Model-Based STPA: Towards Agile Safety-Guided Design with Formalization

The competition for market entry in emerging segments such as Urban Air Mobility highlights the need for efficient and flexible development processes. This is accompanied by the trend towards software-intensive avionics systems due to the requirement for complex and computationally expensive algorithms. Considering the successful application of agile developments in the software domain, one might conclude that the agile paradigm would be the perfect fit to address these issues. However, for highly safety-critical domains such as aviation, multiple conflicts with the agile paradigm exist. Especially the constraint to follow rigorous and well documented processes contradicts the ideas of agile. To bridge this gap, a comprehensive and well documented, but still flexible process is necessary. Accordingly, this paper proposes a first step towards such an agile safety-guided design, by combining Model-Based Systems Engineering with the System-Theoretic Process Analysis. Particularly, focus is placed on enabling an iterative safety-guided design by providing functionality to track design changes to the corresponding safety artifacts. This automated functionality is enabled by a formalized execution of the safety analysis. At first glance, formalization sounds like a contradiction to the agile paradigm. However, we argue that formality and agility are not necessarily contradicting each other. Our theory is that moving the focus of formality from the human activities to the assisting functionality even increases overall agility. The iterative safety-guided design and resulting identification of safety improvements is demonstrated with examples of a flight assistance system.