Quantifying Risk in Commercial Aviation with Fault Trees and Event Sequence Diagrams

The mission of the Federal Aviation Administration (FAA) is to provide the safest and most efficient aerospace system in the world. As the FAA plans and develops the Next Generation (NextGen) Air Transportation System, quantitative risk assessments can help evaluate the impacts of new technologies and changed procedures. The FAA needs to ensure that NextGen changes that could potentially increase capacity or efficiency also maintain or improve safety. A systematic quantitative view of risk of the air transportation system provides the opportunity to fully understand how possible improvements can impact the overall safety of the system. This FAA modeling effort, led by the System Safety Management Transformation program (Sherry Borener, Program Manager), is called the Integrated Safety Assessment Model (ISAM). Within ISAM, event sequence diagrams (ESDs) describe the sequence of events that a flight must encounter for an accident scenario to occur, and a fault tree is developed for each of the pivotal events in the ESDs. The risks identified by the fault trees are linked to identifiable hazards with the goal of managing the hazards and improving system safety. This paper describes the process being used to develop the event sequence diagram-fault tree model, including lessons learned from applying probabilistic risk analysis modeling in the commercial aviation context.