went wrong, investigators discover that a common aircraft insulation material is a fire hazard. thousands of planes are fitted with new insulation. but they still don't know how the fire started. and if other aircraft are at risk. >> when we started to look for potential ignition sources, the very first thing that comes to mind is aircraft wiring. >> the investigators scour more than 150 miles of wiring for signs of an electrical malfunction called arcing. >> arcing events are when electricity jumps from a wire to another piece of metal. it can be another wire or part of the airplane or something else. when it does that it can create tremendous amount of heat. >> finding an arcing event is difficult, and determining if one preceded another is nearly impossible. but six months into writing the final report, the impossible happens.