Model-Based STPA: Enabling Safety Analysis Coverage Assessment with Formalization

Urban Air Mobility introduces safety-related challenges for future avionics systems. The associated need for increased autonomy demands novel functions based on high-performance algorithms. To provide such functionality in future air vehicles of all sizes, the trend is towards centralized and powerful computing platforms. That turns avionics into a complex, integrated, and software-intensive aircraft system. Simultaneously, this increases the need for adapted safety analyses. The System-Theoretic Process Analysis is a promising approach to analyze the safety of software-intensive systems. It enables consideration of interaction and specification issues additional to component failures. However, even when using state-of-the-art analyses such as STPA, claiming the sufficiency of the safety analysis efforts is a challenging tasks for systems with ever-increasing complexity. To address this issue, this paper extends the coverage analysis concepts known from the software development to safety analyses. This is achieved with the utilization of failure graphs, i.e., formalized analysis summaries that can be automatically created during the safety analysis. Failure graphs have two advantages: they provide the possibility for visual analysis state indication and can be used to calculate various statistical metrics. Thereby, they allow to improve the knowledge about the depth, breadth, and state of the safety analysis. Both visual and statistical consideration complement each other to enhance the safety analysis coverage assessment for future avionic systems. To show all capabilities, the analysis of a flight assistance system serves as demonstrator.