The probabilistic approach to safety

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The probabilistic approach to safety - success or failure?

Cherry, R.G.W.

Numerical probabilities have been used as an aid to aircraft systems design and safety analysis for some thirty years. This paper considers the background to the philosophy, its current application and the benefits that might be gained from further developments. It is considered that the use of numerical probabilities has proven to be of great benefit to the systems designer. While no meaningful evaluation has been carried out as to the effect that this approach has had on reducing the accident rate, it has certainly provided a structured approach to the design and safety analysis of aircraft systems and this must have been a contributory factor to the improvements that have been seen in airworthiness related accidents. However, it is considered that the principles employed in the advisory material 'Joint Airworthiness Requirements (JAR)' could be developed further to encompass maintenance analysis, in-service reliability monitoring and criteria for the evaluation of acceptable conditions under which aircraft may be despatched with equipment inoperative. There have been significant improvements in the large transport aircraft accident rate which compares favourably with other modes of transport. Further improvements to the accident record are likely to be achieved by concentrating effort on the human factor issues. It is proposed that this may best be achieved by an in-depth study of accidents and serious incidents with a view to providing guidance in the design of aircraft, the compilation of flight and crew manuals, and flight crew training programmes.