Order to airlines would follow advisory from the plane's maker about danger of using pumps

Thursday, April 12, 2001

By JAMES WALLACE
SEATTLE POST-INTELLIGENCER REPORTER

The Federal Aviation Administration is expected to order the nation's airlines to adopt new safety procedures that would further reduce the chances of a fuel-tank explosion on Boeing's 737, the world's most often-flown jetliner.

An industry source said last night the federal agency will issue an emergency airworthiness directive, perhaps as soon as this week, as a result of the explosion last month of the center fuel tank of a Thai Airways 737 at the Bangkok airport.

The directive would order airlines not to operate two fuel boost pumps in the center tank when the tank does not have fuel in it.

Those pumps were running on the Thai jet even though the center tank was empty, the source said.

The National Transportation Safety Board, in a statement yesterday, said there is no evidence the explosion of the center fuel tank was caused by a bomb or sabotage.

The board also said the air conditioning units on the Thai jet had been running continuously for about 40 minutes while the jet sat on the ground at the Bangkok airport on a hot day.

It is the third known incident since 1990 -- including the TWA Flight 800 catastrophe -- in which the center fuel tank of a Boeing plane blew up after air conditioning units ran for a long time while the planes sat on the ground on a hot day.

Even though fuel tanks explosions are very rare, the safety board has been concerned for some time that a spark inside the center fuel tank could ignite fuel vapors if those vapors are heated to explosive levels.

The air conditioning units are located under the center fuel tank on Boeing-made planes, and the heat given off during ground operations can be get into the tank, raising the temperature of fuel vapors.

A flight attendant was killed and several other people were injured by the blast on the Thai jet, which was first thought to have been a bomb planted to kill the Thai prime minister. He and his son were waiting to board the jet with the other passengers.

Boeing last night sent a telex to all 737 operators recommending the fuel boost pumps in the center fuel tank not be used when the tank is empty. The FAA's airworthiness directive is expected to mirror the Boeing telex but make the recommendation mandatory, the industry source said.

The boost pumps are used to transfer fuel from one tank to another.

It is not known what role, if any, the pumps played in the explosion on the Thai jet.

What is known from the board's statement yesterday is that fuel vapors in the tank would have been heated by running the air conditioning units for so long while the jet was on the ground.

All of Boeing's planes except the 717, formerly the McDonnell Douglas MD-95, have the air conditioning directly below the center wing tank.

McDonnell Douglas jets have air conditioning units in the front or rear of the plane. Airbus jets have air conditioning packs under the center wing tank, but the design incorporates a cooling system.

The safety board last year criticized the design of Boeing jets because of the placement of the air conditioning units and the lack of a ventilation cooling system.

The criticism came as the board ruled that fuel vapors in the center wing tank of TWA Flight 800 had been heated to explosive levels because the air conditioning units on the 747 had run for more than two hours while the 747 was parked on the ground. The tank exploded about 13 minutes after the jet took off from Kennedy airport on a flight to Paris July 17, 1996, killing all 230 aboard.

The board said a spark of unknown origin in the tank apparently ignited the vapors. Had the vapors not been heated, there would have been no explosion, the board said.

Until last month's explosion on the Thai jet, the only known 737 center fuel tank explosion was that of a Philippine Airlines jet in 1990 at the Manila airport. The explosion killed eight people.

As was the case with the Thai jet, it was a hot day and the air conditioning packs on the Philippine Airlines jet had been running for some time when the fuel tank blew up as the plane was being pushed back from the gate.

Last May, Boeing recommended in a service letter to operators of its jets that air conditioning units not be turned on when a plane is on the ground if the outside air temperature is higher than 60 degrees. The FAA subsequently issued an informational bulletin to FAA representatives assigned to each air carrier encouraging the application of Boeing's recommendations.

But the FAA did not issue a rule that would make Boeing's recommendation mandatory. That could change as a result of the Thai explosion, according to people familiar with the matter.

The Seattle Post-Intelligencer reported two weeks ago that the investigation of the Thai Airways incident was focusing on a center fuel tank explosion and not a bomb.

In its statement yesterday, the safety board said the accident occurred at 2:48 p.m. on a day with temperatures the high 90s. The initial explosion of the center tank was followed 18 minutes later by an explosion in the right wing tank, the board 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," the board said.

The safety board last year strongly recommended that inert nitrogen gas be pumped into jetliner fuel tanks to lessen the danger of an explosion.

That recommendation is still under study by the FAA.

The FAA has said that computer modeling shows that a flammable fuel and air mixture exists in the center fuel tank of commercial jetliners about 30 percent of the time.

That would only be reduced to about 25 percent by not running air conditioning units when a plane is on the ground, according to the FAA.

But pumping inert nitrogen into tanks would dramatically reduce the time a dangerous fuel mix is present to about 2 percent, the FAA has said.

The FAA is studying a ground-based system that would pump inert nitrogen into the center fuel tank before takeoff. The cost of that system would be about $1.6 billion over 13 years and would take three years to implement on all U.S. jetliners.

The agency is awaiting a final report by an aviation rule-making advisory committee before deciding how to proceed. That report is expected by July.

P-I reporter James Wallace can be reached at 206-448-8040 or jameswallace@seattle-pi.com