Aviation Investigation Report A98H0003
2.15 Factors Influencing Pilot Decision Making Regarding Initial Odour and Smoke (STI2-2)
The data available regarding aircraft accidents involving in-flight fires illustrates the limited amount of time available to react to the first indications of a potential fire. In the case of SR 111, the pilots noticed an unusual odour in the cockpit at 0110:38, and 20 minutes, 40 seconds, later the aircraft struck the water.
The pilots were at a significant disadvantage when attempting to assess and react to the initial odour and smoke. They did not have detection devices that could have provided accurate information about the source of the odour and smoke. Nor did they have the capability to distinguish with certainty between odour and smoke from an air conditioning source, an electrical source, or a materials fire.
Initially, the pilots sensed only an abnormal smell (see Section 126.96.36.199). About 20 seconds later, at 0110:58, they observed a small amount of smoke entering the cockpit from behind and above them. The initial smoke quickly disappeared. More than two minutes later, they confirmed that smoke had reappeared, likely in the same area. Analysis of the information collected during the investigation indicates that the odour and smoke were migrating from a fire that likely started at the cockpit rear wall above the cockpit ceiling. Assessment of the airflow patterns in the cockpit would support the migration of some smoke from this area into the cockpit via a seam, or some holes, between the upper avionics CB panel and the overhead ceiling panel near the right overhead air diffuser. If the pilots interpreted that the smoke was entering through the overhead air diffuser, this would have contributed to their belief that the odour and smoke originated within the air conditioning system. As the fire initially migrated primarily aft into the area above the forward cabin drop-ceiling, the amount of smoke entering the cockpit would have likely been small and intermittent. The smoke would also have been significantly diluted by mixing with the diffuser air.
Based on the typical awareness level shown by other pilots during interviews, the SR 111 pilots would not likely have been aware of the presence of significant amounts of flammable material in the attic area of the aircraft. As a result, they would not have expected there to be a significant fire threat from that area, or from any other hidden area. There was nothing in their experience that would have caused them to consider the smoke to be associated with an ongoing uncontrolled fire consuming flammable material above the ceiling. Industry norms at the time of the SR 111 occurrence were such that other flight crews, if faced with the same scenario, would likely have interpreted the limited cues available in a similar way.
Based on historical data, it is generally accepted that smoke from an air conditioning system source does not pose an immediate threat to the safety of the aircraft or passengers, and that the threat can be mitigated through isolation procedures. Based on their assessment that the risk from the smoke was relatively low, it appears that the pilots saw no apparent reason to accept the additional risk of attempting an immediate emergency landing. Instead, they established priorities that included obtaining the information needed for the approach and preparing the aircraft for a safe landing. Initially, the amount of smoke entering the cockpit must have been low. Otherwise, it would be expected that they would have attempted to isolate the smoke origin by closing off the conditioned air sources. There is no indication on the aircraft's recorders that they completed any action item in the Air Conditioning Smoke checklist, although, the first action item on that checklist, which is selecting the ECON switch to the OFF position, is not a recorded parameter on the FDR. The first item on the Smoke/Fumes of Unknown Origin Checklist that was recorded on the CVR was carried out about 13 minutes after the odour first became apparent.
The pilots donned their oxygen masks approximately five minutes after they first detected the unusual odour (see Section 188.8.131.52). From what is known of their actions before they donned their oxygen masks, they were not affected by the smoke in any physical way, such as irritation to the eyes or respiratory tract. The materials that were burning would have emitted fumes that would contain noxious and potentially toxic combustion by-products. Exposure to these compounds in sufficient concentrations, particularly through inhalation, could affect performance and judgment.
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