Strategic Lateral Offset Procedure (SLOP)
The world has gone to SLOP. You can (and should) employ 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 the NAT has adopted that (as of July 2019).
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.
Figure: SLOP, from Eddie's notes.
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: EMBR-135BJ N600XL and Gol 1907, and 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.
I've heard some pilots say their aircraft are too aggressive banking into and out of the SLOP offset. I haven't found that to be the case in the G450 as long as you don't initiate or cancel the maneuver too close to the next waypoint. But I've also heard that some pilots in older technology aircraft like to manually establish the offset with a heading mode before latching on to an offset. You should remember that you aren't supposed to apply SLOP unless you have an automatic offset capability, and that you shouldn't be coming off a long range navigation function if you are using a data link system. (ADS-C will report that you have done so and ATC will question you as a result.)
Flight Service Bureau / OpsGroup Note to Members #24 22 Mar 2017
The preliminary report indicates this Challenger 604 was minding its own business and ran into the wake of an A380. The crew managed to recover without losing the aircraft or killing anyone, but there were injuries and the aircraft was damaged beyond repair. Perhaps SLOP could have mitigated the situation.
In January 2017, a Challenger 604 passed 1000 feet underneath an opposite direction Airbus 380, encountered wake turbulence, and lost control, rolling 3-5 times, engines flamed out, and lost 10,000 feet before recovering. The aircraft received damage beyond repair due to the G-forces, and was written off. There is now worldwide concern regarding the effects of A380 wake.
Existing wake guidance
A 2006 European study recommended a new category, Super, for the A380, due to its size. Behind an A380 on approach, you need 6nm in a heavy, 7nm in a medium, and 8nm in a light. For departures, 2 minutes is the minimum for all aircraft taking off behind an A380, increasing to 3 mins for light/medium, and 4 for intersection departures. The same study concluded that the A380 itself did not need any wake separation when following other aircraft, making it the only type to have this ‘out’.
No A380 wake guidance exists en route (in fact, very little en route wake turbulence guidance of any sort exists, which is why we are publishing this note) which is the phase of flight in which the above incident happened. We believe that will change. In the interim, full use of the SLOP offset procedure by all crews can mitigate risk. This will be of particular value to light and medium category aircraft potentially passing through A380 wake en route
Photo: A380, from OpsGroup Note to Member #24
SLOP – Standard Lateral Offset Procedure
SLOP allows an offset, usually 1 or 2nm to the right of track. First introduced as a NAT procedure in 2004, it’s now mandatory there and allowed in many other FIRs. There are two reasons for SLOP. One is reducing collision risk, the other is avoiding wake turbulence, though until now that only really considered wake from traffic ahead.
While the regulatory documents catch up, we offer the following guidance. We do this to help you form your own opinion. Make sure you do that, and not just rely on our say-so. We might be wrong.
We’ve based this guidance on: Flight Service Bureau opinion and research, SKYbrary documentation, existing Aviation Authority guidance, and an official Safety Information Bulletin from EASA that has been sighted but not yet been published. Check ad.easa.europa.eu. The FAA will likely follow suit.
If you can, do it. It may not prevent all situations, especially crossing traffic, but if you’re 2nm right of track you’re a lot less likely to be directly underneath – or behind – another aircraft. Check that SLOP is allowed in the particular airspace you’re in before you make the offset. Always SLOP to the RIGHT.
Our recommendation is not to fly the airway centreline unless you absolutely have to – which will be because ATC mandates it (ie. no SLOP allowed), or because your aircraft can’t (ie. It’s old).
Although SLOP is generally allowed 0nm, 1nm, or 2nm right of track, the 0nm offset is by definition not an offset. Flying centreline:
- increases the risk of collision with opposite direction traffic, either erroneously at the same level as you, or carrying out an emergency descent and not turning away from the centreline as they should.
- increases the risk of wake turbulence from opposite direction traffic also on the centreline.
Visually spotting the wake
Remember that the wake and the contrail are not always, and in fact unlikely to be, in the same place. The contrail comes from the engines, and the wake comes from the wings. While it may move away from the centerline in the same direction as the wake, the contrail will sit at altitude, while the wake drifts down, and, usually, the wake is invisible. NASA says it drifts down at, on average, 350 fpm, and settles about 1000ft below the flight path. Wake vortex decay is much slower below the tropopause.
Just passed one – what now?
Each situation will require your judgment, but if you’ve just gone directly under an opposite direction A380 – or any any aircraft much heavier than yourself:
- First, consider the wind. The danger point is roughly 15-20nm after the crossing point, as this is when the wake will have drifted down 1000 feet. In stronger winds, the wake may have drifted well away from the centreline. A turn away may not be necessary.
- If you’re in SLOP approved airspace, you don’t need a clearance from ATC to turn away from the track you’re on. Turn, if possible into wind – for example from offset R1 to R2. No need to inform ATC (except for Australia).
- If you’re in SLOP unapproved airspace, or you’re not sure, consider talking to ATC and getting a clearance to turn into wind.
- Switch on the seatbelt sign and secure the cabin.
- Check TCAS below and see what’s underneath you, just in case you do encounter an upset.
Flying out of a wake upset
From the FAA “Pilot and Air Traffic Controller Guide to Wake Turbulence”:
An encounter with wake turbulence usually results in induced rolling or pitch moments; however, in rare instances an encounter could cause structural damage to the aircraft. In more than one instance, pilots have described an encounter to be like “hitting a wall”. The dynamic forces of the vortex can exceed the roll or pitch capability of the aircraft to overcome these forces.
During test programs, the wake was approached from all directions to evaluate the effect of encounter direction on response. One item common to all encounters, was that without a concerted effort by the pilot the aircraft would be expelled from the wake.
Counter control is usually effective and induced roll is minimal in cases where the wingspan and ailerons of the encountering aircraft extend beyond the rotational flow field of the vortex. It is more difficult for aircraft with short wingspan (relative to the generating aircraft) to counter the imposed roll induced by the vortex flow.
Pilots of short span aircraft, even of the high performance type, must be especially alert to wake-turbulence encounters.
The following is from an advance copy of an upcoming EASA SIB on En route Wake Turbulence (not yet released)
- It has been demonstrated during flight tests that if the pilot reacts at the first roll motion, when in the core of the vortex, the roll motion could be amplified by this initial piloting action. The result can be a final bank angle greater than if the pilot would not have moved the controls.
- In-flight incidents have demonstrated that pilot inputs may exacerbate the unusual attitude condition with rapid roll control reversals carried out in an “out of phase” manner.
- Avoid large rudder deflections that can create important lateral accelerations, which could then generate very large forces on the vertical stabiliser that may exceed the structural resistance. Although some recent aircraft types are protected by fly-by-wire systems, use of the rudder does not reduce the severity of the encounter nor does it improve the ease of recovery.
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.
Photo: SLOP, photo courtesy of Ivan Luciani.
[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, 126.96.36.199.1, 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
The use of tenths of a nautical mile has, as of this writing, only been authorized in the North Atlantic.
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.
Where is SLOP Authorized?
It is an expanding list. As of 22 Mar 2017, here is what I've found:
- Africa, almost all remote locations employ SLOP. Check the Jeppesen Airways Manual / Air Traffic Control / State Rules and Procedures - Africa) to be sure. Rule of thumb: if you are in radar contact, you probably should not SLOP.
- Australia is another special case. You may only offset in the OCA, and, if you’re still on radar, then you need to tell ATC, both when starting the offset, or changing it. Within domestic CTA airspace, you must fly centerline. (According to Australian guidance in Jeppesen Pages).
- China, on routes A1, L642, M771, and N892 (according to China guidance in Jeppesen Pages). In some areas they employ their unique SLOP offsets, but do allow the standard 1 nm and 2 nm offsets.
- New York, Oakland and Anchorage Oceanic FIRs (according to U.S. FAA guidance).
- Oceanic airspace in the San Juan FIR (according to U.S. FAA guidance).
- North Atlantic Track Region: SLOP is mandatory (according to the North Atlantic Operations and Airspace Manual).
- The Pacific (including the NOPAC, Central East Pacific (CEP) and Pacific Organized Track System (PACOTS)) (according to U.S. FAA guidance).
- South Pacific airspaces (according to U.S. FAA guidance).
Curiously, the one ICAO Document designed to bring up regional differences, ICAO Doc 7030, is remarkably out of date on the subject of SLOP.
Searching for SLOP
Figure: SLOP Example, from JeppsenFD, Guinea Republic, Rules and Procedures, 23 June 2017
A good way to determine if the country you are overflying allows you to SLOP or has particular rules is to download the state rules and procedures and do a search. Let's say you are overflying much of Africa and keep your Jeppesen state pages on an iPad. Here's a way to quickly find every instance of SLOP in Africa:
- Access the JeppFD application.
- Select the text icon.
- Select the Africa manuals.
- Copy to a good PDF reader. I use "GoodReader."
- Execute of search for SLOP.
Using this method we found the example shown above for the Guinea Republic. Here we see that they do use SLOP but that on some airways these are applied in tenths of a mile.
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.
[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:
- The offset must only be applied by aircraft with automatic offset tracking capability.
- 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.
- The offset must only be applied during the en route phase of flight.
- 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.
- The offset must not be used in addition to diversions or other offsets; eg, weather or wake turbulence.
- The offset must not be applied at levels where obstacle clearance would be affected.
- Identified aircraft:
- may continue an offset in OCA; and
- must advise ATC prior to initiating or changing an offset.
Note: Offsets to the left of track are not permitted.
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.
[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - Canada/North Atlantic (NAT) Operations, ¶11.22]
- The Strategic Lateral Offset Procedure (SLOP) is now a standard operating procedure throughout the North Atlantic (NAT) Region. This procedure mitigates collision risk and wake turbulence encounters. Pilots conducting oceanic flights within the NAT Region with automatic offset programming capability are recommended to fly lateral offsets of either 1 or 2 NM right of center line.
P.R. of China Guidance
[Jeppesen Airway Manual/Air Traffic Control/State Rules and Procedures - China/China, P.R. - Rules and Procedures
- Offsets are only applied on routes A1, L642, M771 and N892.
- The following requirements apply to the use of the offset:
- the decision to apply a strategic lateral offset is the responsibility of the flight crew;
- the offset shall be established at a distance of one or two nautical miles to RIGHT of the center line relative to the direction of flight;
- in airspace where the use of lateral offsets has been authorized, pilots are not required to inform ATC that an offset is being applied.
- offsets are only applied by aircraft with automatic offset tracking capability.
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. Controllers can direct you to SLOP in great distances as well. Current guidance, what is out there, limits SLOP to the routes shown above.
[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.
- Operators that have an automatic offset capability should fly up to 2 NM right of the centerline.
- Aircraft that do not have an automatic offset capability (that can be programmed in the LRNS) should fly the centerline only.
North Atlantic Guidance
[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 judgement 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.
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?
Figure: SLOP Track vs. Random, from Eddie's notes.
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.)
Photo: G450 MCDU, Prog Page 1/3, prior to SLOP, from Eddie's airplane.
To begin the SLOP, press the PROG key.
Photo: G450 MCDU, Prog Page 3/3, prior to SLOP, from Eddie's airplane.
Press the PREV key to get to Page 3.
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.
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.
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.
Portions of this page can be found in the book International Operations Flight Manual, Part VIII, Chapter 35.
Advisory Circular 91-70B, Oceanic and International Operations, 10/4/16, U.S. Department of Transportation
Gulfstream Operational Information Supplement For G350, G450, G500, G550 Airplanes, Supplement Number GAC-OIS-13: PlaneView Operational Issues List, Revision 03, May 16, 2014
ICAO Doc 4444 - Air Traffic Management, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016
ICAO Doc 7030, Amendment 1, International Civil Aviation Organization, 8 January 2009
ICAO Nat Doc 007, North Atlantic Operations and Airspace Manual, v. 2019-3, applicable from July 2019
En route Wake Turbulence Note to Members #24, Flight Service Bureau.