OptimalAvailability Studio™ Wiki / FMEA and FMECA
Method
FMEA and FMECA
Failure mode and effects analysis is the disciplined walk through every way an item can fail and what each failure does. FMECA adds a criticality analysis, so the modes can be ranked and the effort aimed at the few that matter.
FMEA, and the C in FMECA
FMEA identifies each failure mode of an item, its causes and its effects, and the controls in place. FMECA adds criticality: a measure that ranks each mode by how bad and how likely it is. The method dates to MIL-STD-1629A and is carried today by SAE J1739, IEC 60812 and the AIAG-VDA FMEA Handbook (2019). It comes in three flavours: functional, design or hardware, and process.
The analysis, step by step
For each item, working down the asset hierarchy:
| Function | What the item is required to do, with its performance standard. |
|---|---|
| Failure mode | The specific way it fails to deliver that function. |
| Effects | The consequence at three levels: local, next-higher assembly and end effect on the system. |
| Cause | The mechanism or root that brings the mode about. |
| Controls | What currently prevents the cause or detects the mode before the effect. |
| Ratings | Severity, occurrence and detection, each scored so the risk can be prioritised. |
Risk: RPN and its successor
RPN = Severity × Occurrence × Detection
Each of the three is scored 1 to 10, so the risk priority number runs 1 to 1000. It is simple and widely used, but flawed: very different risks can share an RPN, the scale has large gaps, and a high severity can hide behind a low product. The 2019 AIAG-VDA handbook therefore replaces RPN with Action Priority (High, Medium or Low), a lookup on the same three factors that weights severity first. OptimalAvailability Studio™ supports both, and leads with severity so a safety-critical mode is never buried.
Criticality analysis
Criticality can be qualitative, a severity-by-likelihood matrix, or quantitative. The MIL-STD-1629A quantitative form is:
Mode criticality Cm = β · α · λp · t
Item criticality Cr = Σ Cm
β is the conditional probability the mode causes the end effect, α the mode ratio (share of the item failure rate), λp the item failure rate, and t the operating time. Summing the modes gives the item criticality, which ranks the hardware for attention.
What FMECA feeds
The FMECA is the shared spine of the reliability model. Its failure modes and effects feed RCM task selection, its criticality drives spares stocking in OptimalSPARES™ and inspection planning in OptimalIntegrity Studio™, and its failure rates feed the RBD and RAM model. Done once, well, it serves the whole estate.
Where OptimalAvailability Studio™ fits
OptimalAvailability Studio™ holds the FMECA as a reusable library keyed to the asset taxonomy, so a pump analysed once is inherited by every like pump, and a change to a mode updates every place it is used: the RCM tasks, the criticality ranking and the system model.