Power Surge Probed as Cause of TWA Jet Crash

The search for what touched off the blast that downed TWA Flight 800 last year is focusing increasingly on the electrical wiring for sensors that measured how much fuel was in the center tank.

National Transportation Safety Board investigators have conceded they may never pinpoint the precise ignition source. But they said Wednesday that a power surge in one of those wires that had somehow been stripped of its protective insulation might have caused a spark that triggered the explosion.

Dave Johnson, a civilian engineer employed by the Air Force, testified during Wednesday’s session of the weeklong NTSB crash hearings that the insulation on a fragment of one sensor wire found in the wreckage apparently had cracked--perhaps because of aging--sometime before the explosion.

Minute traces of chemical residue on the exposed copper wire indicate that it had been exposed to fuel in the tank for some time. A similar wire recovered from the fuel tank of another old 747 showed the same sort of residue.

Electronics experts have testified that the normal voltage in the sensor wires is far too low to create a spark that could have ignited the fumes in Flight 800’s nearly empty tank. But NTSB engineer Bob Swain said his investigators are looking at the possibility that some external power source sent a surge of high-voltage electricity through a damaged wire.

Although the fragment with the cracked insulation showed none of the scorching that would indicate a spark had arced there, Swain pointed out that most of the wiring for the tank’s seven fuel sensors has never been found.

He said a fragment that did arc may lie amid a vast tangle of wire still piled in a hangar in Calverton, N.Y., where the wreckage of Flight 800 has been reassembled. “You search a lot of hay until you find a needle,” Swain said.

The NTSB engineer said his investigators are looking for evidence of several possible scenarios that might account for a power surge from outside the tank. These include some sort of short circuit or inadvertent contact between the sensor wires leading to cockpit fuel gauges and some other high-voltage wire.

One scenario involves induction, a phenomenon in which power from one wire is transferred magnetically to a parallel wire, even though they never actually touch.

Another involves a high-voltage wire that passed close to those same sensor wires where they were routed through the forward cargo hold. This high-voltage wire has never been found. Engineers suggest that if it is, it may turn out to have been damaged by shifting cargo, and that damage might have caused a short circuit that was passed on to the sensor wires.

Several other theories about what might have ignited the explosion off Long Island that tore apart the jetliner, killing all 230 aboard, have fallen from favor or been discounted altogether.

Both the NTSB and the FBI--which joined in the first 16 months of the investigation before concluding there was no evidence of a criminal act--have dismissed the possibility or a bomb or missile, citing a complete lack of supporting evidence.

Engineers say heat from the air-conditioning system increased the flammability of the fuel vapors, but not enough to set them off without a spark.

There is still no evidence that heat from other systems or flames from some other small blaze aboard the plane touched off the explosion.

The difficulty in pinning down the ignition source and tests showing how easily warm fuel vapors can be ignited have led the Boeing Co. to abandon its long-held position that the only way to prevent fuel tank explosions is to eliminate the possibility of a spark.

In what it called “a major philosophical shift,” Boeing said earlier this week that from now on, it will look at both eliminating sparks and at reducing flammability.