Resources - System Safety

Failure Mode and Effects Analysis (FMEA)

FMEAs are important reliability programs tools that provide data usable by the system safety program. This analysis is performed for reliability, safety, and supportability information. Hazard analyses typically use a top down analysis methodology (e.g., Fault Tree). The approach first identifies specific hazards and isolates all possible (or probable) causes. The FMEA may be performed either top down or bottoms-up, usually the latter.

The procedural approach to generating an FMEA is comparable to that of the Fault Hazard Analysis.

The first step is to list all components or low level functions. Then, by examining system block diagrams, schematics, etc., the function of each component is identified.

Next, all reasonably possible failure modes of the lowest component being analyzed are identified. Using a coolant pump bearing as an example they might include frozen, high friction, or too much play. For each identified failure mode, the effect at the local level, an intermediate level, and the top system level are recorded. A local effect might be "the shaft won't turn," the intermediate "pump won't circulate coolant," and the system level "engine overheat and fail."

At this point in the analysis, the FMEA might identify a hazard.

The analyst next documents the method of fault detection. This input is valuable for designing self test features or the test interface of a system. More importantly, it can alert an air crew to a failure in process prior to a catastrophic event. A frozen pump bearing might be detected by monitoring power to the pump motor or coolant temperature. Given adequate warning, the engine can be shut down before damage or the aircraft landed prior to engine failure.

Next, compensating provisions are identified as the first step in determining the impact of the failure. If there are redundant pumps or combined cooling techniques, the significance of the failure is less than if the engine depends on a single pump. The severity categories used for the hazard analysis can be used as the severity class in the FMEA. A comments column is usually added to the FMEA to provide additional information that might assist the reviewer in understanding any FMEA column.

Source: FAA System Safety Handbook, Ch. 9.

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