Protection from Ignition Sources Due to Lightning, EMI and Wire Harness Shorts for Aircraft Fuel Tank Gauging Systems 2000-01-2120

The investigation that followed the TWA 800 tragedy has revealed the probability that the center wing fuel tank exploded and caused structural failure that led to the aircraft plunging into the ocean. Examination of wiring from that aircraft and other Boeing 747 aircraft of similar age has uncovered the existence of wiring insulation faults. These faults have been found inside the tanks between powered leads and ground surfaces, and outside the tanks between Fuel Quantity Indicating System wiring and other wiring. The Boeing series of aircraft have in the past been designed with the FQIS wiring routed together with other wiring such as power leads. Thus, wire harness insulation failures could result in the FQIS wiring being exposed to voltages that might provide an ignition source in the tank. EMI from neighboring wires and lightning may also be present. Fuel gauging systems have not in the past been designed for these contingencies. Smiths Industries has developed a Transient Suppression Unit that is installed in the FQIS wiring at the tank wall which filters out any dangerous electrical transients from entering the fuel tank, while allowing the Smiths digital fuel gauging system to function normally.