Facilitating the Maintenance of Safety Cases

Developers of some safety critical systems construct a safety case comprising both safety evidence, and a safety argument explaining that evidence. Safety cases are costly to produce, maintain and manage. Modularity has been introduced as a key to enable the reusability within safety cases and thus reduces their costs. The Industrial Avionics Working Group (IAWG) has proposed Modular Safety Cases as a means of containing the cost of change by dividing the safety case into a set of argument modules. IAWG's Modular Software Safety Case (MSSC) process facilitates handling system changes as a series of relatively small increments rather than occasional major updates. However, the process doesn’t provide detailed guidelines or a clear example of how to handle the impact of these changes in the safety case. In this paper, we apply the main steps of MSSC process to a real safety critical system from industry. We show how the process can be aligned to ISO 26262 obligations for decomposing safety requirements. As part of this, we propose extensions to MSSC process for identifying the potential consequences of a system change (i.e., impact analysis), thus facilitating the maintenance of a safety case.