. By the request of Vince Galotti (Chief of Air Traffic Management in ICAO Air Navigation Bureau) SASP meeting in May 2007 decided to start investigating possible SLOP provisions for ‘domestic’ airspaces. The Project Team (PT9) was set up with IFALPA SASP representative as a coordinator.
188.8.131.52. Smaller offset values (ie. hundreds of meters or fractions of a nautical mile) have been looked at. This would require support by aircraft manufacturers, as most avionics used today allow offset input only by whole nautical mile (NM) values.
184.108.40.206. Project team have also been considering how far the use of current 1NM or 2NM strategic lateral offset could be progressed in domestic airspace.
2.3. SLOP usage
2.3.1. Published Strategic Lateral Offset Procedures are available ie. in North Atlantic (NAT) and Pacific (PAC) Oceanic Areas.
2.3.2. Most recent introduction of SLOP took place in China in November 2007 parallel with the RVSM implementation.
2.3.3. SLOP execution requires pilot initiation, and hence it has been noted that it is not used as regularly as intended. Figures by NATS Scottish & Oceanic Area Control Centre have shown that majority of the airlines operating in NAT used to be offsetting only in very irregular basis.
Figure showing the Flight Management System display while flying offset.
2.3.4. Awareness rising is being done by ICAO North Atlantic Programme Coordination Office. www.nat-pco.org –website includes a short promotion film and current procedures within NAT area. It has been noted that SLOP utilisation by airlines in NAT area has been increasing recently.
2.3.5. There are some local examples of strategic lateral offset procedures, which are not strictly as the ICAO SLOP, but are aiming at the same goals.
220.127.116.11. In Sweden it is recommended in AIP that capable operators should fly offset right 0.1 NM when flying enroute on ATS routes.
18.104.22.168. In Israel in one congested bi-directional domestic ATS route (J10) 1NM offset is programmed in FMS for safety margin and to avoid merging of radar symbols.
2.4. International Federation of Air Line Pilots’ Associations (IFALPA)
2.4.1. IFALPA has been very proactive in developing and promoting offset procedures. It has several detailed pol-stats and draft policies regarding lateral offsets. Some of them from as early as 1981.
2.4.2. Prompted by Brazilian mid-air collision IFALPA conference in March 2007 issued a press-release calling ICAO member states to implement SLOP in all appropriate airspaces and continue development of advanced off-set tracking procedures.
2.4.3. In autumn 2007 IFALPA ATS committee has been reviewing and updating some of their offset policies to be formalised in their conference in Mexico City 18th-22nd April 2008.
22.214.171.124. These policies propose that aircraft with navigation equipment capable of programming offsets should be allowed and required to use the ICAO SLOP. By this IFALPA is now concentrating more on implementation effort, as they would ultimately prefer SLOP to be the global norm with only differences published.
126.96.36.199. IFALPA requires accordingly that offset value should be taken into account in design of airspace and route network.
188.8.131.52. Additionally IFALPA is striving for development of advanced lateral offset, where offset would be embedded and automatic f.ex in aircraft Flight Management System (FMS).
3.1 Strategic Lateral Offsets are well researched and established procedures as currently published by ICAO to be applicable in oceanic and remote continental airspace. These have been endorsed by IFATCA to enhance safety by mitigating risk of vertical collision and wake turbulence encounters.
3.2 IFATCA policy from 1988 is mostly outdated as SLOP are currently applicable in majority of oceanic RVSM airspaces.
3.3 It has also been recognized that extension to other airspaces than oceanic and remote continental needs extensive research considering at least the controllers function in airspace where ATS surveillance service is provided.
It is recommended that;
4.1 IFATCA policy on page 3 2 7 2 of the IFATCA Manual:
“IFATCA endorses the investigation of the concept regarding the use of lateral offset in Oceanic RVSM airspace.”
4.2 IFATCA Provisional Policy is:
“IFATCA endorses Strategic Lateral Offset Procedures (SLOP) in oceanic- or remote continental airspace where there is no ATS surveillance service provided” “Further research is required before IFATCA can endorse any offset procedure in airspace where there is ATS surveillance service provided”
and is included on page 3 2 3 19 of the IFATCA Manual.
4.3 Study new provisions for Strategic Lateral Offset Procedures is included in TOC work program for the year 2008 – 2009.
IFATCA Manual, Amendment No. 52 – July 2007.
IFATCA WP 107 – Tolouse 1998.
IFATCA Report of the 37th Annual Conference, Tolouse 1998.
IFATCA WP 88 – Geneva 2001.
IFATCA Report of the 40th Annual Conference, Geneva 2001.
IFATCA WP 78 – Cancun 2002.
IFATCA WP 92 – Cancun 2002.
IFATCA Report of the 41st Annual Conference, Cancun 2002.
Member Association Replies to IFATCA questionnaire SLOP.
ICAO Annex 2 Rules of the Air.
ICAO Doc 4444 Air Traffic Management.
IFATCA report ICAO SASP WG-WHL-11, May 2007.
ICAO SASP WG/WHL/12 – WP/21.
ICAO SASP WG/WHL/12 – Appendix F.
ICAO NAT FIG/15 – WP16 (February 2007).
ICAO NAT SPG/43 – Summary of discussions and conclusions (June 2007).
IFALPA Press Release – 08PRL01 (March 2007).
IFALPA ATS 2/2007 Discussion Paper – Review of Offset Tracking Policies (08ATS052).
15.2.4 Procedures for strategic lateral offsets in oceanic and remote continental airspace
Note 1.— Annex 2, 184.108.40.206.1 requires authorization for the application of strategic lateral offsets from the appropriate ATS authority responsible for the airspace concerned.
Note 2.— The following incorporates lateral offset procedures for both the mitigation of the increasing lateral
overlap probability due to increased navigation accuracy, and wake turbulence encounters.
Note 3.— The use of highly accurate navigation systems (such as the global navigation satellite system (GNSS)) by an
increasing proportion of the aircraft population has had the effect of reducing the magnitude of lateral deviations from the route centre line and, consequently, increasing the probability of a collision, should a loss of vertical separation between aircraft on the same route occur.
220.127.116.11 The following shall be taken into account by the appropriate ATS authority when authorizing the use of
strategic lateral offsets in a particular airspace:
a) strategic lateral offsets shall only be authorized in en route oceanic or remote continental airspace. Where
part of the airspace in question is provided with an ATS surveillance service, transiting aircraft should normally be allowed to initiate or continue offset tracking;
b) strategic lateral offsets may be authorized for the following types of routes (including where routes or route systems intersect):
1) uni-directional and bi-directional routes; and
2) parallel route systems where the spacing between route centre lines is not less than 55.5 km (30 NM);
c) in some instances it may be necessary to impose restrictions on the use of strategic lateral offsets, e.g.
where their application may be inappropriate for reasons related to obstacle clearance;
d) strategic lateral offset procedures should be implemented on a regional basis after coordination between all States involved;
e) the routes or airspace where application of strategic lateral offsets is authorized, and the procedures to be followed by pilots, shall be promulgated in aeronautical information publications (AIPs); and
f) air traffic controllers shall be made aware of the airspace within which strategic lateral offsets are authorized.
18.104.22.168.1 The decision to apply a strategic lateral offset shall be the responsibility of the flight crew. The flight crew shall only apply strategic lateral offsets in airspace where such offsets have been authorized by the appropriate ATS authority and when the aircraft is equipped with automatic offset tracking capability.
22.214.171.124.2 The strategic lateral offset shall be established at a distance of 1.85 km (1 NM) or 3.7 km (2 NM) to the right of the centre line relative to the direction of flight.
Note 1.— Pilots may contact other aircraft on the interpilot air-to-air frequency 123.45 MHz to coordinate offsets.
Note 2.— The strategic lateral offset procedure has been designed to include offsets to mitigate the effects of wake
turbulence of preceding aircraft. If wake turbulence needs tobe avoided, one of the three available options (centre line, 1.85 km (1 NM) or 3.7 km (2 NM) right offset) may be used.
Note 3.— Pilots are not required to inform ATC that a strategic lateral offset is being applied.
Last Update: September 29, 2020