Failure Mode, Effects, and Criticality Analysis(FMECA)
Definition
An extension of FMEA that adds a criticality analysis step to quantify the combined frequency and severity of each failure mode's consequences. FMECA augments FMEA by calculating a criticality number for each failure mode based on the failure effect probability, failure mode ratio, part failure rate, and operating time, enabling a more rigorous mathematical prioritization than RPN alone.
Context
FMECA is used when a more quantitative risk ranking is needed beyond the semi-quantitative RPN approach. It originated in military standards (MIL-STD-1629A) and is common in aerospace, defense, and nuclear applications. In industrial asset integrity, FMECA-level rigor is typically applied to safety-critical or high-consequence equipment.
Formula
Criticality Number (Cm) = beta x alpha x lambda_p x t, where beta = failure effect probability, alpha = failure mode ratio, lambda_p = part failure rate, t = operating time
Examples
- A FMECA of a safety instrumented function calculates criticality numbers for each failure mode to demonstrate that the overall system meets SIL requirements.
- The criticality analysis produces a Pareto chart showing that 3 failure modes account for 80% of the total criticality, focusing resources on the vital few.
Applicable Standards
- MIL-STD-1629A
- IEC 60812:2018
- SAE ARP5580
Related Terms
A systematic, proactive methodology for evaluating a process, product, or design to identify all possible failure modes, determine their effects on system performance, rank them by risk priority, and recommend actions to reduce or eliminate the highest risks. FMEA examines each component's potential failure modes, their causes, effects at local/system/end levels, current controls, and risk priority numbers.
Criticality AnalysisA structured evaluation to determine the relative importance (criticality) of equipment items based on the consequences of their failure. Criticality analysis considers safety, environmental, production, and cost impacts to classify equipment into criticality categories that drive maintenance strategy, inspection priority, and spare parts stocking decisions.
Risk Priority Number(RPN)A numerical value that ranks the risk associated with a failure mode by multiplying three independently scored factors: Severity (impact of the failure), Occurrence (likelihood of the failure happening), and Detection (likelihood of detecting the failure before it reaches the end user or causes harm). RPN ranges from 1 to 1,000, where higher values indicate greater risk priority.
Failure Rate(Lambda)The average number of failures per unit of time for a population of items, typically expressed as failures per hour, per year, or per million hours. Failure rate (lambda) is the reciprocal of MTBF for constant-failure-rate systems. It is a statistical average measure, distinct from the instantaneous hazard rate, and is most accurate when the failure rate is approximately constant (beta near 1).
Critical ItemAn equipment item, component, or system whose failure would result in unacceptable safety, environmental, or operational consequences. Critical items are identified through criticality analysis and receive the highest levels of inspection, maintenance, monitoring, and engineering attention. They typically require specific maintenance strategies, dedicated spare parts, and documented management plans.
Where this appears in Reliatic
This concept is used in the FMEA & Risk module.