NTSB Urges Inspections for ‘Unsafe’ 747 Wiring

Reacting to suspect wiring found in the wreckage of TWA Flight 800 and on other planes, federal officials Tuesday called for prompt inspections of Boeing 747s like the one that crashed off Long Island, N.Y., in 1996, killing all 230 on board.

Saying that wires from the fuel monitoring systems of Flight 800 and three other 747s “illustrate unsafe conditions” that may exist in the three oldest models of the jumbo jet, the National Transportation Safety Board urged the Federal Aviation Administration to order the inspections of the planes--about 650 in all--”as soon as possible.”

The NTSB also recommended the replacement of rough-edged Honeywell terminal blocks that may be causing some of the damage to the wiring. The board said that, as a precaution, protective systems should be installed that would prevent power surges to the systems that gauge how full the planes’ fuel tanks are.

Although the FAA responded immediately--saying it “agrees with the intent” of the NTSB recommendations--the response lacked the NTSB’s tone of urgency.

Instead of mandating the suggested inspections right away, the FAA said it would study them first, probably issuing a notice of proposed rule making later this spring.

As for the terminal blocks, the FAA said it has been working with Boeing on a manufacturer’s service bulletin for replacement of the blocks with smoother ones less likely to cause damage. An airworthiness directive that would make the replacements mandatory probably will not be issued until this summer, the FAA said.

Doug Webb, a Boeing spokesman, said it’s impossible to estimate what the proposed inspections and fixes might cost “because right now, we still don’t know what they might entail.”

The NTSB said it has still not determined what touched off the center fuel tank explosion that tore the TWA Boeing 747 apart a few minutes after it had taken off from New York on July 17, 1996, on a scheduled flight to Paris.

However, NTSB investigators said last December that a power surge in fuel-monitoring system wiring that had somehow been stripped of its protective insulation might have caused a spark that triggered the blast.

The investigators testified during hearings on the crash that the insulation on a fragment of one sensor wire found in the wreckage apparently had cracked or was worn away--perhaps because of aging or friction--sometime before the explosion.

Minute traces of chemical residue on the copper wire indicated that it had, in fact, been exposed to fuel in the tank for some time. Wire from the fuel tank of one of the other 747s mentioned in Tuesday’s NTSB recommendations showed the same residue.

Electronics experts said the normal voltage in the fuel-monitoring system wires is far too low to create a spark that could have ignited the fumes in the nearly empty center fuel tank.

However, NTSB engineer Bob Swain said his investigators have been looking at the possibility that some external power source might have sent a surge of high-voltage electricity through the wire.

Several possible scenarios are being studied, most of them involving inadvertent contact between the sensor wires and other, high-voltage wires. The NTSB said that, in places, the sensor wires are routed in bundles with nearly 400 other wires, some of which carry up to 350 volts.

The planes on which wire damage has been found by the NTSB include the Flight 800 aircraft and three retired 747s--two of them in storage since being retired by TWA in 1992 and 1994 and one used for testing by the NTSB and other agencies after it was retired by Air France in 1994.

Officials in Great Britain have told the NTSB similar damage was found on a fifth plane, a 747 being flown by British Airways.

“These findings illustrate unsafe conditions that may exist on other Boeing 747s, and should be addressed by the FAA,” NTSB Chairman Jim Hall said Tuesday in a letter to Jane Garvey, administrator of the FAA.

The NTSB recommendation calls for inspection of all 747s from the 100, 200 and 300 series. Flight 800 was a 747-131. Although 724 of the old 747s were built, about 75 have been permanently retired from service, crashed or been scrapped.

As attention focuses increasingly on the fuel-monitoring system, some other theories about what touched off the fuel tank explosion on Flight 800 have fallen from favor or been discounted altogether.

Despite continuing support from conspiracy theorists and those who would blame errant military maneuvers, both the NTSB and the FBI have ruled out a bomb or a missile, citing a lack of supporting evidence.

There still is no evidence that heat from other systems on the plane or flames from some small blaze ignited the blast.

Analysis of the weather at the time of the crash has ruled out lightning, and astronomers say the odds are that a meteorite will strike a plane roughly once every 55,000 to 77,000 years.