Proficient Pilot: SLOPpy flying

July 1, 2008

Capt. Barry Schiff retired from TWA in 1998.

When first learning to navigate along low-frequency airways in 1955, I was taught to fly on the right side of a leg (or beam). This, I was told, was to avoid a close encounter with opposite-direction traffic on the other side of a leg. This led me to ask, “What about when the beams narrow and converge as they approach the station? Wouldn’t there be danger of collision as two aircraft simultaneously pass over a given station at the same altitude?”

“No one can track a leg of a four-course range that accurately. It is improbable that two pilots would ever arrive over the same point at the same time, even if they tried,” said my instructor. Then came omnirange and VOR navigation, which provides far more accuracy than did low-frequency ranges—accuracy that has lead to occasional midair conflicts directly over VOR stations.

Now that the Global Positioning System provides accuracy to within mere feet, the likelihood of a close or actual encounter has increased significantly for those who fly directly from one navigational fix to another. GPS also increases the risk of a midair collision while tracking along airways. Accidents can occur as the result of one aircraft overtaking another or when one of two opposite-direction aircraft is at the wrong altitude.

A collision occurred on September 29, 2006, when a Gol Airlines Boeing 737-800 collided with an opposite-direction Embraer Legacy 600 business jet over Brazil’s Amazon jungle. Both aircraft were using GPS to track the same route, although one of them obviously was at the wrong altitude. Both aircraft were navigating via GPS, and there is no doubt that the seven-meter horizontal and 12-meter vertical accuracy increased the likelihood of this tragedy. (WAAS improves accuracy to plus or minus one meter.) A less-accurate form of navigation might have enabled these aircraft to pass harmlessly in the night.

There is an interesting antidote to these types of midair threats. It was developed soon after the 1997 implementation of reduced vertical separation minimum, or RVSM, procedures over the heavily trafficked North Atlantic. Instead of high-flying jets being vertically separated by 2,000 feet, RVSM reduced separation to only 1,000 feet. Pilots of aircraft flying in the new RVSM airspace and navigating with GPS began experiencing wake turbulence from aircraft that were 1,000 feet above and slightly ahead. Such wake encounters were rarely if ever experienced when 2,000 feet of vertical separation were used. An aircraft at FL350 flying in the same direction and along the same track as an aircraft at FL360 could wind up being buffeted by wake turbulence all the way across the North Atlantic.

Many pilots also expressed concern about the consequences of altitude-deviation errors when aircraft were navigating so accurately along the same route. The procedure subsequently developed argues against the philosophy that pilots should track the centerline of an airway. It is called the Strategic Lateral Offset Procedure, or SLOP.

SLOP allows pilots flying in certain oceanic and remote-continental airspace to laterally offset their flights by as much as two nm to the right of an airway or designated track. Where approved, such pilots are allowed to apply SLOP without advising air traffic control. The randomness of SLOP reduces wake turbulence encounters as well as the potential for midair collision.

When flying in RVSM airspace, it can be unnerving to observe an opposite-direction aircraft that is approaching head-on along the same route even though it is 1,000 feet below you. Pilots looking out the windshield at high altitude might notice that the Earth’s horizon appears below the horizontal, a phenomenon called dip; the higher one flies, the lower the horizon appears in the windshield.

Consequently, an approaching aircraft that is 1,000 feet below you creates the illusion that it is above the horizon. You receive the erroneous impression that it is above you. As the approaching aircraft gets closer, it eventually appears to “descend” to the horizon. At this point, you get the distinct impression that the approaching traffic is at your altitude and poses a grave threat (at a closure rate of almost 1,000 knots). Soon, though, the approaching aircraft continues to “descend” below the visual horizon at which point you become visually aware (and relieved!) that the aircraft is indeed below you.

Using SLOP eliminates this kind of concern. VFR pilots can utilize lateral-offset procedures at any time to reduce the risk of midair collision. I use it to fly abeam instead of directly over en route VORs. I also use SLOP to avoid tracking the exact centerline of an airway. One of my unspoken fears has been that I might climb or descend into an airplane flying along the same route. Seeing and avoiding other aircraft at such a time can be difficult. Or perhaps another airplane might climb or descend into me. My first aviation employer, Paul Bell, who owned Bell Air Service, was killed when a Piper Tri-Pacer descended into the Cessna 180 he was flying. Both aircraft were flying along the same airway. This was in 1956 when navigational accuracy was nothing like it is today.

A possibly unanticipated consequence associated with GPS accuracy is that there are occasions when it is too accurate. SLOP introduces randomness that artificially degrades the accuracy of GPS navigation and goes a long way toward reducing the potential for a midair collision.