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Repost from April 9, 2003 - TSB Final Report
The following are the key excerpts from the final report having to do with the source of ignition. They are a bit difficult to read, but I think what they tell is the following:

- the most likely ignition source was an arcing event that ignited the MPET
- an arcing event definitely occurred in the IFEN wiring near the location where the fire started
- it (the arcing event) did not trip the circuit breakers
- it is likely that the arcing was the result of chafed insulation, possibly where the wire came in contact with an aluminum wire mounting bracket
- because the mounting bracket was separated from the aircraft structure by a nylon insulator, the insulation on another wire most also have been chafed for the IFEN wire to have arced
- the scenario wherein the IFEN wire arced, but was unrelated to the ignition, is unlikely; had the arced IFEN wire been the result of collateral damage from a dramatic arcing event (i.e. polyamide arc tracking), the damage to nearby bundles would have been serious and CBs would have tripped.
- the missing wire involved in the arcing event is most likely a second IFEN wire

From Section 2.17.1:

Various potential ignition sources, including electrical and non-electrical, were evaluated within the fire-damaged area. It was determined that the most likely ignition source was an electrical arcing event involving breached wire insulation that ignited nearby MPET-covered insulation material.

From Section 2.21:

The forward arc on Exhibit 1-3791 was assessed to determine whether it was, by itself, the lead arcing event that started the fire. For this arc to be the single lead arcing event, the wire would first have to be damaged, for example by chafing, at the location of the arc, to expose the conductor. The exposed conductor would then have to contact grounded aircraft structure, resulting in the arcing event. Although it was possible to position the cable segment (Exhibit 1-3791), and therefore the forward arc, relatively accurately, the extent and nature of the damage required interpretation of the damage patterns. This interpretation allowed for a small range of possible locations in the placement of the wires, as described in Section At the forward end of the possible range, the arc was placed where it would be in contact with an aluminum wire support bracket. However, the chafing of any one wire by itself to this bracket would not result in an arc, as the bracket was isolated from the aircraft structure by a nylon stand-off and would not have provided an electrical path to ground.

There was aluminum found in one copper bead adhering to the wire strands slightly removed from the main arc site of the forward arc bead on Exhibit 1-3791. This suggests that the arc might have resulted from contact with aluminum. Arcing to the aluminum bracket would only be possible if there were two exposed conductors in contact with the bracket. This would provide an opportunity for arcing, as aluminum is a good conductor of electricity. Such a scenario would involve, for example, two phases of a PSU cable chafing separately against this same bracket until both of their conductors became exposed. This scenario could not be ruled out; however, there is no corroborating information to support it. The bracket was not identified in the wreckage. Neither were the two remaining PSU cables, the 16 AWG control wire or other aircraft wires from that area, that may have been involved.

Another potential lead arcing event scenario involving the forward arc on Exhibit 1-3791 would be that the arc occurred directly to another wire of a different electrical potential. This could be either an aircraft wire, or another IFEN wire. In either case, both wires would have to be damaged at the location of the arc, allowing their bare conductors to contact each other. Because the IFEN wires in the STA 383 area were routed separately and not along existing wire bundles, it is less likely that the IFEN wires would be in contact with aircraft wires within the localized area where the fire most likely started; therefore, the more likely candidate wires for this type of scenario would be the other wires in the bundle of four IFEN PSU cables and the 16 AWG control wire. It is known that the other wires in the 1-3791/1-3793 pair did not arc at that forward location. However, the wires from the other pair of PSU cables and the 16 AWG control wire from that area were not identified. Therefore, neither aircraft wires nor other IFEN wires can be ruled out as potentially being involved in such a scenario. Damage to two or more wires in a wire bundle can be caused by chafing contact with the aircraft structure, by inadvertent damage occurring during installation or subsequent maintenance, or by the presence of swarf, such as a metal shaving, that could cut through the insulation on both wires, exposing their conductors. A metal shaving could also act as a conductor. If any of those events occurred, the subsequent arcing that took place on all of the PSU cables and 16 AWG control wire confirms that any arcing on these wires near STA 383 did not trip the associated CB.

An assessment was made to determine whether the forward arcing damage on Exhibit 1-3791 could have resulted from collateral damage; that is, damage from an arcing event involving other wires in the immediate vicinity that was of sufficient magnitude to breach the insulation on at least two other wires, including Exhibit 1-3791. For this to occur, the lead-event wires would have to be in very close proximity to the forward arc on Exhibit 1-3791. An arcing event of sufficient magnitude to damage other wires would likely have tripped the associated CBs. None of the IFEN CBs tripped at the time of the lead arcing event (subsequent arcing occurred on all of the PSU cables and the 16 AWG control wire); therefore, if such an arcing event occurred, it did not involve IFEN wires. Such a lead-arcing event would have to involve aircraft wires, but not result in any electrical anomalies that would be apparent to the pilots and not be recorded on the FDR. Although the possibility of a scenario involving collateral damage to Exhibit 1-3791 could not be ruled out, it appears unlikely that such a scenario occurred.

From 1.16.8

Electrical discharges in the form of arcs or sparks can produce localized temperatures in excess of 500C (932F). A sustained short-circuit event will cause a conventional CB to trip and de-energize the faulted circuit. However, a CB may not trip when an intermittent short circuit
exists. Tests were conducted to characterize the ignition properties and determine whether MPET-covered insulation blankets would ignite when exposed to electrical sparks produced by ground shorts from wires carrying 115 V AC and 28 V DC current. It was observed that MPET-covered insulation blankets would ignite and propagate a flame when exposed to an electrical arc or spark. However, ignition was sporadic in that it sometimes occurred with the
first strike of the arc and other times it was not achieved after numerous attempts.

The arcs were struck by hand and typically resulted in the tripping of the CB. It appears that electrical arcs were sufficiently rapid in onset and localized to overcome the propensity of a cover material constructed with thin-film material, such as MPET, to shrink away from a heat source.
In one such test, an MPET-covered insulation blanket, similar to those in the occurrence aircraft,
was placed between the vertical frames in a section of aircraft fuselage. The blanket was exposed to an intermittent electrical short between an exposed 115 V wire and the grounded fuselage.
The MPET-covered insulation blanket ignited, causing a flame to propagate vertically and horizontally across the face and rear surface of the blanket.
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