Failure Modes, Effects, and Criticality Analysis
Failure Modes, Effects, and Criticality Analysis
This domain tests bottom-up FMECA: identifying modes, effects, severity, occurrence, detection, RPN, and criticality numbers.
Core rules
- FMECA extends FMEA with criticality. Severity ranks consequence from catastrophic to minor; occurrence ranks likelihood; detection ranks ability to catch the mode before escape.
- RPN=S×O×D is a prioritization aid, not a risk acceptance criterion.
- MIL-STD-1629A criticality: Cm=β·α·λp·t, where α is the mode ratio, β is the conditional probability that the failure causes the effect, λp is the part failure rate, and t is operating time.
- Analyze each function, then each mode. Effects are stated at local, next-higher, and end-item levels.
Worked example
A valve mode 'fails to open' has S=8, O=4, and D=3, so RPN=96. Criticality: λp=2 failures/10^6 h, α=0.30, β=0.50, and t=1,000 h. Cm=0.50×0.30×2×10^-6×1,000=0.0003 failures. A second mode with RPN 80 but S=10 outranks it despite a lower RPN because severity drives corrective action.
Traps
- Sorting only by RPN and ignoring severity.
- Reversing the detection scale.
- Treating FMECA as top-down; it is inductive.
- Forgetting human and software failure modes.