The action comes after maintenance checks in the past ten days uncovered a troubling number of problems with fuel pumps inside the center tanks of 747s.
More than a dozen of the pumps removed from planes have shown excessive wear -- a condition which the FAA contends could trigger a lethal spark inside a vapor-filled tank.
The fuel pumps, two in each center tank, are used to transfer fuel to the jet's engines. If the pumps continue to run after the fuel is drawn down, parts of the pumps rubbing against each other "...could cause hot spots and sparks, and a possible explosion," the FAA warns.
Airlines now will have to keep enough fuel in the 747 center tanks to keep the pumps covered. However, the weight of the extra fuel could reduce capacity on cargo planes and limit the range of a few passenger flights.
This is the latest safety order to emerge from the two and half year investigation into the 1996 crash of TWA flight 800. While that jet was downed by a fuel tank explosion, there is no evidence fuel pumps played any role.
The TWA explosion, which occurred on a New York-to-Paris flight and killed all 230 aboard, has prompted a debate in the aviation industry about how to deal with empty center and other fuel tanks.
Boeing, the 747s manufacturer, questions the latest FAA order, saying its testing does not show an explosion risk. But U.S. airlines have no choice but to make the changes, and the rest of the world's carriers will likely go along.
Under Thursday's emergency airworthiness directive, airlines have two options to prevent the dry operation of the 17,000-gallon center tank:
- Load at least 17,000 pounds of fuel into the tank before flight and stop using it when the level reaches 7,000 pounds, or;
- Load at least 50,000 pounds of fuel and stop drawing from it when the level reaches 3,000 pounds. A gallon of fuel weighs about seven pounds.
Operation of the 3,300-gallon horizontal stabilizer tank, unique to the 747-400, must stop immediately because the pumps in those tanks are always drained. It was unclear how the change would affect long-haul carriers that routinely use the tanks on nonstop flights from the United States to Asia.
"We are working with the FAA on an inspection process to allow them to be able to use that tank," said Boeing spokeswoman Kirsti Dunn.
She said that when Boeing and the part supplier tested some of the suspects parts over he weekend, "we were not able to generate a sparking ignition source for fuel vapor."