Is the Boeing 757 a threat to other airliners?
An unusual wake vortex has landed this airliner in a class by itself.
- By Rebecca Maksel
- AirSpaceMag.com, May 27, 2008
NASA Langley Research Center
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As a result, the industry set out to determine if the wake of the Boeing 757 is larger or more hazardous than that of other aircraft. The assessment of the Boeing 757 and the 767 was inconclusive. While the 757's wake decayed faster than that of the 767, its vortex velocity was approximately 50 percent higher—during one test. Because of this single unusual measurement, the FAA placed 757s in the wide-body category for separation rules, along with the larger 747s and 767s. At this time, the 757 is the only narrow-body with this restriction.
Boeing maintains that there is nothing about the wing design that would cause wake turbulence. "It's because [the 757] is so quiet and operationally efficient, and able to fly into airports for the smaller guys, that the [FAA] restriction was levied," says Elizabeth Verdier of Boeing public relations. The 757 is no longer in production, although most of the 1,050 airplanes sold to airlines are still in service.
"The wake from the 757 is no stronger or weaker than you would predict from the weight, span, and speed of the airplane," agrees NASA's David Hinton, who has taken wake measurements at airports around the country and is principal investigator of NASA's NextGen Airportal Project. In the 1990s, engineers tested a number of airplanes to determine safe following distances. "We were really looking for ways to predict wake vortex behavior in general," Hinton says. "There's a formula: You can take the weight, the speed, and the wingspan of an airplane, and then you can predict the circulation strength from that. And we never saw anything [for the 757] that differed from what you would expect."
At about the same time, the National Oceanic and Atmospheric Administration did a study at Idaho Falls to measure velocities in the wakes, and released a report concluding that the 757 wake was the strongest the researchers had ever seen. That led to hearings on Capitol Hill and the unusual spacing regulations for the 757. But the peak velocities measured by NOAA were very localized, according to Hinton. "It was a meteorologist writing on a parameter that does not correlate to the threat to a following airplane," he says. "If you go back and look at the circulation strength of the wake, which does correlate to how hazardous it is [to the trailing aircraft], the 757 wake was no different than you would expect."
Nevertheless, the new regulations stuck. "I can understand why people would think [the 757's wake vortex] was stronger," says Hinton, "because there was so much hysteria at the time. But I would hate to see the debate reignited."