Precautions Ordered for 747 Fuel Tanks

The Federal Aviation Administration on Thursday ordered airlines to keep some fuel in the center tanks of their Boeing 747 jumbo jets during normal operations because of sparks that could touch off an explosion when the tanks are dry.

Citing similar concerns, the agency also ordered the airlines to stop using the tail fuel tanks of all 747 model 400 jetliners.

FAA officials said the orders were prompted by evidence that worn pump shaft bearings could lead to chafing that would throw off sparks. The danger exists when a tank is drained, leaving behind a highly combustible mixture of fuel vapor and air.

Sparks in a drained center fuel tank are believed to have touched off the blast that downed TWA Flight 800 off the coast of New York in 1996--killing all 230 on board, but the FAA stressed that those sparks were not generated by a worn fuel pump bearing.

“That part was recovered after the TWA crash, and there was no damage to it,” said Diane Spitaliere, a spokeswoman for the agency.

The airworthiness directive covering center fuel tanks affects all 246 Boeing 747s currently registered in the United States. Airlines are being required to keep at least 3,000 pounds of fuel--about 430 gallons--in the tanks while the fuel pumps are running.

Scavenge pumps, which are not affected by the order, can still be used in flight to transfer the remaining 430 gallons to other tanks, from which the fuel can then be pumped to the planes’ four engines. However, the transfer is a slow and cumbersome process that is not always practical.

The other directive covers tanks in the horizontal stabilizers of 400-series planes, the late-model 747s with the distinctive, turned-up wingtips. Horizontal stabilizers are the twin, wing-like structures in the tails of the jetliners. Figures were not immediately available on how many 747-400s currently are registered in this country, Boeing said.

The tail tanks in 747-400s are shallow, so there is little fuel covering the fuel pumps, Spitaliere said. Not wanting to run the risk of exposing any worn shafts that could cause sparks, the FAA ordered the airlines to discontinue use of the tail tanks.

Spitaliere said the tail tanks carry about 3,000 gallons of fuel. Discontinuing their use will lop about 250 to 300 miles off the range of the 747-400s.

She said the worn shaft bearings were discovered after several pilots reported warning lights indicating that the pumps were failing to deliver the desired fuel flow.

The airworthiness directives are interim measures that will remain in effect until engineers determine what fuel pump fixes are needed, Spitaliere said.

“Once they do that, there’ll probably be another directive ordering the fixes,” she said.