Trans World Airlines Flight 800 most likely exploded and killed 230 people because an electrical short circuit in the Boeing 747's aging wiring found a path to low-voltage wires in the plane's center fuel tank, the head of the National Transportation Safety Board's aviation safety division said yesterday.

Summarizing staff conclusions during the first session of a two-day board meeting, Bernard Loeb emphatically ruled out that a bomb, missile or any other high-energy, man-made explosion caused the New York-to-Paris flight to plunge into the Atlantic off the southern coast of Long Island.

After the meeting, the five-member board will issue a final "probable cause" report on the July 17, 1996, crash. However, Loeb appears to have telegraphed the board's likely conclusion.

Board Chairman Jim Hall opened the meeting with a denunciation of several groups that have concluded that a missile brought down the plane and that the safety board and the FBI are covering up the truth.

"To the Flight 800 families, I would like to add this," he said. "It is unfortunate that a small number of people, pursuing their own agendas, have persisted in making unfounded charges of a government coverup in this investigation. These people do a grievous injustice to the many dedicated individuals, civilian and military, who have been involved in this investigation."

Hall said that the investigation is technically complicated and that reasonable people can disagree, "but I take exception to those who consistently distort the record and persist in making unfounded charges of a coverup. They do a disservice to us all, but most especially to you, the TWA 800 families, who have suffered so much in this tragedy. And for that, I am sorry."

The safety board has repeatedly asserted that explosive fumes in the plane's nearly empty center fuel tank erupted about 13 minutes into the flight, blowing the front of the huge tank off and ripping the plane's forward section away. But left unanswered has been what provided the spark needed to set off the fumes.

Loeb said investigators had looked at numerous possible scenarios for what supplied the spark--a tire explosion, static electricity, a bomb, a fuel pump, a hot missile fragment, a meteor, lightning--and dismissed them.

"One ignition scenario that we could not deem unlikely . . . was that a short circuit involving electrical wiring outside the center wing tank somehow transferred excess voltage to fuel quantity indication system wiring leading to the center wing tank," he said. "Although the voltage in the fuel quantity indication system wiring is limited by design to a very low level, a short circuit from higher-voltage wires could allow excessive voltage to be transferred to fuel quantity indication system wires and enter the tank."

The fuel quantity indication system is like an automobile gas tank gauge system except that a 747's fuel quantity is measured by numerous long metal rods sticking up from the bottom of the fuel tank. Each rod is energized by a low-voltage current that could not provide enough of a spark to ignite fuel vapor. In earlier hearings, the board noted that wiring leading to these rods was often packaged in wire bundles containing high-voltage wires.

Loeb said wiring recovered from the 25-year-old aircraft "showed definite signs of deterioration and damage."

Inspections of older planes showed that deteriorated wiring is common. "It became clear from our investigation that current maintenance practices do not adequately protect aircraft electrical wiring, especially with regard to older airplanes," he said.

The board made six recommendations to the Federal Aviation Administration in 1998 to address potentially hazardous conditions found on aircraft wiring, Loeb said, and "the FAA has taken action toward addressing all of those recommendations."

Unlike many past NTSB accident reports, the TWA 800 report appears to be unusually kind to the FAA. The board, which investigates accidents but can only make recommendations, is often critical of the FAA, which has the power to impose safety rules on airlines and the aviation system.

The FAA has issued more than three dozen rules, called "airworthiness directives," on issues growing out of the TWA 800 crash and is considering others. Like many crashes, the TWA 800 case is leading to improvements in aviation safety, even in areas that appear to have nothing to do with the cause of the crash.

One development the FAA is moving toward is nitrogen inerting of fuel tanks using ground-based nitrogen-producing equipment.

Loeb noted that the TWA 800 center tank could have been explosive for only a short time during its flight--when the mixture of oxygen, fuel vapors and temperatures are right and a spark is present. As the plane climbed and the tank cooled, the tank would not have been potentially explosive.

Loeb said he agrees with the FAA that some form of nitrogen inerting would be the best way to prevent future fuel tank explosions.

Loeb noted that physical evidence from the plane "indicated irrefutably that a missile did not strike the airplane." He said that during the two days of hearings, board investigators will show that eyewitness reports of flare-like objects rising to meet the plane do not indicate a missile.

Loeb said "burning fuel from the airplane" as the rear section continued to fly for 30 seconds after the explosion likely appeared as streaks of light to the witnesses.

Robert Swaim of the board's aviation engineering division said the board gradually eliminated most of the possible causes of the spark until only the fuel quantity system appeared to be the likely suspect. He said he would probably never be able to say that was the cause for sure.

Swaim said low-voltage wires that run from the in-tank fuel gauge system to the cockpit were joined into one wire bundle with higher-voltage wires, including those for the 350-volt cabin lighting system, providing a possible source of a high-voltage charge. Board investigators also found that much of the wiring recovered from the plane had damaged insulation, raising the possibility that two wires in the bundle could have come in contact.

In addition, he said melted wiring was found in two places--under the tank and under the forward galley where the bundle would have been routed. Nearby was a large metal patch where mechanics had drilled holes for rivets, scattering in the area metal shavings that tests showed could lead to insulation cuts and electrical arcing.

But more intriguing was something the pilots said just before the crash that investigators had earlier thought was inconsequential. They noted a "crazy fuel-flow indicator." Swaim said tests showed that could be an indication of the beginning of arcing in the wire bundle containing the fuel quantity indicator wires.

Also, just before the explosion, the natural background electrical "noise" on a wire leading from the captain's microphone to the cockpit voice recorder was briefly interrupted twice; the channel was suddenly unusually clear of background hum. Investigators believe that may be evidence of the spark that caused the explosion. "Almost all of that energy went somewhere," Swaim said.

Flight 800

TWA Flight 800 most likely exploded because an electrical short circuit in the plane's wiring found a path to low-voltage wires in the plane's nearly empty center fuel tank, where it set off fumes.

The 747 plunged into the ocean off Long Island on July 17, 1996, killing 230 people.