Strategic Lateral Offset Procedure (SLOP) is an en route high altitude procedure used by pilots of turbojet and turboprop aircraft navigating an assigned route along an airway or between published navigational fixes by adjusting their course to parallel the assigned route either 1 NM or 2 NM to the right of the airway or course centerline.
In the North Atlantic Region pilots are expected to fly along the Oceanic Track centre-line or 1 or 2 nautical miles to its right, randomly choosing one of these three offsets on each entry to Oceanic airspace. The aim is to achieve an overall even distribution of one third of all flights on each of the three possible tracks, thus lowering the overall risk of collision should an aircraft move vertically away from its assigned level. This randomisation has the advantage over a planned assignment of offsets to each individual aircraft in that it mitigates the collision hazard for same direction flights should an aircraft be erroneously flown along a track that was not assigned by ATC.
SLOP is recommended for use in modern Flight management system based, RVSM (Reduced Vertical Separation Minima) equipped aircraft operations to mitigate the midair collision hazard which is amplified by the accuracy of modern aircraft navigational technology and onboard flight instruments.
Global Positioning System (GPS) based lateral navigation (left/right) and RVSM quality altimetry (up/down) are each so accurate in their own dimension that opposite direction aircraft which are erroneously flying the same altitude on the same navigational path are very likely to collide.
In addition to mitigating en route midair collision hazard, SLOP is utilized to reduce the probability of high altitude wake turbulence encounters. During periods of low wind velocity aloft aircraft which are spaced 1000 feet vertically but pass directly overhead in opposite directions can generate wake turbulence which may cause either injury to passengers/crew or undue structural airframe stress. This hazard is an unintended consequence of RVSM vertical spacing reductions which are designed to increase allowable air traffic density. Rates of closure for typical jet aircraft at cruise speed routinely exceed 900 knots.
Wake turbulence is thought likely to be experienced by the lower of two aircraft when it arrives approximately 15-30 nm behind an opposite direction aircraft which has crossed directly overhead on the same route.
Further information on SLOP can be found in the North Atlantic MNPS Operations Manual (section 8.5) - see link to ICAO site below.