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Air safety: the wind shear factor

When Pan Am Flight 759 crashed during takeoff in New Orleans last July 9, safety specialists blamed a recently discovered weather phenomenon: the microburst.

Scientists link microbursts directly with three such US jetliner crashes over the past seven years. Now they say they're learning enough about these sudden, fierce downbursts of air to prevent similar accidents from happening in the future.

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As part of the $3 million Joint Aircraft Weather Studies (JAWS) project, researchers just spent three months probing 5,000 square miles of skies around Denver's Stapleton International Airport. With the aid of five experimental aircraft, a vast network of weather stations, and a high-flying U-2 jet, observers recorded more than 100 microbursts. ''We didn't even expect to see 25, '' says John McCarthy, one of three project leaders.

Dr. McCarthy estimates that the data will not be fully analyzed until 1985. But a number of significant findings have already surfaced.

* The microburst phenomenon is much more common than previously thought. A number of accidents and close calls attributed to wind shear - an extremely abrupt change in the direction of wind acting much like a riptide - might be more specifically credited to the microburst, the source of wind shear. Previously, scientists attributed wind shear to ''gust fronts,'' blustering winds accompanying a storm front. JAWS researchers now say it may account for up to 98 percent of all hazardous wind shear.

* Microbursts appear to occur in groups. Those associated with fairly dry weather seem especially inclined to appear in one-hour batches. Thus, the sighting of one microburst may indicate the imminent formation of more and provide a reliable alert for pilots.

* Profiles of 50 of the microbursts observed are detailed enough to be used to program flight simulators. Airline officials says they consider this a major step toward training pilots to handle wind shear. Before JAWS, such profiles did not exist.

* The $12 million Low Level Wind Shear Alert System (LLWSAS), a ground-based array of wind gauges installed by the Federal Aviation Administration (FAA) at 58 airports including New Orleans, is inadequate for detecting microbursts. Many of these downbursts occur in a runway-sized area - too local to be observed by the gauges - or are too far aloft to affect them.

* The Doppler microwave radar may be the key to a comprehensive microburst watch. Operating on the same principle as police traffic radar, the Doppler takes 500,000 measurements a minute, vertically and horizontally. This is compared with the LLWSAS's 1,000 ground-level measurements a minute.

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The focus of early findings is on detection and warning. It's believed that formal recommendations about a comprehensive detection system will be made to the FAA in June. Included in this will be a proposal for a comprehensive microburst watch patterned after the severe storms watch.

Just as critical, Dr. McCarthy says, will be the need to make pilots aware of this issue. ''We'll be urging them to look hard at all the signs'' from not only the LLWSAS but the Doppler and other warning devices. Recommendations also will be made on ways to speed the wind-shear information around the airport. ''There's no question that the word should get around fast if a wind shear is there.''

At this time, microburst prediction is not seen as realistic. The short lifespan of a microburst - as little as two minutes - is a major stumbling block. ''By the time we know it's there, it's gone,'' says another JAWS leader, T. Theodore Fujita, a meteorologist at the University of Chicago. Dr. Fujita first described the microburst in 1980.

Another barrier is a lack of knowledge about its causes. ''Until we know why there is a microburst,'' adds Fujita, ''trying to predict it will be like trying to predict something where there's nothing.''

Scientists emphasize that the understanding of the microburst is still a long way off. This will be consuming most of the JAWS data analysis through 1985. However, even those results are not expected to answer all of the meteorologists' questions. ''These are problems for a lifetime,'' Dr. McCarthy comments.

The most severe microbursts were thought to occur randomly in thunderstorms. But some of the Colorado study's worst microbursts appeared in virga - wisps of rain evaporating as they stream into dry air. Evaporation cools the air as it plunges through warmer air below. Several hundred feet above the ground, this frigid, highly concentrated column begins to spread radially, until, at speeds measured up to 80 knots, it is horizontal. The effect is something like water running from a faucet onto a flat surface.

At high altitudes, such a burst will do little more than bump the airplane. At low altitudes during landings and takeoffs, however, it can be lethal.

A jet flying into this situation encounters, in quick sequence, a headwind, a downdraft, and a tailwind. The headwind, speeding airflow over the jet's wings, increases lift and pushes the plane upward. If caught by suprise, the pilot will instinctively compensate by cutting his speed. But suddenly, the tailwind takes away this lift possibly slowing the plane to a stall. ''The plane's got wings,'' says McCarthy, ''but it's a rock.''

A pilot attempting to fly out of this atmospheric riptide would be hampered by a further phenomenon. A characteristic of the landing jetliner is such, McCarthy found, that it will oscillate very slowly if jolted about every 35 seconds. Coincidentally, this is about the same length of time it takes to fly through a microburst. Socked first by a strong headwind, then by a tailwind, the plane will tend to rock slightly, making it more difficult to control.

''It's another damaging factor that, combined with the others, sums up to one big negative number,'' he adds.

The chief problem, scientists say, is the lift-robbing tailwind. Jets are quite vulnerable as their slow acceleration prevents pilots from escaping the tailwind quickly. By the time the jet reaches full power, it may be too late.

Of seven warnings systems tested, one of the most promising was Britain's Laser True Airspeed System. Aiming an infrared beam from the nose of the plane, it provide pilots with a three-second warning of oncoming microburst - enough time, they say, to effectively react in most cases.

The JAWS program, jointly administered by the National Center for Atmospheric Research and the University of Chicago, is funded primarily by grants from the National Science Foundation.