Strategic Lateral Offset Procedure (SLOP)

Strategic Lateral Offset Procedures (SLOP)

Note 1.— SLOP are approved procedures that allow aircraft to fly on a parallel track to the right of the centre line relative to the direction of flight to mitigate the lateral overlap probability due to increased navigation accuracy and wake turbulence encounters. Unless specified in the separation standard, an aircraft’s use of these procedures does not affect the application of prescribed separation standards.

Note 2.— Annex 2, 3.6.2.1.1, requires authorization for the application of strategic lateral offsets from the appropriate ATS authority responsible for the airspace concerned.

16.5.1 Implementation of strategic lateral offset procedures shall be coordinated among the States involved.

Note.— Information concerning the implementation of strategic lateral offset procedures is contained in the Implementation of Strategic Lateral Offset Procedures (Circular 331).

16.5.2 Strategic lateral offsets shall be authorized only in en-route airspace as follows:

a) where the lateral separation minima or spacing between route centre lines is 42.6 km (23 NM) or more, offsets to the right of the centre line relative to the direction of flight in tenths of a nautical mile up to a maximum of 3.7 km (2 NM); and

b) where the lateral separation minima or spacing between route centre lines is 11.1 km (6 NM) or more and less than 42.6 km (23 NM), offsets to the right of the centre line relative to the direction of flight in tenths of a nautical mile up to a maximum of 0.9 km (0.5 NM).

16.5.3 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). 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. Route conformance monitoring systems shall account for the application of SLOP.

16.5.4 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.

Note 1.— Pilots may contact other aircraft on the inter-pilot 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 to be avoided, an offset to the right and within the limits specified in 16.5.2 may be used.

Note 3.— Pilots are not required to inform ATC that a strategic lateral offset is being applied.

Source: ICAO Doc 4444, ¶16.5

While ATC clearances are designed to ensure that separation standards are continually maintained for all traffic, errors do occur. Neither flight crews nor controllers are infallible. Gross Navigation Errors (usually involving whole or half latitude degree mistakes in route waypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by the controller. Ironically, when such errors are made, the extreme accuracies of modern navigation and height keeping systems themselves increase the risk of a collision. Within an ATS Surveillance environment where VHF communications are available, controllers alerted to such errors will intervene using VHF voice communications. In areas (surveillance or otherwise) where VHF voice communication is not available, controllers rely on voice and data link position reports augmented by ADS‐C and ADS‐B transmissions to monitor conformance. Controllers, when alerted to errors, will intervene using HF, CPDLC, SATVOICE or any other means available. Given the potential delay in intervention, it has been determined that encouraging aircraft operating in the NAT to fly self‐selected lateral offsets provides an additional safety margin and mitigates the risk of traffic conflict when non‐normal events (such as aircraft navigation errors, height deviation errors and turbulence induced altitude‐keeping errors) do occur. Collision risk is significantly reduced by application of these offsets. These procedures are known as “Strategic Lateral Offset Procedures (SLOP)”.

Source: NAT Doc 007, ¶8.5.8.

This procedure provides for offsets within the following guidelines:

a) an aircraft may fly offsets right of centreline up to a maximum of 2 NM; and

b) offsets left of centreline are not permitted.

Source: NAT Doc 007, ¶8.5.9.

Distributing aircraft laterally and equally across all available positions adds an additional safety margin and reduces collision risk. SLOP is now a standard operating procedure for the entire NAT region and flight crews are required to adopt this procedure as is appropriate. In this connection, it should be noted that:

a) Aircraft without automatic offset programming capability must fly the centreline.

b) Aircraft able to perform offsets in tenths of nautical mile should do so as it contributes to risk reduction.

c) It is recommended that flight crews of aircraft capable of programming automatic offsets should randomly select flying centreline or an offset. In order to obtain lateral spacing from nearby aircraft (i.e. those immediately above and/or below), flight crews should use whatever means are available (e.g. ACAS/TCAS, communications, visual acquisition, GPWS) to determine the best flight path to fly.

d) An aircraft overtaking another aircraft should offset within the confines of this procedure, if capable, so as to minimize the amount of wake turbulence for the aircraft being overtaken.

e) For wake turbulence purposes, flight crews should fly one of the offset positions. Flight crews may contact other aircraft on the air‐to‐air channel, 123.450 MHz, as necessary, to co‐ ordinate the best wake turbulence mutual offset option. (Note. It is recognized that the flight crew will use their judgment to determine the action most appropriate to any given situation and that the pilot‐in‐command has the final authority and responsibility for the safe operations of the aircraft. See also Chapter 13).

f) Flight crews may apply an offset outbound at the oceanic entry point and must return to centreline prior to the oceanic exit point unless otherwise authorized by the appropriate ATS authority or directed by the appropriate ATC unit.

g) There is no ATC clearance required for this procedure and it is not necessary that ATC be advised.

h) Voice Position reports should be based on the waypoints of the current ATC clearance and not the offset positions.

i) Aircraft shall not apply SLOP below F285 in the Reykjavik CTA and Bodo OCA.

j) The offset should be applied from the time the aircraft reaches its cruising level until top of descent.

Source: NAT Doc 007, ¶8.5.10.

Your SOPs should include SLOP for all oceanic crossings. NOTAMs, State AIPs, and other flight planning guidance will indicate where exceptions apply and where procedures differ.

• This procedure was developed to reduce the risk associated with an altitude deviation and two highly accurate navigation systems navigating to the same point.
• SLOP also replaced the contingency procedure developed for aircraft encountering wake turbulence. Depending upon winds aloft, coordination between aircraft to avoid wake turbulence may be necessary.
• This procedure, which distributes traffic between the route centerline and up to 2 NM right of centerline, greatly reduces risk by the nature of its randomness.
1. Operators that have an automatic offset capability should fly up to 2 NM right of the centerline.
2. Aircraft that do not have an automatic offset capability (that can be programmed in the LRNS) should fly the centerline only.

Source: AC 91-70B, ¶D.2.6.5