Science: Dangerous Wake

Wise pilots do not have to be told to fly clear of the wakes of nearby aircraft, especially big ones. They know that the turbulent air behind big, fast planes may be full of invisible, wing-racking bumps. And the danger has been growing worse as airliners boost both speed and size. Last week Aerodynamicist William A. McGowan of the National Aeronautics and Space Administration reported just how dangerous jet-age wake bumps can be.

When the air flows past an airplane’s wings, it slips sideways past the tips and swirls into two twisting eddies that linger in the plane’s wake for as long as a minute. Even after the airplane that made them is miles away, the eddies spin with surprising violence. A modern swept-wing jetliner, flying at 220 m.p.h. as it slows down approaching an airport, generates two twirling cornucopias of air with cores 22 ft. in diameter and outside layers rotating at 35 m.p.h.

An airplane that hits this invisible turbulence would be slammed upward and downward as if it had flown through a miniature thunderstorm. A light airplane flying through the core itself, says Mc-Gowan, “can experience loading conditions that exceed the design ultimate load factors,” i.e., can be torn apart. Although no supersonic airliners are flying yet, McGowan looks forward to their take-off with some trepidation. Their wake will be strong enough to knock the wings off a good-sized commercial airliner.