Editing TWA Flight 800 (section)

====Fuel quantity indication system====
Because a combustible fuel-air mixture will always exist in fuel tanks, Boeing designers attempted to eliminate all possible sources of ignition in the 747's tanks. To do so, all devices are protected from vapor intrusion, and voltages and currents used by the fuel quantity indication system (FQIS) are kept very low. In the case of the 747-100 series, the only wiring located inside the CWT is that associated with the FQIS.{{citation needed|date=July 2020}}

For the FQIS to have been Flight 800's ignition source, a transfer of higher-than-normal voltage to the FQIS would have needed to occur, as well as some mechanism whereby the excess energy was released by the FQIS wiring into the CWT. The NTSB concluded, "the ignition energy for the CWT explosion most likely entered the CWT through the FQIS wiring."<ref name="Final Report" />{{rp|294}}<ref>{{cite book |title=In-flight breakup over the Atlantic Ocean, Trans World Airlines Flight 800 Boeing 747-131, N93119, near East Moriches, New York, July 17, 1996 |year=2000 |url=https://books.google.com/books?id=Sd9rSiFtSwIC&pg=PA294 |publisher=Diane Publishing |isbn=978-1-4289-6207-1 |pages=294– |access-date=March 10, 2021 |archive-date=July 23, 2021 |archive-url=https://web.archive.org/web/20210723174657/https://books.google.com/books?id=Sd9rSiFtSwIC&pg=PA294 |url-status=live}}</ref>

Though the FQIS itself was designed to prevent danger by minimizing voltages and currents, the innermost tube of Flight 800's FQIS compensator showed damage similar to that of the compensator tube identified as the ignition source for the [[Imperial Iranian Air Force Flight 48|surge tank fire that destroyed a 747 near Madrid in 1976]].<ref name="Final Report"/>{{rp|293–294}} This was not considered proof of a source of ignition. Evidence of arcing was found in a wire bundle that included FQIS wiring connecting to the center wing tank.<ref name="Final Report"/>{{rp|288}} Arcing signs were also seen on two wires sharing a cable raceway with FQIS wiring at station 955.<ref name="Final Report"/>{{rp|288}}

The captain's CVR channel showed two "dropouts" of background power harmonics in the second before the recording ended (with the separation of the nose).<ref name="Final Report"/>{{rp|289}} This might well be the signature of an arc on cockpit wiring adjacent to the FQIS wiring. The captain commented on the "crazy" readings of the number 4 engine fuel flow gauge about 2&thinsp;1/2 minutes before the CVR recording ended.<ref name="Final Report"/>{{rp|290}} Finally, the CWT fuel quantity gauge was recovered and indicated 640&nbsp;pounds instead of the 300&nbsp;pounds that had been loaded into that tank.<ref name="Final Report"/>{{rp|290}} Experiments showed that applying power to a wire leading to the fuel quantity gauge can cause the digital display to change by several hundred pounds before the circuit breaker trips. Thus, the gauge anomaly could have been caused by a short circuit to the FQIS wiring.<ref name="Final Report"/>{{rp|290}} The NTSB concluded that the most likely source of sufficient voltage to cause ignition was a short circuit from damaged wiring, or within electrical components of the FQIS. As not all components and wiring were recovered, pinpointing the source of the necessary voltage was not possible.