FASR: Automated Identification of Unsafe Control Actions in STPA

Abstract: The System-Theoretic Process Analysis (STPA) is a well-established hazard analysis technique that has been applied to a wide range of safety-critical systems. Despite its popularity, there is relatively little automation support for STPA, and most of its steps are carried out manually by a human analyst, which can be time consuming and error prone. This paper investigates the potential use of model-based engineering and formal methods to assist human analysts in efficiently and accurately carrying out STPA. The proposed tool, called FASR (Formalizing and Automating STPA with Robustness), enables automated, complete identification of unsafe control actions (UCAs), leveraging recent advances in robustness analysis to identify UCAs as undesirable deviations in the controller's actions. The use of the tool is demonstrated on a case study involving a Braking System Control Unit (BSCU) in an avionics system. As a preliminary exploration of the potential benefits and limitations of the tool, the paper reports on a user study involving nine participants with varying backgrounds in STPA, model-based engineering, and formal methods; the study found that most participants considered the tool a useful aid in identifying UCAs, while suggesting improvements that would make a tool such as FASR usable and applicable to a wider range of systems and analysts.