1 FAULT TREE ANALYSIS 1. Introduction
FTA is a deductive, failure-based approach. As a deductive approach, FTA starts with an undesired event, such as failure of a main engine, and then determines (deduces) its causes using a systematic, backward-stepping process. In determining the causes, a fault tree (FT) is constructed as a logical illustration of the events and their relationships that are necessary and sufficient to result in the undesired event, or top event. The symbols used in a FT indicate the type of events and type of relationships that are involved. The FT is a qualitative model that provides extremely useful information on the causes of the undesired event. The FT can also be quantified to provide useful information on the probability of the top event occurring and the importance of all the causes and events modelled in the FT. This handbook leads the reader through FTA. Particular details can be skipped if the reader desires only an overview of FTA and instead wants to focus on its uses to assist decision-making. In addition to FTA, inductive approaches are also used in safety analysis and in risk and reliability analysis. In contrast to the deductive approach used in FTA, inductive approaches are forward-stepping approaches that begin with a basic cause or initiating event and then investigate (induce) the end effects. Both FTA and inductive approaches are failure-based. The advantages of failure-based approaches are also discussed. A FT can be transformed into its logical complement; a success tree (ST) that shows the specific ways the undesired event can be prevented from occurring. The ST provides conditions that, if assured, guarantee that the undesired event will not occur. The ST is a valuable tool that provides equivalent information to the fault tree, but from a success viewpoint. Techniques for transforming the FT to its ST are described along with uses of the ST. The uses of FTA to assist decision-making are described in this AFTH. FTA provides critical information that can be used to prioritize the importance of the contributors to the undesired event. The contributor importance provided by FTA vividly shows the causes that are dominant and that should be the focus of any safety or reliability activity. Mo re formal riskbenefit approaches can also be used to optimally allocate resources to minimize both resources expenditures and the occurrence probability of the undesired event. These risk approaches are useful for allocating resource expenditures, such as safety upgrades to complex systems like the Space Shuttle. FTA can be applied to both an existing system and to a system that is being designed. When it is applied to a system being designed for which specific data do not exist, FTA can provide an estimate of the failure probability and the important contributors using generic data to bracket the design components or concepts. FTA can also be used as an important element in the development of a performance-based design. When applied to an existing system, FTA can be used to identify weaknesses and to evaluate possible upgrades. It can also be used to monitor and predict behaviour. Furthermore, FTA can be used to diagnose causes and potential corrective measures for an observed system failure. The approaches and tools to obtain this information and the applications of this information in decision-making are important topics of the AFTH. The second part of the AFTH contains examples of the application of FTA in studies that have been previously performed. The focus is on aerospace applications. The examples include the rupture of a pressure tank (a classic FTA example), failure to initiate and terminate thrust in a monopropellant propulsion system, failure of a redundant container seal (design analysis), and a dynamic FT analysis of a mission avionics system.
GRAPH BASED METHODS
2 FAULT TREE ANALYSIS
1.1 The Fault Tree Approach FTA can be simply described as an analytical technique,...
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