Urgent new safety recommendations by the National Transportation Safety Board signal that the agency may be closing in on mechanical failure as the cause of the TWA Flight 800 crash.
In a letter to the Federal Aviation Administration Friday night, the NTSB outlined a "series of urgent recommendations" for several design and operational changes to protect aircraft fuel tanks from sources of heat and sparks.
The NTSB cautioned that it had not concluded what caused the Boeing 747 to explode off Long Island on July 17, killing all 230 aboard. But it said the investigation had uncovered possible sources for a spark that could have ignited the plane's center fuel tank.
The independent government agency, charged with determining causes of airplane accidents, usually does not issue urgent recommendations until it has solid and corroborating evidence of a problem.
The announcement has irritated the FBI, a co-partner in the investigation. James Kallstrom, the FBI deputy director in charge of the case, said that the bureau is still investigating the crash as a criminal case and, in a criticism of the NTSB, added that "it is not prudent nor professional to speculate on what might or might not be the cause of this tremendous tragedy."
"The possibility of some criminal act providing the source of ignition still is being investigated," says FBI spokesman James Margolin. "There is insufficient evidence to say whether it was a criminal act or some other catastrophic event."
Investigators have known since August that the plane's center fuel tank, which carried about 50 gallons of fuel in a 12,890-gallon tank, exploded. But investigators have not been certain whether that explosion was the only one or whether it was set off by a previous explosion. A safety board member, however, now has said that the reconstruction of the plane showed the center wing's partially full fuel tank to be deformed "consistent with an explosion originating within the tank."
Moreover, the Seattle-based Boeing Company has produced evidence that the center fuel tank could become hot enough to vaporize the fuel - meaning the vapor could have burst into flames in the presence of an ignition source, such as a spark. The company conducted the test on a 747 in California's Mojave desert about a month after the crash.
NTSB officials now speculate that a spark may have been produced by a buildup of static electricity in the cross-feed manifold, the pipe that connects the five main fuel tanks on a 747. A break in the pipe's insulation at a joint could allow a spark to jump the gap, or a pinhole in an O-ring could cause fuel to spray and build up a static charge.
An engineer who has worked on the 747 says that it can be difficult to determine what goes wrong with older planes. Aircraft are tested for problems with all new parts. And planes are designed to last about 20 years, although most experts agree that with regular inspections and maintenance, they can be used much longer. The plane on TWA Flight 800 was 25 years old.
"If you crawl under a 30-year-old car, especially if it was owned by several people over the years, it is not necessarily going to look the way the manufacturer built it," the engineer says. He has seen planes from all over the world that would "really shake you up" because of the poor quality of work that had been done on them.
The FBI's Mr. Kallstrom says neither the bureau nor the NTSB has any evidence to prove or disprove any of the theories - that the explosion was caused by a bomb, a missile, or a mechanical malfunction.
NTSB and FBI members met Saturday with about 100 family members of crash victims. A spokesman for the families, Jose Cremades, said that both NTSB and FBI officials still insist they are considering all three theories. But NTSB officials told them that most of the center fuel tank had been recovered and that no evidence of a bomb or a missile was apparent.
In response to the NTSB recommendations, Boeing, maker of 60 percent of the world's commercial jets, said: "It is important to remember that these are recommendations only, and do not presume to determine the cause of the accident." Still, said a company spokesman, Boeing "will take the appropriate action to support any directives from the FAA."
The FAA is not required to act, but in the past the agency has implemented 90 percent of the NTSB's urgent recommendations. The new recommendations including keeping center fuel tanks about half full to prevent vapor buildup and adding temperature gauges that can be read by pilots.