System Safety Assessment process objectives

A scientific approach to safety assessment, such as the one presented in this chapter, does have proponents and adherents. Such an approach requires those involved in both the management and conduct of the safety assessment process to continually question (and test) both the efficacy and the validity of their evaluation systems and processes. More to the point, it requires recognition of the fact that we have always done it this way is not a reason for continuing to do so. This approach asks first what is the objective behind the testing, and then it asks how well our testing is meeting this objective. 

Having identified the appropriate risks with respect to the design under consideration, each risk should be the subject of a specific study to examine and document the simultaneous or cascading effect(s) of each risk. The objective is to ensure that any SR effects are either eliminated or the risk is shown to be acceptable. A particular risk assessment is required for aircraft airworthiness certification by the FAA the process is documented in SAE/ARP-4761, Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment, 1996. 

This whole process of safety analysis is regulated by the ARP 4761. Identification of safety objectives is given by a fimctional approach, documented in a functional hazard analysis demonstrating compliance with these objectives is achieved by identifying the combinations of failiues, and this is documented in the System Safety Assessment (SSA). 

The HSA is a problem-solving tool that considers both quantitative and qualitative issues within the framework of the defined system. It is useful for problems which can be well defined, fairly limited in extent and with agreed objectives (defined future state, such as a system which meets its safety targets). The HSA process is iterative in nature and is summarised in Fig. 8.5 The hard systems approach can be tailored to the safety assessment process as shown in Fig. 8.6. 

Some would say that this is the current state of the art. Much of the necessary library could be assembled from test systems that have been extensively evaluated and have already undergone extensive validation (Gad, 2000, 2001). Three critical steps must be taken for the eventual fulfillment of these objectives (1) acceptance of a scientific approach to the problem of safety assessment (2) development of an operative validation and acceptance process for new test procedures (3) clear enunciation of an acceptance criterion for new test designs by regulatory authorities. 

Those analytical processes that assess system and aeroplane safety in an objective nonnumerical manner [AC23.1309E]. 

The safety case, particularly for computer based systems, traditionally contains diverse arguments that support its claims. These arguments are sometimes called the legs of the safety case and are based on different evidence. Just as there is defence in depth in employing diversity at system architecture level, so we see an analogous approach within the safety case itself Another important feature of the safety case process is independent assessment. The objective of independent assessment is to ensure that more than one person or team sees the evidence so as to overcome possible conflicts of interest and blinkered views that may arise from a single assessment. The existence of an independent assessor can also motivate the... 

If these questions do not have satisfying answers, the organization may wish to modify its objectives. These examples describe different vantage points to use in assessing objectives for the process safety system and should play a role in defining the metrics system objectives. 

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