Mark Mallari writes from Manila, the Philippines, with a question about one of the most successful passenger jets in history: "I've heard from more than one source that aerodynamic peculiarities cause the Boeing 757 wake turbulence to be much greater than other airliners, such that pilots exercise additional caution when following one. I can see no obvious reason, and, to me, the 757 looks just like any other airliner. Could you please enlighten us?"
Even the experts disagree a fair amount on this question, says Kamran Rokhsaz of the department of aerospace engineering at Wichita State University in Kansas. "You can find very credible aerodynamicists arguing on both sides," he says. "Some feel that there is nothing special about the 757, while others do believe that it has a much stronger wake vortex than other aircraft of the same size."
When an aircraft wing generates lift, it also creates horizontal, tornado-like vortices that can knock a trailing airplane, especially a smaller one, out of controlled flight. According to NASA, the severity of the wake vortex hazard depends on several things: the size, geometry, and operating conditions of both the leading and trailing aircraft; the distance between the two; the angle and altitude of the aircraft in the encounter; and the atmospheric conditions, such as wind, that influence the strength and decay of vortices.
In 1969, after the first jumbo jets appeared, the Federal Aviation Administration tried to determine the wake characteristics of large aircraft. This led the agency to come up with a separation standard, or following distance, of 10 miles for aircraft trailing an Air Force C-5 cargo transport or a Boeing 747 commercial transport. (This figure was later revised to five miles for smaller aircraft trailing those with takeoff weights of more than 300,000 pounds.)
Then, in January 1983, the Boeing 757 entered service.
"Even though the 757 is a narrow-bodied jet airliner [the fuselage has a single aisle], it is treated as a large wide-body by air traffic controllers to protect small aircraft," says Robert van der Linden, chairman of the aeronautics division at the National Air and Space Museum. "The 757 features a very efficient supercritical wing, which, during certain brief periods of flight during takeoff or landing, can produce a wake vortex stronger than that of a much larger Boeing 747 jumbo jet. Therefore, the [required] separation is longer than [for] other narrow-bodied airliners."
The supercritical wing, conceived by Richard Whitcomb in the 1960s, gives aircraft increased cruising speed, greater fuel efficiency, and a greater range. It's now commonplace on subsonic commercial transports.
In the 1990s, 757s were involved in a series of accidents during airport approaches, which called attention to the wake vortex problem. In 1992, a Cessna Citation trailing a 757 into Billings Logan International Airport in Montana crashed, killing two crew members and six passengers. In 1993, an Israel Aircraft Industries Westwind, also trailing a 757, crashed at the John Wayne Airport in Santa Ana, California, killing two crew members and three passengers. Additional 757-related accidents involved a Cessna 182, a McDonnell Douglas MD-88, and a Boeing 737; fortunately, there were no fatalities.
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."