Thursday, May 25, 2006 - Page updated at 02:27 PM

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Information in this article, originally published May 25, 2006, was corrected on May 25, 2006. An earlier version of this story said that a proposed new FAA rule on aircraft fuel tank safety won't apply to Boeing 727 jets because they have no center fuel tank. In fact 727s do have a center fuel tank. The rule will not apply to them because the tank is not near a heat source.

By Dominic Gates

Seattle aerospace reporter

A team of National Transportation Safety Board investigators is flying to Bangalore, India, to assist in the investigation of a reported wing fuel-tank explosion on a Transmile Airlines Boeing 727-200.

Transmile is a Malayasian air-cargo company. The incident occurred May 4 while the airplane was on the ground in Bangalore. There were no passengers and no one was injured.

The investigation comes just ahead of the 10th anniversary of the loss of TWA Flight 800 off Long Island, N.Y., with the deaths of all 230 people aboard that Boeing 747.

The plane was brought down by a midair explosion inside the center fuel tank — not the wing tank. The cause of the ignition inside the tank was never identified. Since then, prevention of fuel-tank explosions has been an intense focus at the National Transportation Safety Board (NTSB) and at the Federal Aviation Administration (FAA).

"The tragic TWA 800 accident in 1996 highlighted the vulnerability of transport aircraft fuel tanks," said NTSB acting chairman Mark Rosenker in a statement Wednesday.

"A decade later, the issue remains a major concern of the Safety Board and is on our Most Wanted List of safety improvements. I am hopeful what is learned in this investigation may provide added impetus for a resolution of this problem without further delay."

Fuel-tank explosions have been confirmed in two other deadly incidents. In 1990, the center fuel tank in a Philippine Airlines 737-300 exploded while the aircraft was taxiing for departure, killing eight passengers. Chafed wiring inside the tank was suspected.

In 2001, a Thai Airways 737-400 exploded at the gate at the Bangkok airport after its center fuel-tank pumps were left running when the tank was dry. Passengers had not boarded but a flight attendant died and six others were injured.

From a series of safety studies after the TWA 800 disaster, the FAA has issued more than 100 airworthiness directives to prevent potential ignition sources inside the tanks.

It has also proposed a rule that would mandate fuel-tank "flammability-reduction systems" on all commercial aircraft that have a center fuel tank and require retrofits on in-service aircraft. Center tanks are heated and more vulnerable to explosions than wing tanks.

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"The risk is very small," said FAA spokeswoman Alison Duquette. "We feel that reducing the flammability of the tank is a safety net."

The most likely way of meeting the mandated requirement is the installation of inerting systems, which release nonflammable nitrogen gas into the space in the tank as the fuel level goes down in flight.

Boeing is now testing such an inerting system and expects in about six months to have it certified and ready to install on all new jets coming off its assembly lines.

Boeing's new 787 will have an inerting system in the wings as well as in the center tank. The A380 superjumbo will not have an inerting system, but Airbus intends to meet the reduced flammability requirement in other ways, said the FAA's Duquette.

The comment period for the FAA proposal ended earlier this month. The Air Transport Association, representing the airlines, submitted comments opposing the rule as not cost effective. The required retrofits on commercial fleet, which would be paid for by the airlines, would be costly at a time when airlines are losing billions of dollars.

Last fall, the FAA gave a preliminary estimate for the total cost for the U.S. fleet at approximately $808 million over 49 years, including $313 million for retrofitting.

If implemented, the new rule would apply to all Boeing jets and to Airbus A320s and A330s. Boeing air-safety spokeswoman Liz Verdier said the company will produce retrofit kits whether or not reduced flammability is made mandatory.