Center wing fuel tank suspected in Thai Airways explosion

One person was killed in the explosion that happened less than an hour before takeoff

From Mike Ahlers
CNN

WASHINGTON (CNN) -- A Boeing 737 that exploded on the ground at Bangkok Airport last month shows no traces of a bomb, and investigators are now focusing on the possibility that the explosion originated in the plane's center wing fuel tank, possibly heated by hot air conditioning units underneath it.

A similar scenario helped bring down TWA 800 -- a Boeing 747-100 -- off the coast of Long Island, New York -- during the summer of 1996, killing all 230 people aboard.

The Thai Airways International plane exploded on March 3 -- just 35 minutes before it was to depart with the prime minister of Thailand, prompting speculation that the explosion was an assassination attempt.

But the U.S. National Transportation Safety Board, which is providing technical support to Thailand, said Wednesday, "Despite a thorough examination of the wreckage by Thai and American bomb experts, no physical evidence of a bomb has been found to date."

The 9-year-old Boeing 737-400 was sitting at a gate at Don Muang International Airport in Bangkok when it exploded. A flight attendant aboard the plane was killed and seven people were injured in the blast and subsequent fire, which destroyed the airplane. No passengers were on the plane at the time.

"The accident occurred at 2:48 p.m. on a day with temperatures in the high 90s Fahrenheit," the NTSB said. "Air conditioning packs, which are located directly beneath the center wing tank and generate heat when they are running, had been running continuously since the airplane's previous flight, including about 40 minutes on the ground."

While investigators are quick to say they are far from establishing a cause for the explosion, they acknowlege air conditioning packs were deemed a factor in the TWA 800 explosion.

In that case, the TWA 800 sat on a hot airport ramp with its air conditioners running for more than an hour before taking off. It exploded 13 minutes after takeoff, splintering apart and plunging into the Atlantic.

Last August, the NTSB concluded the probable cause of the TWA 800 accident was "an explosion of the center wing fuel tank, resulting from ignition of the flammable fuel/air mixture in the tank." The NTSB said it could not determine the source of ignition, but that a contributing factor was the placement of the heat-generating air conditioning units under the fuel tanks.

The plane had been delayed for slightly more than an hour on the ground and had its air conditioners running, NTSB records show. The outside temperature was 71 degrees Farenheit.

Nearly a year ago on May 5, Boeing advised airlines worldwide to use ground air conditioning units to cool planes when they are on the ground for extended periods and the temperature is above 60 degrees Farenheit. The advisory did not specify the length of time planes can use their own cooling units while on the ground.

Airlines typically follow the manufacturer's guidance, but are not required to.

The FAA says that it did not issue an Airworthiness Directive, which would have given the voluntary recommendation the force of law. But it did issue a Flight Standards Information Bulletin advising FAA inspectors of Boeing's recommendation.

Boeing spokesman Tim Neale said it is too early to speculate on the cause of the explosion. "There's still plans to do more testing. As we've seen in so many accidents, it can start to look like one thing and then the evidence pops up that shows its something else," Neale said.

The FAA says it has issued or proposed issuing nearly 50 Airworthiness Directives to address safety concerns with center fuel tanks. It also has begun studying ways to make fuel tank fumes inert, rendering them safe from ignition sources.

Former FAA Chief of Staff Michael Goldfard said the similarities between TWA 800 and the Bangkok explosion are only partial.

"The similiarities are that it was a hot day in New York (when TWA 800 exploded). It was a hot day on this crash. The air conditioning system was running in New York. The air conditioning system was running in this crash on the ground which created a flammable mixture. What caused the spark that led to the explosion in this crash ... is yet to be determined.

"There could be a whole host of things that could have happened on that aircraft that led to this crash that has nothing to do with the others," Goldfard said.

The Boeing 737 is "the workhorse of the industry," he said. "It's one of the world's safest aircraft. And so for people who fly this is not one thing they should worry about."

Thai leader uncertain if explosive caused plane fire
March 4, 2001

National Transportation Safety Board
FAA
Thai Airways International
Boeing 737 Family

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