Aviation Investigation Report A98H0003

2.17.3  Airflow, Fire Propagation, and Potential Ignition Locations

The pre-fire airflow patterns were assessed to determine potential locations from which odour and smoke could enter the cockpit as noted by the pilots, and not be noticed in the passenger cabin. Each of these potential locations was then assessed to determine whether a fire originating at that location would match the known circumstances of the SR 111 fire. Factors that were considered included the presence of potential ignition sources and flammable materials; the likelihood that the fire could propagate from that area in the known time frame, given the amount of flammable material available; the likelihood that fire propagation from that area could result in the known sequence of aircraft systems anomalies; and the likelihood that a fire propagating from that area could produce the observed fire damage.

Of all of the potential locations analyzed, one area, on the right side close to the cut-out in the top of the cockpit rear wall just forward of STA 383 was found to match all the known circumstances involved in the fire. From this area, odour and smoke could migrate into the cockpit at a location where it could be interpreted as smoke coming from the air conditioning system via the right overhead diffuser outlet. Initially, smoke generated in this area would not likely be evident elsewhere in the aircraft. A fire could propagate rearward from the area near STA 383, and based on known material flammability characteristics and airflow patterns as described below, the fire could return with more intensity into the cockpit after a time delay. This succession of events could occur before the fire would affect the passenger cabin environment or aircraft systems.

A fire starting farther forward in the cockpit on the underside of the over-frame insulation blankets, ahead of the wires and wire bundles that cross laterally in front of STA 383, would likely have initially propagated over a larger area within the cockpit. If the fire were to start at a more forward location, fire-induced system malfunctions and failures would likely have occurred earlier and the associated symptoms would likely have been evident to the pilots early in the fire sequence and potentially been captured on the recorders. In addition, smoke would be expected to initially enter the cockpit from locations where the pilots would not likely have associated it with a conditioned air source. The cockpit ceiling liner would also be susceptible to early fire penetration and melting, inasmuch as the liner is fabricated from a thermoformable plastic that has a relatively low forming temperature.

Similarly, if the fire had started at a more inboard location on the underside of the over-frame insulation blankets, close to the aircraft centreline, the fire would be essentially bounded on both the left and right sides by wire runs that are routed in a fore and aft direction. These parallel wire runs enter the overhead switch panel housing through the oval holes on the aft side of the housing. The wire runs would be expected to act as fire barriers that would channel flame spread in a fore and aft corridor. As shown during flight tests, smoke would be expected to be drawn into the openings of the overhead panel housing and into the cockpit through passageways, such as the engine fire shut-off handle slots in the overhead panel. Again, smoke penetration into the cockpit through locations such as these would likely not be associated with having originated from conditioned air sources. The same reasoning would also apply to fires originating on the left side of the cockpit attic. These factors negate the plausibility of the fire originating elsewhere in the hidden areas of the cockpit.

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