Integrating Safety into Architecture Selection and System Trade Studies

4 Integrating Safety into Architecture Selection and System Trade Studies 

How are these decisions made The selection process usually involves extensive tradeoff analysis that compares the different feasible architectures with respect to some important system property or properties. Cost, not surprisingly, usually plays a large role in the selection process while other properties, including system safety, are usually left as a problem to be addressed later in the development lifecycle. Many of the early architectural decisions, however, have a significant and lasting impact on safety and may not be reversible after the basic architectural decisions have been made. For example, the decision not to include a crew escape system on 

A Frequent B Moderate C Occasional D Remote E Unlikely F Impossible 

High-level hazards are first identified and, for each identified hazard, a qualitative evaluation is performed by classifying the hazard according to its severity and likelihood. 

The increasing use of software in most complex systems complicates the situation further. Much or even most of the software in the system will be new and have no historical usage information. In addition, statistical techniques that assume randomness are not applicable to software design flaws. Software and digital systems also introduce new ways for hazards to occur, including new types of component interaction accidents. Safety is a system property, and, as argued in part I, combining the probability of failure of the system components to be used has little or no relationship to the safety of the system as a whole. 

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