Eintime Conversion for education and research 10-20-2007 @ 07:24:19
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Microburst air downdraft could have caused Thailand holiday isle crash

KUALA LUMPUR, Malaysia (AP) — The Thai jetliner that crashed on Phuket island could have been hit by a microburst, a vicious form of wind shear that deceives pilots into slowing the plane perilously close to the ground and forcing it to drop like a stone, experts say.

Thai officials have said that the pilot of the One-Two-Go budget aircraft was warned of wind shear at Phuket airport as he was coming in to land on Sunday in a thunderstorm. The McDonnell Douglas MD-82 hit the runway, skidded into an embankment and burst into flames, killing 89 of the 130 people on board.

But it is not clear if the pilot was told about a possible microburst, or whether the airport had a system to detect the phenomenon, which is among the most dangerous of all weather-related threats to flying.

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"In fatal crashes it has been one of the worst causes," G. Brant Foote, director of the U.S. Research Applications Laboratory, which runs a program to develop weather technologies for aviation industry, said late Monday. The RAL is part of the National Center for Atmospheric Research in Boulder, Colorado.

"Anywhere you have thunderstorms you will have a microburst problem," said Foote. But aircraft have to be very unlucky to be caught in a microburst, and need to be in the wrong place at the wrong time, he said.

"You need to line up an aircraft, runway and microburst all at the right time to have a simultaneous encounter," Foote told The Associated Press in a telephone interview.

Microbursts are produced when a massive block of cumulonimbus cloud — the dark, wind- and rain-laden mass typically seen during thunderstorms — starts to decay. The cold air in the center of the cloud collapses and hits the ground at a high speed. The air then spreads out in all directions.

Aircraft flying through a microburst during takeoff or landing encounter a strong headwind followed by a downdraft, which shows as a sharp spike in the plane's airspeed.

"The natural reaction of the pilot then is to slow down the plane" to compensate for the spike, said David Learmount, a flight safety expert in Britain.

Because of this "not only you are being thrown into the ground (by the downdraft) but you are also losing speed," said Learmount, a former Royal Air Force transport pilot.

As the plane emerges from the microburst, it encounters a tail wind and the airspeed drops dramatically further. Coupled with the slowdown initiated by the pilot earlier, the result can be disastrous.

"That can make a difference between being able to fly and not being able to fly," said Learmount, the operations and safety editor of Flight International, a leading aerospace weekly magazine.

Microbursts, a well-known phenomenon for the last 20 years, have been blamed for at least six previous fatal crashes worldwide. But such tragedies were reduced to zero in the United States with the help of a microburst warning system developed by the National Center for Atmospheric Research for the Federal Aviation Authority between 1983 and 1988.

"In the U.S., all major airports have them ... But foreign governments have been slow in adopting them," said Foote, the RAL expert. "It is something that any airport should seriously consider. It gives them an extra margin of safety," he said.

The Thai Air Transport Department's director-general, Chaisak Ungsuwan, said the pilot of the ill-fated Flight OG268 decided to land in Phuket even after being informed about the wind shear.

"There was a strong wind and heavy rain and the visibility was poor. The pilots attempted to make a landing but they couldn't touch the runway and they decided to pull up. The plane lost balance and crashed," he said.

Vuttichai Singhamanee, director of the flight standard bureau of the Transport Ministry's Aviation Authority Department, said three out of six available solar-powered low-level wind shear alert systems were out of power at the time. He said they use the same system to detect microbursts.

Vuttichai said the systems could have made it difficult for pilot Arief Mulyadi to judge whether it was safe to land.

Mulyadi, who died in the crash, had come under fire from some Transport Ministry officials for landing despite wind shear warnings from the tower.

Foote, Learmount and other experts warn that it is too early to pinpoint any cause, and only data in the black boxes could provide clues to the disaster.

"Two things will be asked. One, did the pilot make a good decision to go ahead. Two, what was the quality of weather information that was being given by air traffic controller," said Learmount.

But ultimately, the buck stops in the cockpit, and it is up to the pilot to decide whether to land or not.

"If a pilot says he doesn't like the weather he can go somewhere else, or come back after 10 minutes. Deciding to die and deciding not to die is a very easy choice," Learmount said.

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10-20-2007 @ 07:24:19