a warhead could have detonated in such a location where a fragment could penetrate the CWT without other missile fragments impacting the surrounding airplane structure, leaving distinctive impact marks. Similarly, the investigation considered the possibility that a small explosive charge placed on the CWT could have been the ignition source. Testing by the NTSB and the British Defence Evaluation and Research Agency demonstrated that when metal of the same type and thickness of the CWT was penetrated by a small charge, petalling of the surface occurred where the charge was placed, with pitting on the adjacent surfaces, and visible hot-gas washing damage in the surrounding area. Since none of the recovered CWT wreckage exhibited these damage characteristics, and none of the areas of missing wreckage was large enough to encompass all the expected damage, the investigation concluded that this scenario was "very unlikely". Other potential sources The NTSB also investigated whether the fuel-air mixture in the CWT could have been ignited by lightning strike, meteor strike, auto ignition or hot surface ignition, a fire migrating to the CWT from another fuel tank via the vent system, an uncontained engine failure, a turbine burst in the air conditioning packs beneath the CWT, a malfunctioning CWT jettison/override pump, a malfunctioning CWT scavenger pump, or static electricity. After analysis, the investigation determined that these potential sources were "very unlikely" to have been the source of ignition. Fuel quantity indication system Because a combustible fuel-air mixture will always exist in fuel tanks, Boeing designers had 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. 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." 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