SLOP Mitigates Collision Risk Posed by GPS Navigation Paradox

By Scott Spangler on November 2nd, 2020

SLOP-FSBwikipediaAviators live and die by their acronyms, so reading one unfamiliar motivates a frenzy of catch-up research. A short news item about changes ICAO recently made to special procedures for in-flight contingencies in oceanic airspace focused on something know as SLOP. Airliners flying in oceanic airspace such as the North Atlantic follow precise prescribed tracks to maintain separation from other airplanes doing the same thing. When they have a problem that prevents them from maintaining their spot in the track, they must leave it to address the problem, and ICAO reduced the offset to 5 miles from 15 miles and prohibited turn-arounds until the afflicted jet was below Flight Level 290 or above FL 410.

But it didn’t really explain what SLOP is, and why it is important. The OpsGroup blog introduced not only Strategic Lateral Offset Procedures but also the Navigation Paradox posed by the position accuracy GPS makes possible. With positional accuracy measured in feet, GPS guides jets through the flight management system, which flies an airplane with a level of precision no hand-flying pilot can duplicate. In other words, jets can precisely fly the oceanic airspace tracks, which increase the risk of collisions.

An AIAA paper, A Stochastic Conflict Detection Model Revisited, applies uncertainty modeling to improve the estimation of traffic conflicts for air traffic control. The possibility of a midair collision was six-times more likely for an aircraft precisely flying the prescribed hemispherical cruising altitude than an airplane whose altitude compliance wasn’t so accurate. SLOP made its debut in 2004 to introduce this randomness to the world’s busiest non-radar airspace, the North Atlantic airways that connect North America to Europe. As a bonus, it also reduced encounters with wake turbulence from preceding aircraft on the same track.

slop code7700Simply put, GPS accuracy makes it possible to stack jets vertically on a prescribed airway, and any deviation from their flight level increases the risk of a collision. When pilots command the FMS to fly a randomly selected SLOP, measured in tenths up to 2 nautical miles to the right of the assigned track, those aircraft are no longer precisely vertically stacked.

Flights must always SLOP right because offsetting them left would set them up for potential head-on collisions with jets following airways in the opposite direction. And that’s what led ICAO to narrow its contingency procedures. To increase oceanic airspace capacity, the overseers of the world’s airspace reduced the lateral separation standards that in the North Atlantic sometimes were 20 nautical miles or less.

Given this lateral proximity, airplanes making a 15-mile contingency offset would violate the safety space of an airplane on the next airway, but a 5-mile offset would preserve it. The same logic applies to the new turn-around altitudes, because a cruising jet can’t comfortably make a 180-degree course change in the new narrower confines. (If you’re curious to learn more, Code7700.com is a good start.) – Scott Spangler, Editor

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5 Responses to “SLOP Mitigates Collision Risk Posed by GPS Navigation Paradox”

  1. SLOP Mitigates Collision Risk Posed by GPS Navigation Paradox – My Blog Says:

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  4. Mal Gormley Says:

    This problem reminded me of a similar dilemma experienced by WWII bomber crews. The bombsights were so accurate that bombardiers would begin their bombing run miles from the target–a factory, say, with hundreds of airplanes all approaching the same precise point in space to release their bombs. The result was each plane was slowly closing in on all the other planes in the formation. This led to some disastrous results–bombers dropping their loads on other bombers just below them.

    To solve the problem, the procedure was changed so that every nearby bomber would release at the same moment the lead bomber would.

    My dad was often assigned as the lead bombardier on his B-24 squadron’s missions over Nazi Europe, told me about this curious bit of air history trivia when I was a kid.

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