Model-Based Safety Assessment for the Three Stages of Refinement of the System Development Process in ARP4754A

Model Based Safety techniques have been developed for a number of years, though the models have not been customised to help address the safety considerations/ actions at each refinement level. The work performed in the MISSA Project looked at defining the content of “safety models” for each of the refinement levels. A modelling approach has been defined that provides support for the initial functional hazard analysis, then for the systems architectural definition level and finally for the systems implementation level. The Aircraft functional model is used to apportion qualitative and quantitative requirements, the systems architectural level is used to perform a preliminary systems safety analysis to demonstrate that a system architecture can satisfy qualitative and quantitative requirements. Finally the systems implementation level intends to take behavioural models that can be used to generate qualifiable code from a supplier and to demonstrate that the implemented system conforms to the requirements specified at the systems architectural level. A technique has also been developed to compare the results between the systems architectural level qualitative analysis results with those of the systems implementation level results. This paper describes the construction of each of these models, the intended use, and provides a simple example to show how they fit together.