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Jul 5, 2011

Criticality

Some failures are more critical than others, and the audit should highlight those critical failures most deserving of attention and scarce resources. Failure Mode, Effects and Criticality Analysis (FMECA) is one way to spotlight critical (or potentially critical) failure opportunities.

FMECA, like Failure Mode and Effect Analysis (FMEA), is usually performed during the reliability apportionment phase. Also like FMEA, FMECA consists of considering every possible failure mode and its effect on the product. However, FMECA goes one step beyond FMEA in that it also considers the criticality of the effect and actions which must be taken to compensate for this effect. Typical criticality categories are similar to that grouped under the heading of "defects," e.g., critical (loss of life or product), major (total product failure) and minor (loss of function). Preferably, FMECA will result in a design modified to eliminate unwanted seriously deleterious effects. A contingency plan will be prepared for dealing with those effects that cannot be removed from the design. (Pyzdek, 1996)

The National Aeronautics and Space Administration (NASA), for example, uses criticality analysis as a way of categorizing non-conformities discovered during the course of both its Process-Product Integrity Audits (PPIA) and Supplier Process-Product Integrity Audits. (See Appendix H for an in-depth look at NASA’s Supplier Process-Product Integrity Audit [SPPIA].) For both PPIAs and SPPIAs, audit findings are categorized by criticality in the following manner:

Category 1—Instances where hardware or work product was produced using incorrect work instructions, specifications or drawings with no subsequent inspection operations or tests. When such instances are identified, immediate steps must be taken to identify the problem and allow the normal material review reporting system to be invoked.

Category 2—Instances where hardware or work product was produced correctly, but the work instructions, drawings or both were incorrect. Correction is required before the next application of the operation.

Category 3—Audit findings that were to be incorporated at the next normal revision. Such findings include better practices (enhancements rather than corrections to the engineering, requirements definition, or work instructions) and changes that require further study.