Towards Normal Design for Safety-Critical Systems

Normal design is, essentially, when an engineer knows that the design they are working on will work. Routine ‘traditional’ engineering works through normal design. Software engineering has more often been assessed as being closer to radical design, i.e., repeated innovation. One of the aims of the Problem Oriented Software Engineering framework (POSE) is to provide a foundation for software engineering to be considered an application of normal design. To achieve this software engineering must mesh with traditional, normal forms of engineering, such as aeronautical engineering. The POSE approach for normalising software development, from early requirements through to code (and beyond), is to provide a structure within which the results of different development activities can be recorded, combined and reconciled. The approach elaborates, transforms and analyses the project requirements, reasons about the effect of (partially detailed) candidate architectures, and audits design rationale through iterative development, to produce a justified (where warranted) fit-for-purpose solution. In this paper we show how POSE supports the development task of a safety-critical system. A normal ‘pattern of development’ for software safety under POSE is proposed and validated through its application to an industrial case study.