The role of safety architectures in aviation safety cases

Highlights: • Safety architectures (SAs) are formalized to extend Bow Tie Diagrams (BTDs). • SAs represent a barrier model of safety giving a rigorous basis for risk analysis. • SAs should be a core component of aviation safety cases. • The AdvoCATE toolset provides support for SAs and other aspects of safety cases. -- Abstract: We develop a notion of safety architecture (SA), based on an extension to Bow Tie Diagrams (BTDs), to characterize the overall scope of the mitigation measures undertaken to provide safety assurance at both design time and during operations. We motivate the need for SAs, whilst also illustrating their application and utility in the context of aviation systems, through an example based upon a safety case for an unmanned aircraft system mission that successfully underwent regulatory scrutiny. We elaborate how SAs fit into our overall safety assurance methodology, also discussing the key role they play in conjunction with structured assurance arguments to provide a more comprehensive basis for the associated safety case. We give a formal semantics as a basis for implementing both BTDs and SAs in our assurance case tool, AdvoCATE, describing the functionality afforded to support both the related safety analysis and subsequent development activities, e.g., enforcement of well-formedness properties, computation of residual risk, and model-based views and transformations.