PANEL SUMMARY: Possible Ignition Sources - Internal (Section B)

An NTSB aircraft systems investigator told a public hearing into the crash of TWA flight 800 that investigators were actively looking into four possible energy sources that could have triggered the explosion that brought down the Boeing 747.

Bob Swaim said the four areas involve the electrical system that measures the jet fuel in the plane’s center wing tank. The NTSB has determined that the plane’s center fuel tank exploded but has not yet determined the source of ignition.

Swaim told an NTSB public meeting into the accident 16 months ago that from the beginning numerous possible ignition sources had been studied, but now all but four have been set aside, at least for the present, while more promising ones are pursued.

He said the four are:

• - A possible short circuit to the fuel quantity indication system (FQIS) wiring, outside the fuel tank, combined with latent failures not apparent during operation of the plane, or copper sulfide deposits on FQIS components in the fuel tank.
• - Energy induced into the FQIS combined with latent failures, foreign materials or copper sulfide deposits in the fuel tank.
• - Damage to wiring above the forward cargo compartment. In the flight of a different aircraft, unrelated to the accident, investigation has found that a cargo container may have struck the wiring in this area and created a short circuit. In wreckage recovered from the accident aircraft, a portion of that wiring is missing.
• - Possible short circuit in other parts of FQIS wiring, some of which has not been recovered from flight 800.

Swaim said that among the possible ignition causes set aside for now were: fire in the center fuel tank touched off by bleed air from the jet engines, which can reach 1000 degrees; a fire in the landing gear bays; short circuits in power cables that could have ignited the fuel vapors in the partially filled center fuel tank; a fire originating in the wing center section’s dry bay, which carries no fuel, and spreading to the bays containing fuel; short circuits in the dry bay; and weather within 300 miles that could have caused lightning to create arcing in the tank.

Swaim said that while these potential ignition sources have been set aside for now, NTSB investigators might return to them in the case of new evidence.

George Slenski, an engineer at the Wright Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio, said he has studied the probes into fuel tanks from other aircraft that measure the quantity of fuel remaining and found that if there was a short in them, it should not create enough energy to ignite the fuel.

He said the deterioration found in the other aircraft he examined was found to be associated with aging, creating corrosion and residues. The accident aircraft was 25 years old. Slenski said that semi-conductive copper-sulfur and silver-sulfur residues were found on aged fuel probe components and wiring and the residues can reduce the insulation resistance between fuel probe components and wiring. Slenski added that residues, which can create an electrical path, can typically sustain only low current and voltage levels without rupturing. He said it was better to use nickel for wiring, less of a conduit for electricity than is the standard copper wiring wrapped in silver.

Boeing officials told the hearing that the company has issued a Service Bulletin to operators of the 970 747s flying worldwide to inspect fuel tanks and fuel gauges when the carriers do their next scheduled heavy maintenance, or within two years, whichever comes sooner. They added that Boeing is widening the Service Bulletin to include inspection of the wiring insulation on the fuel-measuring components inside the tanks.

USAF’s Slenski reported on studies he conducted of wire from the accident aircraft. He noted how hot stamping of characters onto the wire, used for identification, had penetrated the insulation in some cases. He therefore recommended that investigators take a close look at this process.

Swaim said that he and his associates are continuing to examine recovered wire, looking for signs of arcing from this or other potential causes.

Another possible ignition source is transient electrical fields between adjacent wires. Jerry Hulm, Boeing Manager of Electrical Systems, described a series of tests on this phenomenon, now in progress. These tests have been conducted in the laboratory and also in an aircraft currently in service.

Mike Collins, with the FAA, and Ivor Thomas, Boeing Chief Engineer of Fuel Systems and Auxiliary Power Units, discussed how unlikely it would be for a fuel pump to cause a catastrophic explosion. Collins said he is not aware of any such occurrence. Thomas pointed out the safeguards in pump design. But under questioning from Swaim, he acknowledged that as a result of the TWA accident, Boeing is considering at least one design modification, a flame arrestor in inlets of all pumps.