A complete fmeca also includes an estimate of the

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Unformatted text preview: roject. Even if the documented lessons learned from previous projects are not applicable at the system level, there may be valuable data applicable at the subsystem or component level. FMECAs, FMEAs, Digraphs, and Fault Trees. Failure Modes, Effects, and Criticality Analysis (FMECA), Failure Modes and Effects Analysis (FMEA), digraphs, and fault trees are specialized techniques for safety (and/or hazard) risk identification and characterization. These techniques focus on the hardware components that make up the system. According to MIL-STD-1629A, FMECA is "an ongoing procedure by which each potential failure in a system is analyzed to determine the results or effects thereof on the system, and to classify each potential failure mode according to its severity." Failures are generally classified into four seventy categories: • • • • Category I—Catastrophic failure (possible death or system loss) Category II—Critical failure (possible major injury or system damage) Category III—Major failure (possible minor injury or mission effectiveness degradation) Category IV — Minor failure (requires system maintenance, but does not pose a hazard to personnel or mission effectiveness). A complete FMECA also includes an estimate of the probability of each potential failure. These probabilities are usually based, at first, on subjective judgment or experience factors from similar kinds of hardware components, but may be refined from reliability data as the system development progresses. An FMEA is similar to an FMECA, but typically there is less emphasis on the severity classification portion of the analysis. Digraph analysis is an aid in determining fault tolerance, propagation, and reliability in large, interconnected systems. Digraphs exhibit a network structure and resemble a schematic diagram. The digraph technique permits the integration of data from a number of individual FMECAs/FMEAs, and can be translated into fault trees, described in Section 6.2, if quantitative probability e...
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