Nov. 6, 2000 A warning system for detecting windshear at airports might have been able to help the pilot of Singapore Airlines flight SQ006, had it been completed. "If the Taiwan Low-Level Windshear Alert System (LLWAS) at Chiang Kai-Shek airport had been completed, it would have been able to provide windshear information to the pilot for the conditions that were occurring at the time of the accident," says Bill Mahoney of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Windshear is a rapid change of wind speed or direction over a short distance. One form of windshear that is particularly dangerous to aircraft are microbursts, which are very short-lived, intense, downdrafts of air. Windshear is also associated with frontal systems that are the result of thunderstorms, where the front marks an abrupt transition in wind speed and direction.
NCAR is working with the Taiwan Civil Aeronautics Administration to modernize the weather detection systems at three Taiwan airports. Mahoney is the program manager for installing LLWAS at Chaing Kai-Shek (CKS) airport. Mahoney says the LLWAS is scheduled to be completed by the end of this year.
LLWAS was originally developed by the FAA in the 1970's to detect large scale wind shifts (by comparing readings from a number of wind sensors), after an accident at JFK airport in New York when a plane landed during a wind shift created by the interaction of a sea breeze and thunderstorm outflows.
In the 1980's NCAR determined that microburst windshear events were very dangerous to aircraft below 1000 feet. To address the problem, NCAR developed a new LLWAS system that detected and located microburst events. "The system was designed to provide alerts specific to each runway in operation," Mahoney says.
Now in its third generation, most LLWAS systems have between 12 and 16 wind sensors, but the largest one, at Denver International Airport, has 32 sensors. Each airport site is assessed to determine the geometry needed for the sensor network.
While it is still unclear what role the weather played in the Taiwan crash, Mahoney says "the weather that was occurring at the airport could certainly have included windshear."
According to Kelvin Droegemeier, Director of the Center for Analysis and Prediction of Storms at the University of Oklahoma, the threat of windshear is greatest when airplanes are close to the ground. Microbursts occur when a downdraft of air hits the ground and spreads out radially, creating a thin layer of fast-moving air near the ground. "When the airplane descends into this layer, the flow over the wings can increase or decrease abruptly depending on the plane's direction," says Droegemeier. This can cause the plane to suddenly rise or fall, forcing the pilot to make a correction. However, as the plane crosses the other side of the microburst, the flow over the wings changes again, and, "if not recognized and corrected, the plane may not have time to recover."
In addition to LLWAS systems at airports that have reduced the danger of windshear events, Droegmeier points out that, "tremendous strides have been taken to improve pilot training and understanding of wind shear."
Third generation LLWAS systems, like the one being installed in Taiwan, already exist at 9 U.S. airports, and are being put in place in Korea, Singapore, Saudi Arabia and Kuwait. Phase-2 LLWAS systems (which have 5-6 sensors, rather than 12-16), are in place at 100 U.S. airports. Mahoney points out that, "a Phase-1 or Phase-2 LLWAS is not designed to detect microbursts per se, but if the flow is large and strong, it may alert [the airport controller]."
LLWAS is not the only system for detecting windshear. The Federal Aviation Administration also uses Terminal Doppler Weather Radar and other radar solutions to detect windshear events.
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