A Structured Framework for Prioritizing Unsafe Control Actions in STPA: Case Study on eVTOL Operations

Systems Theoretic Process Analysis (STPA) is a widely recommended method for analysing complex system safety. STPA can identify numerous Unsafe Control Actions (UCAs) and requirements depending on the level of granularity of the analysis and the complexity of the system being analysed. Managing numerous results is challenging, especially during a fast-paced development lifecycle. Extensive research has been done to optimize the efficiency of managing and prioritising the STPA results. However, maintaining the objectivity of prioritisation and communicating the prioritised results have become common challenges. In this paper, the authors present a complementary approach that incorporates inputs from both the safety analysts and domain experts to more objectively prioritise UCAs. This is done by evaluating the severity of each UCA, the impact factor of each controller or decision maker that issues the UCA, and the ranking provided by the subject matter experts who assess the UCA criticalities based on different factors. In addition, a Monte Carlo simulation is introduced to reduce subjectivity and relativity, thus enabling more objective prioritisation of the UCAs. As part of the approach to better communicate the prioritisation results and plan the next steps of system development, a dynamic-scaling prioritisation matrix was developed to capture different sets of prioritised UCAs. The approach was applied to a real project to improve the safe operations of Electric Vertical Take-off and Landing (eVTOL). The results highlighted critical UCAs that need to be prioritised for safer eVTOL operation. 318 UCAs were identified in total. Based on the application of the prioritisation methodology, 110 were recognized as high-priority UCAs to strengthen the system design.