Figure: SLOP, from Eddie's notes.

Eddie Sez:

There are SLOP ultra-denyers and SLOP ultra-believers. Me? I'm a SLOP realist.

  • The SLOP ultra denyers say it is just one more thing to get wrong, let the other guy apply it. Tell that to the survivors of EMBR-135BJ, and more importantly to the victims of Gol 1907, DHL 611, and Bashkirskie Avialinii 2937. More about those: Mishaps / EMBR-135BJ N600XL and Gol 1907, and Mishaps / DHL 611 and Bashkirskie Avialinii 2937.

  • The SLOP ultra-believers are so in tune with the procedure that they apply it everywhere. If it could have prevented the Brazil midair, what about in the remote regions of China, Africa, and Australia? I'm not so sure, the procedure is designed for where tracks are separated by at least 30 NM and if the controlling authority hasn't authorized SLOP, they might have separation issues in mind.

  • I believe in the concept but it isn't yet approved worldwide. The ICAO says it should be allowed in ADS-C airspace, but only in the NAT is that actually in writing.

Clearly it can save your life and it is therefore worth using. But you need to know where it is authorized and how to apply it correctly. Also note that while usually no clearance is required to initiate SLOP and no call is needed when returning to centerline, this isn't the case worldwide.

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.

ICAO Guidance

Photo: SLOP, photo courtesy of Ivan Luciani.

[ICAO Doc 4444 - Amendment No. 2, ¶16.5] Strategic Lateral Offset Procedures (SLOP) in Oceanic and Remote Continental Airspace

16.5.1. 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. An aircraft's use of these procedures does not affect the application of prescribed separation standards.

Note 1.— The use of highly accurate navigational 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.

Note 2.— The following incorporates lateral offset procedures for both the mitigation of the increasing lateral overlap probability due to increase navigation accuracy, and wake turbulence encounters.

Note 3.— Annex 2,, requires authorization for the application of strategic lateral offsets from the appropriate ATS authority responsible for the airspace concerned.

16.5.2. The following shall be taken into account by the appropriate ATS authority when authorizing the use of strategic lateral offsets in a particular airspace;

  1. 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 allow to initiate or continue offset tracking;

  2. strategic lateral offsets do not affect lateral separation minima and 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 the route centre line is not less than 55.5 km (30 NM);
  3. 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;

  4. strategic lateral offset procedures should be implemented on a regional basis after coordination between all States involved;

  5. 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

  6. air traffic controllers shall be made aware of the airspace within which strategic lateral offsets are authorized.

16.5.3. 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.

16.5.4. 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 inter-pilot air-to-air frequency 123.45 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, 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.

The ICAO recommends the procedure but does not mandate it and even notes the ATS authority must approve the practice before pilots can opt to use it. Clearly it is up to the pilot to know when it is and isn't authorized.

Where is SLOP Authorized?

It is an expanding list. As of 31 Dec 2013, here is what I've found:

Curiously, the one ICAO Document designed to bring up regional differences, ICAO Doc 7030, is remarkably out of date on the subject of SLOP.

African Guidance

Various locations throughout Africa authorize SLOP in specified areas or routes. You will have to check each country's state pages. For example:

[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - Africa / Madagascar - Rules and Procedures] Madagascar has implemented in its upper airspace between FL280 and FL460 the strategic lateral offset procedures (SLOP) in its entire continental Antananarivo FIR. The pilot must report to the controller when normal navigation is resumed after a lateral deviation of 1 or 2 NM right of the axis of the nominal route. Pilots may contact other aircraft on the Interpilot frequency 123.45MHz to coordinate offset.

Australian Guidance

[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - Australia/General Flight Procedures]

6.4.1. Aircraft operating in OCA within Australian administered airspace are authorized to use strategic lateral offset procedures (SLOP) in accordance with the requirements detailed below.

6.4.2. The following requirements apply to the use of SLOP:

  1. The offset must only be applied by aircraft with automatic offset tracking capability.

  2. The offset must be established at a distance of 1 NM or 2 NM to the RIGHT of track relative to the direction of flight.
  3. Note: Offsets to the left of track are not permitted.

  4. The offset must only be applied during the en route phase of flight.

  5. The offset may only be used in OCA. Pilots must fly the centerline for any portion within CTA. Pilots must return to centerline before leaving OCA or, where the subsequent state does not allow SLOP, prior to leaving Australian administered airspace.

  6. The offset must not be used in addition to diversions or other offsets; eg, weather or wake turbulence.

  7. The offset must not be applied at levels where obstacle clearance would be affected.

  8. Identified aircraft:
    1. may continue an offset in OCA; and

    2. must advise ATC prior to initiating or changing an offset.

6.4.3. The decision to apply SLOP is the responsibility of the pilot in command — a clearance is not required. Except when an identified aircraft initiates or changes a lateral offset, pilots are not required to notify ATC that SLOP are being applied.

6.4.4. The use of SLOP is recommended in OCA for aircraft cruising at levels not in compliance with the Table of Cruising Levels.

Canadian Guidance

[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - Canada/North Atlantic (NAT) Operations, ¶11.22]

P.R. of China Guidance

[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - China/China, P.R. - Rules and Procedures

Be careful dealing with SLOP in the People's Republic of China. The rules have changed several times over the years and the guidance out there is contradictory. SLOP used to be at the discretion of ATC and was to the left as well as the right. Current guidance, what is out there, limits slop to the routes shown above.

FAA Guidance

[AC 91-70A, ¶3-9.] Pilots should use the Strategic Lateral Offset Procedure (SLOP) as standard operating practice in the course of normal operations to mitigate collision risk and wake turbulence. The SLOP is in force throughout the New York, Oakland and Anchorage Oceanic FIRs and in oceanic airspace in the San Juan FIR. Internationally, operators implement the SLOP in the NAT, the Pacific (including the NOPAC, Central East Pacific (CEP) and Pacific Organized Track System (PACOTS)) and South Pacific airspaces. Use this procedure for both the heightened risk of collision when non-normal events such as operational altitude deviation errors and turbulence-induced altitude deviations occur due to highly-accurate navigational systems and to mitigate wake vortex encounters.

  1. Guidelines. Apply SLOP using the following guidelines:
    • Make strategic lateral offsets and those executed to mitigate the effects of wake turbulence to the right of a route or track only.

    • In relation to a route or track, there are three positions that an aircraft may fly: centerline (CL) and 1 or 2 NM right.

    • Offsets are not to exceed 2 NM right of CL.
  2. Reducing Risk. The intent of this procedure is to reduce risk (increase the safety margin) by distributing aircraft laterally and equally across the three available positions. In this connection, pilots must take into account the following:
    • Aircraft without automatic offset programming capability must fly the CL.

    • Aircraft programmed with automatic offsets may fly the CL or offset 1 or 2 NM right of CL to obtain lateral spacing from nearby aircraft.

    • Pilots should use whatever means are available (e.g., TCAS, communications, visual acquisition, ground proximity warning system (GPWS)) to determine the best flight path to fly.

    • Any aircraft overtaking another aircraft is to offset within the confines of this procedure, if capable, so as to create the least amount of wake turbulence for the overtaken aircraft.

    • For wake turbulence purposes, pilots are also to fly one of the three positions in the second bullet above and never offset to the left of the CL nor offset more than 2 NM right of CL, appropriate to any given situation and have the final authority and responsibility for the safe operation of the aircraft. You may use air-to-air frequency 123.45 to coordinate the best wake turbulence offset option.
    • NOTE: The FAA recognizes that the pilot will use his/her judgment to determine the action most appropriate to any given situation and has the final authority and responsibility for the safe operation of the aircraft. You may use air-to-air frequency 123.45 to coordinate the best wake turbulence offset option.

    • Pilots may apply an offset outbound at the oceanic entry point, but must return to CL at the oceanic exit point.

    • Aircraft transiting radar-controlled airspace (e.g., Bermuda) may remain on their established offset positions.

    • This procedure does not require ATC clearance and it is not necessary to advise ATC.

    • Base voice position reports on the current ATC clearance and not the offset positions.

[AC 91-70A, Appendix 2, ¶2.g.(4)] The SLOP should be SOP for all oceanic crossings. This procedure reduces the risk from highly accurate navigation systems or operational errors involving the ATC clearance. 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 of flying centerline (CL), 1 NM or 2 NM right of CL, greatly reduces the risk to the airspace by the nature of the randomness. Aircraft that do not have an automatic offset capability (that can be programmed in the LRNS) should fly the CL only. SLOP is not for operators to use only in contingency situations.

North Atlantic Guidance

[NAT Doc 007, ¶8.5.1.] ATC clearances are designed to ensure that separation standards are continually maintained for all traffic. However, the chain of clearance definition, delivery and execution involves a series of technical system processes and human actions. Errors are very rare but they do occur. Neither pilots nor controllers are infallible. Gross Navigation Errors (usually involving whole latitude degree mistakes in route waypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by the controller. When such errors are made, ironically, the extreme accuracies of modern navigation and height keeping systems themselves increase the risk of an actual collision. Within an SSR environment the controller is alerted to such errors and can, using VHF voice communications, intervene in a timely fashion. This is not the case in Oceanic airspace, such as the North Atlantic, where the controller's awareness of the disposition of a significant proportion of the traffic is reliant largely upon pilot position reports through communication links utilising HF or SATCOM Voice via third party radio operators. And furthermore, even among that proportion of traffic utilising data link for automated position reporting, and perhaps ATS communications, navigation errors continue to occur. Consequently, it has been determined that allowing aircraft conducting oceanic flight to fly self-selected lateral offsets will provide an additional safety margin and mitigate 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, ¶8.5.2.] This procedure provides for offsets within the following guidelines:

[NAT Doc 007, ¶8.5.3.] Distributing aircraft laterally and equally across the three available positions adds an additional safety margin and reduces collision risk. Consequently, SLOP is now a standard operating procedure for the entire NAT Region and pilots are required to adopt this procedure as is appropriate. In this connection, it should be noted that:

[NAT Doc 007, ¶8.5.4.] SLOP has been implemented as a standard operating procedure in the NAT region since 2004. An indication of the proportion of pilots adopting a SLOP offset here is obtained through study of ADS-C position reports. Such study has shown that during 2012 more than 40% of aircraft flying in the NAT MNPS Airspace selected the 1NM Right option and about 20% chose the 2NM Right option. As indicated above, system safety would be further enhanced if aircraft were more evenly distributed between the centreline, 1 and 2 NM Right options. As proposed in paragraph 8.5.3 b) above, Pilots should attempt to determine the offsets (if any) being flown by aircraft immediately ahead on the same track one flight level above and one flight level below. And then select an offset which differs from those. If this is not possible or practical, then pilots should randomly choose one of the three flight path options.

[NAT Doc 007, ¶8.5.5] The previously mentioned study of ADS-C position reports has also shown that some aircraft continue to adopt an offset LEFT of cleared track centre-line. The standard SLOP procedures are designed to provide safety enhancements for both uni-directional and bi-directional flows. On bi-directional routes a LEFT offset will INCREASE collision risk rather than decrease it. There are areas in the NAT Region where bi- directional traffic flows are routinely used. And there are times when opposite direction traffic may be encountered in any part of the Region. Pilots must therefore recognise that LEFT offsets from the cleared track centre-line must not be adopted. After the introduction of RVSM and before the adoption of SLOP, a NAT offsetting procedure was promulgated for wake-turbulence avoidance. This procedure allowed both right and left offsets to be flown. The procedure was developed primarily with a view to the unique traffic flows of the NAT OTS, where uni-directional traffic occupied every flight level from FL310 to FL390. That procedure is no longer in place. The avoidance of wake turbulence (even in the OTS) can be accomplished effectively within the confines of the SLOP procedures, as specified in paragraph 8.5.3 d) above. Pilots should communicate with the other aircraft involved to coordinate a pair of mutual offsets from within the allowed three options, in order to mitigate any wake-turbulence issue.

Random Route SLOP?

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

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?

G450 Example

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

To begin the SLOP, press the PROG key.

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

Press the PREV key to get to Page 3.

Photo: G450 MCDU, Prog Page 3/1, R2 in scratch pad, from Eddie's airplane.

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

Photo: G450 MCDU, Prog Page 3/1, 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.

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.

Book Notes

Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 35.


Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

ICAO Doc 4444 - Amendment No. 2, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 2, 19/11/09

ICAO Doc 7030, Amendment 1, International Civil Aviation Organization, 8 January 2009

NAT Doc 007, North Atlantic Operations and Airspace Manual Doc 007, Edition 2013