What is FMECA?
FMECA adds a criticality analysis to FMEA. It is the preferred method when reliability data exists and decisions about redundancy, maintenance intervals or spare parts depend on how often each failure mode will really occur. The results feed reliability-centered maintenance, design reviews and safety cases.
Criticality analysis
For each failure mode, Cm = β × α × λp × t. With λp in failures per million hours, Cm is the expected number of occurrences of that failure mode with that effect per million missions. Summing Cm per item and severity class gives the item criticality Cr. The criticality matrix plots each failure mode by severity class (horizontal) and Cm (vertical, log scale): the further up and to the right, the more critical.
FMECA example
The example covers the blower and breathing circuit of a portable ventilator for a 24-hour mission. The most critical failure modes are in class I (catastrophic): blower bearing seizure (Cm = 86) and winding open circuit (Cm = 58), both causing loss of ventilation. A stuck-closed exhalation valve is also class I, but much less likely (Cm = 9). Blower speed drift is class II with Cm = 72. The analysis points to blower motor reliability, and to the detection and backup provisions for loss of ventilation, as the top priorities.
Acting on the results
- Class I and II, high Cm: design changes, redundancy or derating to reduce λp or β.
- Hidden failures: add detection (alarms, built-in tests) or periodic inspection.
- Maintenance: set preventive replacement intervals for wear-out failure modes.
- Visualize the controls: use a bowtie diagram to show the barriers for the worst failure modes.
How to use this FMECA tool
- Define the system and mission. Name the system and describe the mission or operating profile, including operating time.
- List items and failure modes. For each item, list its failure modes and their end effects on the system.
- Classify severity. Assign severity class I to IV to each failure mode based on its worst credible end effect.
- Enter reliability data. Enter the failure rate λp, mode ratio α, effect probability β and operating time t.
- Read the criticality matrix. Failure modes high and to the right are the most critical: act on them first.
- Export. Export the worksheet, item criticality and matrix to Excel or PDF.
Frequently asked questions
What is FMECA?
Failure mode, effects and criticality analysis (FMECA) is an FMEA extended with a criticality analysis. Each failure mode gets a severity classification and a criticality number based on failure rate data, so failure modes can be ranked by both how bad and how likely they are. It is defined in MIL-STD-1629A and widely used in aerospace, defence, medical devices and energy.
What is the difference between FMEA and FMECA?
FMEA rates occurrence and detection on ordinal 1 to 10 scales and ranks by RPN or Action Priority. FMECA quantifies the probability of each failure mode from failure rates, mode ratios and operating time, and plots the results on a criticality matrix against severity class.
How is the failure mode criticality number calculated?
Cm = β × α × λp × t, where λp is the part failure rate (here per million hours), α the failure mode ratio (the share of the part's failures that occur in this mode), β the conditional probability that the failure mode causes the end effect, and t the operating time per mission in hours. Item criticality Cr is the sum of Cm for the item's failure modes in one severity class.
What are the severity classes?
MIL-STD-1629A defines Category I Catastrophic (death or system loss), II Critical (severe injury or major system damage), III Marginal (minor injury or damage, mission degradation) and IV Minor (less than minor injury or damage, unscheduled maintenance).
What values should β have?
The standard suggests 1.00 for actual loss, 0.10 to 1.00 for probable loss, above 0 to 0.10 for possible loss, and 0 for no effect. The tool offers 1.0, 0.5, 0.1 and 0 as practical choices.
Where do failure rates come from?
From field and test data where available, otherwise from reliability handbooks and databases such as MIL-HDBK-217, SN 29500, NPRD/EPRD or supplier data. Document the source for each rate.