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Wind Shear Detection System
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13871 |
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Section : |
NATURAL SCIENCE
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| Issue
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12 / 1988 |
3,460 Words |
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Joann T. Dennett
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The steady regularity of aircraft crashes in the United States—about once every 18 months during the 1970s—led atmospheric scientists and accident investigators to suspect some sort of short-lived, low-altitude air disturbance was at fault. The list is long. In 1973, an Iberian Airlines DC-10 with 167 people aboard crashed during its approach to Boston's Logan International Airport. In 1975, an Eastern Air Lines crash at John F. Kennedy International Airport killed 113 people. The aircraft hit the "outflowing winds and downdraft associated with thunderstorms," according to the accident report. In the next two years, there were three more incidents: a Continental Airlines plane at Denver's Stapleton International Airport (August 7, 1975), an Allegheny Airlines flight at Philadelphia (June 23, 1976), and another Continental flight at Tucson, Arizona (June 3, 1977).
After defining the problem as wind shear—a sudden change in wind speed or direction—scientists devised the Low Level Wind Shear Alert System (LLWAS), a ground-based network of wind instruments arrayed at airports, in 1976. But LLWAS was operating in New Orleans the day a Pan American Airways flight crashed on takeoff in 1982. The death toll: 153. It was clear that LLWAS was not the answer, and researchers stepped up their efforts to perfect such new technologies as the Doppler radar systems the new systems were well into development in 1985 when the killer struck again, downing Delta Air Lines flight 191 at Dallas and killing 134 people.
And these are only the big crashes. There have been many other "incidents" over the years. Robert L. Ireland, a staff engineer with United Air Lines (UAL), describes one: "We had an encounter at Chicago about the same time as the Pan Am [crash] in New Orleans. In August of 1983, a United plane taking off hit a rain squall that hit the ground just as the aircraft arrived." The United plane, on its takeoff climb, descended to below 50 feet. "The only difference from the Pan Am incident," Ireland said, "was there were no trees." So, while the United flight recovered, the Pan Am aircraft hit trees and crashed.
The Most Powerful Source of Wind Shear
Excerpts from the National Transportation Safety Board's (NTSB) report on the 1985 crash at the Dallas-Fort Worth International Airport point to the killer: "About 17 seconds before initial impact, the airplane encountered rapid reversals in the lateral, horizontal, and vertical winds. … The accident was not survivable for persons seated forward of Row 40 …." Such a description most
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