The Strategical Lateral Offset Procedure (SLOP)

Introduction

The Strategical Lateral Offset Procedure (SLOP) is a byproduct of improved navigational accuracy. Stay with me we'll get to the explanation soon!

Lets go back to the days before GPS navigation was widely accepted as a primary means of aeronautical navigation, the days when VORs & NDBs were used to navigate on airways. Due to the inherent inaccuracy, compared to GPS navigation, aircrafts flying on the same airway where actually scattered along the airway being slightly offset from the center of the airway. This randomness of  aircraft positions flying along an airway caused by reduced accuracy was actually beneficial to safety. Getting there...

The introduction of Reduced Vertical Separation Minima (RVSM), in 1982, caused aircrafts flying reciprocal tracks on the same airway to fly closer to each other than ever before. BUT thanks to the imperfection of VOR & NDB navigational accuracy aircrafts were almost always laterally separated.

Inaccuracies in lateral positions were statistically sufficient to almost eliminate the possibility of having two aircrafts flying reciprocal tracks come dangerously close to each other. Then came GPS navigation with it's unparalleled accuracy. GPS navigation has eliminated the small but significant lateral randomness of position of aircrafts flying along an airway.

The almost pin point lateral accuracy has caused an increased safety risk for aircrafts flying along the same airway on reciprocal tracks for the following two reasons:

1. Human or equipment error: causing the aircraft to fly an incorrect altitude/ flight level (e.g. incorrect selection, incorrect assignment of FL or Alt by ATC, pressure altimeter errors, etc...). The combination of an extremely accurate lateral position combines with an error in altitude/flight level is a formula for a potential disaster.
2. Wake Turbulence (aircrafts passing exactly on top of  each other are more susceptible to encountering wake turbulence). Once again, improved accuracy has increased the susceptibility to experiencing wake turbulence from overflying aircrafts. 

The SLOP

SLOP is a procedure whereby the pilot purposefully offsets the aircraft's flightpath laterally away from the center of the airway and parallel to the airway. The available paths tend to be a 1nm and 2nm offset to the right of the center of the airway, some airspace allows for smaller increments of offsets (e.g. 0.5 nm & 1.5 nm). 

In airspace were SLOPs are implemented, the pilot gets to randomly choose by how much they want to offset, if they want to offset in the first place. ATC clearance is usually not required in airspace that implements such a procedure. Leaving the decision, of how much to offset, completely up to the pilot takes the burden away from ATC and leaves things random.

SLOP reintroduces the randomness of the lateral position of aircrafts flying on the same airway, this in turn returns the safety risk to an acceptable level.

Where do we apply SLOP?

You can find out more detailed information about airspace where SLOP is approved through a country's Aeronautical Information Publication (AIP) or through the Jeppesen Airway Manual. But one main airspace were SLOP not only recommended but is mandatory is the North Atlantic Organized Track System (NAT-OTS).

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