Wind shear lurks in the clouds worrying aircrew

Press, 5 August 1985, Page 6

Wind shear lurks in the clouds worrying aircrew

By

H. JOSEF HEBERT

of

the Associated Press (through NZPA) Washington

The United States Government has long studied wind shear, a suspected culprit in the crash of Delta Air Lines flight 191, but radar that could warn pilots of impending danger is still years away from installation at airports. More than 100 large airports have sensors that are supposed to detect sudden shifts of wind, but critics say the sensors have been of little help in preventing a number of wind-shear accidents in recent years because their range and effectiveness is severely limited. Sensors were in use at Dallas-Fort Worth airport when flight 191 crashed on Saturday and were at New Orleans airport where a Pan American World Airways Boeing 727 crashed two years ago when it encountered a powerful wind shear just beyond the runway soon after takeoff. Government researchers and aviation safety experts say the answer to windshear detection is the socalled doppler radar. It can scan the skies, detect the movement of air masses and pinpoint the shears well before they become a hazard to an un-

suspecting aircraft. Unlike the "low level wind shear alert system” which has sensors that concentrate on wind movements close to the runways, the doppler radar is able to scan into the skies and along critical flight paths several kilometres beyond the airport.

The Delta Tri Star crashed about 400 m from the airport. The system, which “has been satisfactorily tested” at a Colorado airport, was not expected to be installed at United States airports until the 19905, said a Federal Aviation Authority spokesman, Edmond Pinto. The wind shear — the most powerful and dangerous variation being the socalled “microburst” — is considered by pilots one of the most ominous dangers in the sky. Lurking in storm clouds, it is almost impossible to deal with when a plane is taking off or landing.

It is caused by a sudden change in wind direction or speed, causing a strong flow of air straight downward, much like water pouring from a tap. When the air hits the ground it flows outward in all directions.

As a plane approaches a shear it meets an increasing headwind that provides the aircraft lift. As it passes through the strong funnel-

like down draft, the plane is pushed downward. Finally, as it moves away from the funnel, what was a headwind becomes a tailwind, causing the speed of the air travelling over the wings to drop and reduce the plane’s lift.

The sudden loss of lift combined with the force of the down draft can push a large aircraft to the ground. It all happens within a few seconds giving pilots little time to make adjustment in engine power and, say some aviation experts, may not even given the engines time to respond.

“Successful wind-shear penetration may suggest a higher level of airmanship than can be expected from the average pilot,” says Robert Stengel, a professor of aerospace engineering at Princeton University who has studied the phenomenon. Air safety experts say that shears ate believed to have been responsible for at least 16 airline accidents in the last 20 years.