Application of Common Cause Failure Methodology to Aviation Safety Assessment Model

The Federal Aviation Administration (FAA) has been developing the Integrated Safety Assessment Model (ISAM) to provide a baseline risk assessment for the National Airspace System and to evaluate the safety impact of proposed changes to the system. ISAM consists of a set of event sequence diagrams and underlying fault trees for various accident scenarios. In the current model, all basic events in the fault trees are assumed to be independent. However many basic events throughout the model appear with the same descriptive label. Such events might have some dependence, rather than being completely independent as is currently assumed. This paper evaluates the dependency between basic events having the same label in order to see the overall impact on accident risk. A common cause failure (CCF) methodology is applied to the event sequence diagrams (ESDs) in ISAM. A modified beta-factor model is applied, and a binary decision diagram method is implemented to evaluate end-state frequencies of an ESD. Accounting for CCFs, this paper observes a wide range of changes in accident frequency relative to the current assumption of independent events. Results for different ESDs range from a decrease in accident frequency by 50% to an increase by more than a factor of 1,000.