Strategic Lateral Offset Procedure (SLOP)

International Operations Manual

Eddie sez:

The world has gone to SLOP. You can (and should) employ a Strategic Lateral Offset Procedure (SLOP) in almost every oceanic area. There is a debate about using SLOP over land, but the legal and official answer is you cannot with only very rare exceptions. Most of the world only permits SLOP to the right and usually only of 1 or 2 NM. The ICAO has permitted "micro SLOP" in tenths since 2014, but only a few regions have adopted this. Remember that some aircraft may not SLOP: particularly those that do not have an automatic offset capability.

Everything here is from the references shown below, with a few comments in an alternate color.

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Figure: The view from 2 nm right with the contrails of 1 nm right and on course to the left, from Frank Flood (B-777), April 9, 2017.

Last revision:

2021-02-26

Cover Story:

2017-06-19


Why?

In the days before GPS, we would be lucky if our oceanic position was within a couple of miles of where we should have been. In the days before INS, being within 10 miles was a major accomplishment. But there were not a lot of airplanes flying oceanic and the accuracy wasn't all that important. These days, the skies are packed with airplanes but GPS allows us to navigate within a tenth or so of course centerline. It is almost all good.

It is almost all good because there is a chance a navigation or altitude error can end very badly. If someone programs the wrong course or levels off at the wrong altitude, they may be precisely on the wrong course, in position to hit someone who is properly and precisely on course. The altitude error is precisely what happened to the victims of Gol 1907, DHL 611, and Bashkirskie Avialinii 2937. More about those: EMBR-135BJ N600XL and Gol 1907, and DHL 611 and Bashkirskie Avialinii 2937.

Of course those were not oceanic midair collisions, but they serve to illustrate the point. Flying a little right of course can save you when you or someone else makes a mistake.

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Figure: SLOP, from Eddie's notes.
Click photo for a larger image

Regulatory

The ICAO

[ICAO Doc 4444, ¶16.5] 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.

The North Atlantic

[NAT Doc 007, ¶8.5.8.] 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)”.

[NAT Doc 007, ¶8.5.9.] 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.

[NAT Doc 007, ¶8.5.10.] 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.

The US FAA

[AC 91-70B, ¶D.2.6.5] 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.
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Photo: SLOP, photo courtesy of Ivan Luciani
Click photo for a larger image

Searching for SLOP

Using a Country's Aeronautical Information Publication

Theoretically, the best place to look is a country's AIP, but not all countries include any mention of SLOP and those that do are inconsistent about where they put this information. In general, look in the ENR section. If you have a PDF copy, do a word search for "lateral" and start from there. Here are a few examples:

  • Australia: ENR 2.2, ¶3.
  • Canada: None found.
  • Greenland: None found.
  • Ireland: ENR 2.2.8
  • United States: ENR 7.1-6, ¶8.

Using Jeppesen

Your JeppFD App on the iPad can help you find out the SLOP rules for the region you are flying. Let's say, for example, you are unsure about the rules off the coast of South Africa.

1. Bring up the JeppFD App, look at the High IFR map, and find the FIR label. The first two letters give you a clue as to what region you are in. In the example, the FACA FIR tells you this is in the Africa region and that the South Africa Republic would be a good place to look. You may have to hunt around countries that border the oceanic region. Not every country publishes SLOP rules, even if the region does allow it.

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Photo: Find SLOP Rules in JeppFD
Click photo for a larger image

2. Press the text icon in the upper left. If the "Route" is in reverse video, you will want to select "Pubs" instead. At this point you might see whatever you had viewed previously; if so, go to higher menus by selecting the left arrow until you see "Route Notes" with various Regions shown below.

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Photo: Find SLOP Rules in JeppFD
Click photo for a larger image

3. From here, select "Africa" from the list of regions. Look for the Air Traffic Control section and select State Rules and Procedures.

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Photo: Find SLOP Rules in JeppFD
Click photo for a larger image

4. Scroll down the list and select your desired country. At this point you will see a page for the rules and procedures from that country.

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Photo: Find SLOP Rules in JeppFD
Click photo for a larger image

5. Scroll down until you see Strategic Lateral Offset.

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Photo: Find SLOP Rules in JeppFD
Click photo for a larger image

Random Route SLOP?

Many Gulfstream pilots feel immune to the SLOP issue, saying they always fly above the tracks and they usually fly random routing. As you can see from the drawing, there is one problem with that argument. If you are on the track, chances are you are flying the same direction as your nearest neighbor and while the sky is considerably more crowded, the chance of a collision is reduced. The guy behind you might make an altitude error but he is behind you and likely to stay behind you.

You might be on the random track because you are flying an unusual city pair or for some other reason. But what if there is somebody flying the same city pair in the opposite direction? Now what if that guy makes an altitude error? Wouldn't an extra mile of separation be nice?

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Figure: SLOP Track vs. Random, from Eddie's notes.

G450 Example

Remember that if your airplane is pre-ASC 909B you should not SLOP because of the threat of an FMS reset. (See GAC-OIS-13, Item 33.)

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Photo: G450 MCDU, Prog Page 1/3, prior to SLOP, from Eddie's airplane.

To begin the SLOP, press the PROG key.

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Photo: G450 MCDU, Prog Page 3/3, prior to SLOP, from Eddie's airplane.

Press the PREV key to get to Page 3.

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Photo: G450 MCDU, Prog Page 3/3, R2 in scratch pad, from Eddie's airplane.

Enter R2 in the scratch pad. You can also enter R1.

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Photo: G450 MCDU, Prog Page 3/3, with 2 nm right SLOP, from Eddie's airplane.

At this point the aircraft should begin a gentle turn to the right to intercept a 2 nm right track. Video: G450 SLOP.

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Photo: G450 Nav Display, with 2 nm right SLOP, from Eddie's airplane.

The course on the Nav display lies to the left and the screen indicates OFFSET in the top right. The CDI on the PFD indicates the offset course as the new desired course.

To remove the SLOP, repeat the process and enter DELETE as the offset amount.


See Also:

ICAO Doc 4444 - Air Traffic Management, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 10 November 2016

ICAO Nat Doc 007, North Atlantic Operations and Airspace Manual, v. 2021-1, applicable from February 2021

Gulfstream Operational Information Supplement For G350, G450, G500, G550 Airplanes, Supplement Number GAC-OIS-13: PlaneView Operational Issues List, Revision 03, May 16, 2014