Failure Mode and Effects Analysis is a systematic method for identifying how a process or product can fail (failure modes), what the consequences are (effects), and what causes the failure (mechanisms). Each failure mode is rated on Severity (impact), Occurrence (likelihood), and Detection (ability to catch it). The ratings drive prioritization of prevention and detection actions.

What is the difference between RPN and Action Priority?

The traditional approach multiplied Severity × Occurrence × Detection to get a Risk Priority Number (RPN). The newer AIAG/VDA methodology uses an Action Priority (AP) table that considers the interaction between S, O, and D more intelligently. For example, a high-severity failure mode always gets high priority regardless of occurrence — because the consequence is unacceptable even if rare. RPN could mask this (S=9, O=1, D=1 = RPN of 9, which appears low).

When should FMEA be done?

Before production starts (during process design/planning), and updated whenever the process changes, a new failure mode is discovered, or a quality escape occurs. An FMEA that is written once and filed is worthless. A living FMEA that is updated with field data and drives ongoing improvement is invaluable.

FMEA for Practitioners: Anticipate Failures Before They Occur

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Statistical Process Control Root Cause Analysis Toolkit

S×O×D

Classic RPN

1–10

Rating Scale

H/M/L

Action Priority

Living

Document Status

The FMEA Process: 7 Steps

Step 1: Define Scope

What process or product are you analyzing? A process FMEA (PFMEA) examines manufacturing steps. A design FMEA (DFMEA) examines the product design. For lean practitioners, PFMEA is most common. Define the boundaries: which operations are included?

Step 2: Identify Process Steps

List every operation in the process. For each operation, identify: its function (what it should do), its requirements (how you know it is done correctly).

Step 3: Identify Failure Modes

For each operation: how could it fail? What could go wrong? A failure mode is the way the operation fails to meet its requirement. Example: “Hole drilled undersize,” “Sealant applied with insufficient fillet,” “Fastener installed with incorrect torque.”

Step 4: Determine Effects and Severity

For each failure mode: what is the impact on the customer (next operation, final assembly, or end user)? Rate Severity on a 1–10 scale. S=1 is no effect. S=9–10 is safety hazard or regulatory violation. In aerospace, structural failures and FOD are typically S=9 or 10.

Step 5: Identify Causes and Occurrence

For each failure mode: what causes it? Rate Occurrence (likelihood) on a 1–10 scale. O=1 means nearly impossible. O=10 means almost certain. Base ratings on actual data (defect rates, historical occurrence) when available, not gut feel.

Step 6: Identify Current Controls and Detection

What controls exist to prevent the cause or detect the failure? Rate Detection on a 1–10 scale. D=1 means the control will almost certainly detect. D=10 means no control exists or detection is impossible. SPC improves detection. Poka-yoke improves both prevention and detection.

Step 7: Determine Action Priority and Assign Actions

Use the S×O×D Action Priority table (or RPN if your organization uses the older method) to prioritize. Assign specific actions with owners, deadlines, and verification methods. After implementation, re-rate to confirm the risk has been reduced.

Rating Scales

Rating	Severity	Occurrence	Detection
1	No effect	Nearly impossible (≤1 in 1,000,000)	Almost certain to detect
2–3	Minor annoyance	Very unlikely	High chance of detection
4–6	Moderate impact, rework needed	Occasional	Moderate detection capability
7–8	Major impact, customer affected	Frequent	Low detection capability
9–10	Safety hazard / regulatory	Very frequent / certain	No detection / cannot detect

Worked Example: Sealant Application PFMEA

📊 Process FMEA — Wing Panel Sealant Application Aerospace

Failure Mode	Effect	S	Cause	O	Current Control	D	RPN	Action
Insufficient fillet radius	Corrosion path at joint; rework	7	No standard work for technique	6	Visual inspection at next station	5	210	Create standard work + TWI training
Sealant uncured	Joint failure in service; safety	9	Shop temp below cure minimum	3	Inspector checks adhesion	4	108	Add temp monitoring + auto-alert
Foreign object in sealant	FOD risk; corrosion initiation	8	Dirty application area	4	Pre-seal clean per SOP	6	192	Add poka-yoke clean verification step
Wrong sealant type applied	Chemical incompatibility; rework	8	Multiple sealant types at station	2	Operator reads work order	7	112	Color-code sealant tubes by type; poka-yoke storage

Priority order (by RPN): Insufficient fillet (210) → FOD (192) → Wrong type (112) → Uncured (108). The highest-RPN item gets action first — standard work and TWI training for sealant application technique.

💡 The FMEA Is a Living Document

The most common FMEA failure: it is created during APQP, filed in a binder, and never updated. A real FMEA is updated when: (1) A new failure mode is discovered in production. (2) A customer complaint reveals an undetected failure. (3) A process change adds new failure possibilities. (4) Countermeasures are implemented and ratings need to be re-scored. Review the FMEA at least quarterly or whenever the process changes.

🎯 The Bottom Line

FMEA is proactive quality — anticipating failures before they occur instead of reacting to defects after. The 7-step process identifies failure modes, rates their risk (Severity × Occurrence × Detection), and prioritizes prevention actions. Focus on high-severity items first (regardless of occurrence), drive occurrence down with process improvement, and drive detection up with SPC and poka-yoke. Keep the FMEA alive by updating it with field data and new discoveries. Next: Poka-Yoke Design — error-proofing techniques that eliminate failure modes at the source.

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