Method 1: 5-Why Analysis
The simplest and most frequently used RCA method. Start with the problem, then ask “Why?” repeatedly until you reach a root cause that is actionable.
| Level | Question | Answer |
|---|---|---|
| Problem | What happened? | Drill bit broke during wing rib machining |
| Why 1 | Why did the drill bit break? | Excessive heat buildup during drilling |
| Why 2 | Why was there excessive heat? | Coolant was not reaching the cutting zone |
| Why 3 | Why was coolant not reaching the zone? | The coolant nozzle was clogged with chip debris |
| Why 4 | Why was the nozzle clogged? | The chip guard was removed during a repair and never reinstalled |
| Why 5 | Why was the chip guard not reinstalled? | There is no post-repair checklist to verify all guards are in place before returning the machine to production |
Root cause: No post-repair verification checklist. Countermeasure: Create a maintenance return-to-service checklist that includes all guards, covers, and safety devices. Verify with a sign-off before the machine resumes production.
⚠️ 5-Why Pitfalls
Stopping too early: “Why did the drill break? Because the operator used the wrong speed.” That is Why 1, not a root cause. Keep going. Blame trail: If every Why leads to “because the operator didn’t...” you are finding human error, not system failure. The system should prevent the error. Single path: Problems often have multiple contributing causes. Branch your 5-Why when “Why?” has more than one valid answer.
Method 2: Ishikawa (Fishbone) Diagram
The fishbone diagram organizes potential causes into categories, ensuring you consider all dimensions of the problem. The standard manufacturing categories are the 6M’s:
| Category | What to Examine | Example Causes for Sealant Rework |
|---|---|---|
| Man (People) | Training, skill, experience, fatigue, technique variation | No standard work for bead application; 3 operators use 3 different techniques |
| Machine | Equipment condition, calibration, maintenance, tooling | Sealant gun nozzle worn, dispensing inconsistent bead width |
| Material | Raw material quality, shelf life, lot variation | Sealant batch from vendor B has higher viscosity than vendor A |
| Method | Process design, work instructions, sequence | Work instruction says “apply sealant” but does not specify bead width or technique |
| Measurement | Gage capability, inspection method, criteria clarity | Fillet radius acceptance criteria is ambiguous (“adequate” vs. “0.125 ± 0.020 in”) |
| Mother Nature (Environment) | Temperature, humidity, lighting, contamination | Shop temperature below 65°F causes sealant to thicken and resist flow |
The fishbone is a brainstorming tool — list every possible cause under each M, then use data to narrow down to the actual root cause(s). It prevents the team from fixating on the first cause that comes to mind.
Method 3: Is/Is-Not Analysis
Is/Is-Not is powerful when you can observe the problem selectively — it happens sometimes but not always, in some locations but not others, with some operators but not all. The method systematically maps the boundaries of the problem to narrow the cause.
| Dimension | IS (problem occurs) | IS NOT (problem does not occur) | Distinction |
|---|---|---|---|
| What | Hi-Lok fasteners in Zone 3 | Hi-Lok fasteners in Zones 1, 2, 4 | Zone 3 only |
| Where | Aft fuselage section | Forward fuselage, wing | Aft section only |
| When | Night shift | Day shift | Night shift only |
| Who | All night shift operators | Day shift operators | Not person-specific |
Hypothesis from distinctions: Zone 3 on aft fuselage, night shift only. Investigation reveals: the night shift torque wrench for Zone 3 was out of calibration (last calibrated 6 months ago). Day shift uses a different wrench that is current. The root cause is not operator error — it is a calibration management gap.
Method 4: Fault Tree Analysis (FTA)
FTA works top-down: start with the undesired event (“top event”) and decompose it into all possible cause combinations using AND/OR logic gates. It is the most rigorous RCA method and is used for safety-critical analysis in aerospace (required by many AS9100 and MIL-STD processes).
| Gate | Symbol | Meaning |
|---|---|---|
| OR gate | ⋀ | The output event occurs if ANY input event occurs (either/or) |
| AND gate | ⋂ | The output event occurs only if ALL input events occur simultaneously |
FTA is more complex than 5-Why or fishbone and is typically used for high-consequence failures (safety incidents, field failures, MRB-level quality escapes). For daily shop floor problem solving, 5-Why and fishbone are sufficient.
Choosing the Right Method
| Situation | Best Method | Why |
|---|---|---|
| Simple problem, daily use | 5-Why | Fast (5–10 min), no special tools needed, good for daily standup |
| Complex problem, multiple possible causes | Fishbone | Structured brainstorming, ensures no category is overlooked |
| Problem occurs selectively | Is/Is-Not | Systematically narrows the field using boundary conditions |
| Safety-critical, high-consequence | Fault Tree | Exhaustive, logic-based, identifies all possible cause combinations |
| A3 Section 5 | 5-Why + Fishbone | Fishbone to brainstorm, 5-Why to dig into the top candidates |
🎯 The Bottom Line
Root cause analysis is the engine behind every lean improvement. 5-Why gets you to actionable causes quickly. Fishbone ensures you consider all dimensions (6M’s). Is/Is-Not narrows the field when the problem occurs selectively. Fault Tree provides exhaustive analysis for safety-critical failures. Use the three root cause tests (remove cause → problem stops? cause present → problem always? logical chain clear?) to verify you have found the real root cause, not a symptom. Next: Statistical Process Control — using data to detect problems before they become defects.
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