Toward a Probabilistic Risk Assessment (PRA) Method for Assessing Mishaps in Legacy Systems Using Mishap Reports

Abstract There is a significant delta between the acknowledged probability of potential mishaps under the current safety assessment approach derived from Military Standard 882E (MIL‐STD‐882E), Department of Defense Standard Practice of System Safety, and what is observed from actualized mishaps being reported. When assessing systems safety during the design process, the approaches used that comply with MIL‐STD‐882E simplify the mishap scenario by assuming a single initiating mechanism. By decomposing mishap reports from legacy systems, fault and event trees are used to identify common failure modes that were not adequately assessed under the current process. Each of the mishap reports assessed has identified more than one initiating mechanism. As such, this work suggests a greater mishap probability than was originally acknowledged. To address the current limitation, this work develops (1) an approach for decomposing mishap scenarios for legacy systems to identify mishaps resulting from multiple failure modes, (2) a method for systematically implementing the mishap decomposition on the assessed system or newly designed systems, and (3) a list of characteristics for an improved systems safety method.