A Pragmatic Approach to Reasoning about the Assurance of Safety Arguments

The development of safety critical systems is guided by standards. Many standards require the development of a safety case to demonstrate the acceptability of Safety Critical Systems. The safety case must provide confidence that the system is acceptably safe to operate. For system components where it is not possible to quantify the associated risks (e.g. software), current standards in the aerospace, rail and defence sectors identify design and safety processes for different Safety Integrity Levels (SILs) or Development Assurance Levels (DALs). The assumption is that components developed against the requirements of higher SILs/DALs will be less prone to critical failures and thus have a lower impact on the safety of the overall system. This paper questions this assumption and instead discusses assurance of the safety argument as a method of demonstrating the confidence that can be placed in a safety case. An industrial case study from the aerospace sector is presented to demonstrate the practical use of the concept.