Dangerous Wake: Wing vortices yield a deadly secret

A new mathematical analysis of an aeronautical hazard known as wake turbulence might eventually lead to improved air safety and increase the number of flights at major airports, scientists say.

BACK OFF. Smoke from flares dropped by this Air Force C-17 transport highlights the swirling vortices (red arrows) trailing it. U.S. Air Force

Airplanes get their lift from the pressure difference between air flowing above and below the wings. At the wings’ tips, the high-pressure air from below the wing spills out and upward, creating two swirling tubes of air. These powerful vortices–one rotating clockwise and the other, counterclockwise–can trail behind an aircraft for more than 30 kilometers, says Philip G. Saffman, an aerodynamicist at the California Institute of Technology in Pasadena.

The strength of a plane’s trailing vortices depends on its size, weight, and speed. The core of each vortex spilling from a hefty airliner can be several meters in diameter and contain air swirling at a speed of about 100 meters per second. Small aircraft that encounter such gusts can be tossed about like confetti, but even large planes can take a violent beating from vortices of other planes. The wake vortex from a Boeing 747 airliner can send a 737, its smaller cousin, into a sudden 45 bank, says Saffman.

Using a mathematical model, Saffman and his Caltech colleague David J. Hill examined how such vortices behave when they encounter wind shear, a natural phenomenon in which adjacent layers of air move at different speeds or in different directions. The researchers found that when the rotation that results opposes an aircraft-trailing vortex, it can cause the vortex to dissipate more quickly than usual. However, when the wind shear rotates in the same direction as a vortex, the swirl persists longer than it does in the absence of wind shear. Saffman and Hill report their analyses in the July 8 Proceedings of the Royal Society of London A.

The Federal Aviation Administration’s current rules regarding the spacing between aircraft as they take off, fly, and land stem from years of experience, says George C. Greene of the agency’s Office of Aviation Research in Hampton, Va. He points out that strong crosswinds–which can create the wind shear that Saffman and Hill found would make a trailing vortex persist–would tend to sweep the vortex hazard out of the flight path of following aircraft.

Nevertheless, says Greene, the team’s finding may someday influence operations at airports where planes take off and land simultaneously on parallel runways.

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