UDRI Researchers Make a Key Finding in FAA Study of 1996 Explosion of TWA Flight 800

Airplane fuel tank residues could cause an explosion under certain conditions, according to tests performed for the Federal Aviation Administration (FAA) by a team of organizations led by SRI International, including the University of Dayton Research Institute (UDRI).

Specifically, researchers found that the residues that form in the presence of low-sulfur jet fuel, water and silver-coated surfaces are conductive, and can ignite jet fuel when exposed to electrical power as low as that from a radio battery.

The FAA commissioned the research in response to National Transportation Safety Board (NTSB) recommendations after the 1996 airplane fuel tank explosion of TWA Flight 800. The crash off Long Island, N.Y., killed 230 people shortly after take-off. Conductive residues were found on wires recovered from the accident site.

NTSB investigators determined the accident was caused by an explosion of the center wing fuel tank. The source of ignition could not be determined; however, the NTSB report concluded that "the ignition energy for the center wing fuel tank (CWT) explosion most likely entered the CWT through the fuel quantity indication system (FQIS) wiring, and, although it is possible that the release of ignition energy inside the CWT was facilitated by the existence of silver-sulfide deposits on an FQIS component, neither the energy release mechanism nor the location of the ignition inside the CWT could be determined from the available evidence."

"We do not know if we have identified the ignition source for the TWA 800 crash or similar fuel tank explosions," said research chemist Robert Kauffman, who led UDRI's participation in the project. As reported to the FAA, the project team believes research has identified a credible ignition source for fuel tanks. The findings have been reproduced by several of the team members.

Kauffman first presented the team's work at the FAA's International Aircraft Fire and Cabin Safety Research Conference in Atlantic City, N.J., last fall and will present the results at an international conference on fuels in Germany in January 2003. His co-author is SRI International's Michael McKubre, the project's lead investigator.

At least two other aircraft have experienced center wing fuel tank explosions in the last dozen years. In 1990, a Philippines Airlines 737 plane exploded while being pushed back from the gate. Last year, a similar fate met a Thailand Airlines 737 plane while standing at the gate.

"In each center fuel tank explosion, the tanks were almost empty, and the remaining fuel was heated by use of air conditioning packs located under the center wing fuel tank," Kauffman said. "If the heated fuel vapors in the center wing tanks reached their flash point temperatures, they would become explosive in the presence of a sufficient ignition source."

Kauffman's research, performed under a three-year, $340,000 subcontract to SRI International, shows that the conductive fuel residues found on terminal blocks from fuel tanks contained silver or silver oxides with an outer layer of fuel gums. Not surprisingly, they were only found on terminal blocks with silver-plated nuts. Additional tests revealed that environmentally friendly low-sulfur fuels, used primarily in Europe and Asia, react with silver or silver oxides to produce what Kauffman described as "conductive gums" that can only be removed by scraping or dissolving them with solvents.

"These conductive residues are well known. They've caused fuel indicators to misread for years, but they've been considered more of a maintenance problem than a safety hazard," Kauffman said. "Our job was to find out why they formed. When they formed, were they a combustible hazard?"

In UDRI's surface and fluids analysis laboratory, Kauffman and colleagues analyzed fuels removed from center wing fuel tanks of commercial airliners after they landed at U.S. airports from the U.S., Europe, South America and Asia; tested parts from retired airplanes and airplanes that experienced faulty fuel indicators; and videotaped the results of fuel ignition tests with conductive residues made in the laboratory.

"Silver gave the residues their conductivity, and these residues formed with certain low-sulfur fuels. If you added very low electrical power, even as low as from a nine-volt battery, these residues would ignite fuel drops," Kauffman said. "It's counter-intuitive to think that low electrical power could be more of a problem than high electrical power."

Of the 150 fuels studied, just 12 could be characterized as the low-sulfur fuels capable of forming conductive residues, according to Kauffman. Of those, nine came from Europe and three from China.

The FAA is requiring silver-plated components, and the potential hazard identified by this research, be addressed in the safety reviews under a Special Federal Aviation Regulation (SFAR No. 88). The FAA and NASA have also been testing equipment that adds noncombustible (inert) gases to fuel tanks that can prevent the explosion if an ignition source is present.

Kauffman also recommends additional research into the effects of low-sulfur fuels. "If researchers could come up with an additive to put in the fuel that deactivates the silver, that would be best," he said. "But you would need additional research to make sure that doesn't cause other safety or maintenance problems."

The University of Dayton operates the largest non-medical research facility on any Catholic university campus. The University of Dayton Research Institute performs approximately $48 million of sponsored research annually and ranks among the top 20 in the nation in Department of Defense contracts. Much of its research focuses on flight safety.

October 18, 2002