The following article on Strategic Lateral Offset Procedure (SLOP) is an excerpt from the Part 91 Hawaii Flight Course and the book version of the course, Flying Part 91 To Hawaii: A Pilot's Guide
ICAO does not allow unwarranted lateral deviations in excess of 2nm from your cleared route. However, you are permitted to fly a parallel track that is either 1 or 2 miles to the right of your cleared route. This offset is called a Strategic Lateral Offset Procedure, or SLOP, and it is very common and even encouraged in oceanic flying. Why might one do this?
Advances in technology have meant that we are able to fly much more precisely than in the past, but that doesn’t mean technology is infallible or that it never makes errors - and we humans are still the same error-prone species we’ve always been! In fact, a study concluded that this increase in the ability to navigate with such great precision actually increases the risk of midair collisions. This idea is known as the Navigation Paradox. I’d encourage you to read about it in detail because it's interesting stuff, but essentially, since we can now maintain such a precise altitude and lateral tracks, and since so many aircraft are using these same closely-spaced routes, a single error by a computer or human is enough to result in two aircraft being in the same spot at the same time. During the days of less precise flying, we were likely to be far enough off from our desired track and altitude that even if some kind of error led to two aircraft’s desired flight paths converging on the same space at the same time, the imprecision of navigation and automated flying would result in a miss.
Today, thanks to RVSM, two heavy aircraft flying the same route in opposite directions with a closing rate of over 1,000 mph might miss each other by a mere 5,280 feet (or less if both aircrafts altimetetry systems are not perfectly calibrated). This “near miss” is a completely acceptable and legal occurrence, and it happens many times a day, all over the world. What if there was a malfunction in the altimetry system that wasn’t caught? What if an assigned altitude had been input incorrectly? What if the aircraft had been cleared to climb but an altitude capture mode was not engaged? 99% of the time, our procedures and equipment allow us to catch errors so that nothing comes of them. But not 100% of the time. So, adding one or two miles of lateral offset from the course centerline adds an additional margin of safety for when things inevitably don’t go as expected.
This offset is also a great way to reduce the risk of encountering wake turbulence from other aircraft. Recall wake turbulence descends around about 500 feet per minute. In a zero-wind situation, this means there are good odds that you will hit wake turbulence within a few moments of passing directly under opposite direction traffic that is 1,000 feet above you on your same route. In fact, this is exactly what happened to a Challenger 604 (post-incident photos above) a few years back when it flew into the wake of an opposite-direction Airbus 380 that had passed 1,000 feet above it 48 seconds beforehand along the same track. The Challenger rolled multiple times, lost 9,000 feet, and passengers were severely injured. No one died, but the airframe was totaled.
Following this event, there was the introduction of the “SUPER” wake turbulence category, as well as revised ICAO and FAA policy on SLOP procedures. Of these changes, the FAA now recommends using SLOP, and because the decision to utilize SLOP in oceanic airspace is at the pilot's discretion, there are few reasons not do it. In order to use SLOP, your aircraft needs to have the ability to automatically offset by 1 or 2nm parallel from your cleared route. Modern FMS and many older ones will have this ability. You do not need ATC clearance to use SLOP - it is permitted at all times between the coastal fixes. Depending on your FMS, this mode may only be available under certain conditions, such as when flying between two fixes and not on an arrival or departure procedure, or while flying a course direct-to a fix.