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Winter/Spring 2008 Vol. 8 No. 1

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In Their Wake


Turbulence Trailing Planes Is Challenge in Crowded Skies

The Next Generation Air Transportation System, or "NextGen," is an interagency federal effort to accommodate increasing demand for air travel, which is expected to double or even triple by 2025. One goal of NextGen, for example, is to take advantage of GPS to safely permit planes to fly more closely spaced.

Unfortunately, GPS does not solve the problem of wake turbulence, the counter-rotating vortex of air -- an inevitable byproduct of lift -- that trails aircraft and is a danger for planes, especially smaller ones, flying too close behind. The heavier an aircraft, the stronger its wake, and the greater the wingspan, the longer a wake lasts. Thus, FAA's standards for how close planes can fly to each other at a similar altitude, or between takeoffs and landings, are based on size. To illustrate, heavy aircraft must stay four miles behind other heavy aircraft, while large planes need to remain five miles back and smaller ones must keep six miles distance.

A National Research Council committee asked by Congress to study the issue found that the current separation requirements, while conservative and safe, indeed prevent taking full advantage of GPS and other technologies that would allow closer flying. However, there is no meaningful metric to tell by how much the spacing could be safely reduced.

While wake turbulence is not the only obstacle to increased air capacity, the committee concluded that a robust wake turbulence R&D effort is needed to maximize the air transportation system's efficiency. For starters, wake vortices of new airplanes need to be characterized prior to the crafts' introduction into service so that appropriate separation criteria can be established. And the current heavy, large, and small designations are extremely broad, with planes varying in weight by hundreds of thousands of pounds within the same category. Among several technical challenges identified by the committee is recategorizing planes to account for detailed characteristics. More accurate wind monitoring and prediction, along with modeling and measurement of wake vortices, would permit pilots to continually adjust their spacing and allow for closer parallel approaches when two runways are available for landing. Vortex alleviation ideas should be explored as well.

Historically, FAA and NASA shared leadership of federal wake turbulence research, but given the space agency's current funding priorities, the committee recommended that FAA take the lead and become a vocal advocate for sustained R&D in this field.   -- Bill Kearney

Wake Turbulence: An Obstacle to Increased Air Traffic Capacity.Committee to Conduct an Independent Assessment of the Nation's Wake Turbulence Research and Development Program, Aeronautics and Space Engineering Board, Division on Engineering and Physical Sciences (2008, approx. 70 pp.; ISBN 0-309-11379-2; available from the National Academies Press, tel. 1-800-624-6242; $21.00 plus $4.50 shipping for single copies).

The committee was chaired by Anthony Broderick, former associate administrator for regulation and certification at the FAA. The study was funded by NASA.

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Copyright 2008 by the National Academy of Sciences