Now the report's conclusion wasANY historical precedent for that happening?The fuel/air vapor in the ullage of the TWA flight 800 center wing tank was flammable at the time of the accident.
Being a CE, I can ohly go Hmmm.
Yes, I haven't often gone Hmmm.
The National Transportation Safety Board determines that the probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT) resulting from ignition of the flammable fuel/air mixture in the tank. The source of ignition energy for the explosion could not be determined with certainty but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the center wing tank that allowed explosive voltage to enter it through electrical wiring associated with the fuel quantity indication system.
During the TWA hearing (Dec. 97), several potential ignition sources for the explosion that occurred in the center wing tank (CWT) were addressed. Many were eliminated and of the remaining (fuel pumps, fuel quantity indicating system and electrostatics), only one, the Fuel Quantity Indicating System (FQIS), was discussed in any detail. For the FQIS to be a credible potential ignition source, two independent failures must occur. The leading scenario presented involved copper-sulfide deposits on the FQIS wiring inside the CWT and an induced voltage onto the FQIS system wiring outside the CWT created by switching on and off power in adjacent wiring. Although the supply test voltage used was much higher than available on the aircraft, only a maximum of 0.6 millijoules (mj) of energy at 1000 volts was induced into the CWT FQIS wires during the demonstration (Exhibit 9A, Page 8). Copper sulfide (CuS-covellite melting point is 103C and Cu2S-chalcocite is 1100C) and silver sulfide (Ag2S melting point is a maximum of 175C for acanthite and 825C for argentite) were identified as possible deposits creating a likely site for ignition inside the CWT. Specific knowledge of the melting temperature and boiling temperature of the deposits could be useful in assessing these deposits as possible ignition sources. These deposits must provide an electrical path to ground or to another wire for current to flow. The resulting energy at this location must either heat these deposits to well over 900C for hot surface ignition of the fuel vapors (See U.S. Bureau of Mines Bulletin 680 dated 1985, Figure 49, A heated area of about two square cm assumed) or must vaporize the material into an incendiary arc (A "spark" is a high voltage event, whereas an "arc" is a low voltage event requiring much more total energy for ignition). Total energy and energy density of the spark/arc are both very important variables for ignition. As an example, a corona discharge may have more than the minimum ignition energy but not the energy density necessary to be an incendiary ignition source. Corona discharges occur quite often during refueling operations. A third scenario should also be considered involving the sulfide deposits heating the wire insulating material to its ignition temperature that in turn ignites the fuel vapors. I believe heat transfer calculations will show none of these three scenarios is possible with only 0.6 millijoules of total energy available. Another FQIS scenario needs some discussion. This scenario involves chafing of the FQIS wires both inside the CWT and outside the CWT. Either of these two events should independently provide some indication of a malfunction to the flight crew or maintenance personnel. The probability of these two events occurring simultaneously without some malfunction indication is astronomical. The probability of them occurring simultaneously for the first time when the CWT is flammable is even higher. In addition, inspection of the available TWA 800 FQIS wiring revealed no evidence of arcing. The final FQIS scenario involved chafing of the FQIS wires outside the CWT and copper sulfide deposits inside the CWT. During tests of a new fuel quantity indicator (Exhibit 9A, Page 107), over-voltages above about 85 volts were applied to the indicator and evidence of electrical stress resulted. No electrical stress or heat exposure was evident during detail inspection of the CWT fuel quantity indicators (Exhibit 9A, Page 108). Again, the question that must be answered is, can an over-voltage of less than 85 volts cause the copper sulfide deposits to produce hot surface ignition, an incendiary spark or ignition of the insulating material? For reference, the 0.6 mj discussed previously was the result of a voltage greater than 1000 volts in the FQIS wiring.