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RedditMark 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.
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At end of Feb. 2009, a Turkish Airlines Boeing 737 has made a crash landing at Amsterdam - Holland, killing the cockpit crew of 3 , 1 hostess and 5 passengers, just 3 Km. short of the runway. The result of the investigation is not yet published as 3 of March , but we have a Boeing 757, which landed just 2 minutes before, passengers who tell that they 'dropped' a thousand feet seconds before the crash, and an ex-army witness who heard a strange engine sound some seconds before the crash, where the plain was not there. Looks like, we have ( unfortunately ) a solid case at hand. Ismail Sarioglu from Istanbul
Posted by Ismail Sarioglu on March 3,2009 | 07:01AM
Ismail, Speculation about wake turbulence from the 757 has since been thoroughly discredited. Adequate separation was maintained between the two aircraft. The report as it stands now shows that while the aircraft was still ~2000ft above ground level a radio altimeter suddenly reported an incorrect height (-8 ft) which caused the auto-throttle to retard the throttles as if it were touching-down on the runway. This by itself should not have been a fatal problem as the pilot flying could have easily disconnected the auto-throttle and flown the remainder of the approach by hand. But for reasons we don't know yet none of the three pilots in the cockpit noticed the loss of thrust and consequent loss of airspeed. This inattention to basic airmanship is of much greater concern in that incident than any mechanical malfunction or effects from preceeding aircraft.
Posted by Jeff Buckles on March 8,2009 | 04:58PM