Aviation Investigation Report A98H0003

2.3.3  Non-fire-hardened Aircraft Systems

Prior to the time of the SR 111 occurrence, regulators and manufacturers perceived minimal in-flight fire threat in areas other than the passenger cabin areas and designated fire zones. Therefore, certification standards did not account for the potential consequences of a fire-related breach or failure of an aircraft system in areas such as the attic space. This deficiency allowed systems to be constructed in a way that a fire-related component failure could potentially exacerbate the fire.

In a fire environment, a breach in a system, such as the hydraulic, oxygen, or air environmental systems, could significantly add to the severity of the fire by increasing the amount of combustible material, adding oxygen, or modifying the airflow in the area. For aircraft certification, defences against such failures are typically put in place as a result of a system safety analysis of the potential hazards. The system safety zonal analysis conducted on the MD-11 for the area where the fire occurred in SR 111 did not include the hazards resulting from system or component failures caused by a fire-in-progress. Regulations did not ensure that such hazards be included in the system safety analysis. The breach of an elastomeric end cap in the air conditioning duct system, and possibly a failed aluminum cap in the flight crew oxygen system, would have allowed these systems to exacerbate the in-flight fire.

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