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 organisations 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-sulphur 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, New York 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 centre wing fuel tank. The source of ignition could not be determined; but the NTSB report concluded that ‘the ignition energy for the centre 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-sulphide 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.
At least two other aircraft have experienced centre wing fuel tank explosions in the last 12 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 centre fuel tank explosion, the tanks were almost empty, and the remaining fuel was heated by use of air conditioning packs located under the centre wing fuel tank,’ Kauffman said. ‘If the heated fuel vapours in the centre wing tanks reached their flash point temperatures, they would become explosive in the presence of a sufficient ignition source.’
Kauffman’s research 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 low-sulphur 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 analysed fuels removed from centre wing fuel tanks of commercial airliners after they landed at US airports from the US, Europe, South America and Asia. They then 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-sulphur 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 characterised as the low-sulphur 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 non-combustible gases to fuel tanks that can prevent the explosion if an ignition source is present.