Failure Mode, Effects, and Criticality Analysis (FMECA)

FMECA enhances the qualitative aspects of FMEA by introducing a quantitative criticality assessment, as defined in standards like MIL-STD-1629A. Criticality is typically calculated in two ways: modal criticality and item criticality. Modal criticality is calculated for each failure mode and can be expressed as [latex]C_m = lambda_p alpha beta K_e[/latex], where [latex]lambda_p[/latex] is the part failure rate, [latex]alpha[/latex] is the failure mode ratio (the fraction of part failures attributable to this mode), [latex]beta[/latex] is the conditional probability that the failure effect will result, and [latex]K_e[/latex] is the severity factor. Item criticality is then the sum of the modal criticalities for all failure modes of a particular item.

This quantitative approach is particularly valuable in high-risk industries like aerospace, defense, and nuclear power, where a more granular and defensible méthode than the standard RPN is required. The output is often a criticality matrix, which plots severity categories against probability of occurrence levels. This matrix provides a clear visual representation of the highest-risk items, allowing engineers and managers to focus resources on mitigating the most critical potential failures, especially those that could lead to catastrophic system loss or loss of life.