- Strategic Lateral Offset Procedure
Strategic Lateral Offset Procedure (SLOP). An enroute 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.
Midair Collision. SLOP is recommended for use in modern FMS based, RVSM 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.
The GOL Flight 1907 mid-air collision mishap over the Amazon jungle is a tragic example of this hazard which has only recently been fully recognized by experienced aircrews and flight operations managers.
Wake Turbulence. In addition to collison hazard considerations, SLOP is also utilized to reduce the probability of high altitude wake turbulence encounters between properly spaced aircraft causing injury to passenger/crew and undue structural airframe stress.
During conditions of low velocity winds at flight level altitudes the adoption of an RVSM vertical separation of 1000 feet, typical GPS lateral navigational accuracy, and high rates of aircraft closure have all combined to increase the probability that high altitude wake turbulence may be encountered.
Wake turbulence is thought likely to be experienced by the lower of two aircraft when it arrives approximately 15-30nm behind an opposite direction aircraft which has crossed directly overhead on the same route.
This distance assumes both a 450kt true airspeed by each aircraft (each traveling about 7-8 Nm per minute) and a 500 fpm sink rate for the wake turbulence generated by the aircraft passing overhead.
The navigational course offset adjustment to implement SLOP can be easily programmed into and deleted out of the "route page" of the Boeing 737NG series, the Boeing 747-400, the 757/767 and the 777"' Flight Management Systems (FMS).
Offset is established by entering an R1.0 or R2.0 - for one mile right or two miles right - and then confirming or "executing" the offset. The LNAV lateral navigation mode and the selected autopilot will immediately turn the aircraft to the right and command an intercept of the parallel course depicted as a broken magenta line. This broken offset course line lies next to the preprogrammed route centerline which continues to be depicted as solid magenta.
Use of SLOP is currently recommended either as a "best practice" technique or as a standard operating procedure in certain global regions by major international air carriers.
Gol Flight 1907
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