What is root cause analysis?

Root cause analysis (RCA) is the systematic process of identifying the fundamental reason a problem exists — not just the symptom or proximate cause, but the deepest cause that, if eliminated, would prevent the problem from recurring. A symptom is 'Station 7 rework rate is 18%.' A proximate cause is 'the sealant bead is too thin.' A root cause is 'no standard work exists for sealant application, so each operator uses a different technique.'

How do you know when you have found the root cause?

Three tests: (1) If you remove this cause, does the problem stop? (2) If this cause is present, does the problem always occur? (3) Can you explain the logical chain from this cause to the observed symptom? If you cannot pass all three tests, you are at a symptom, not a root cause. Keep digging.

Which RCA method should I use?

5-Why for simple, single-cause problems (quick, good for daily use). Fishbone for complex problems with multiple potential cause categories (structured brainstorming). Is/Is-Not for problems where you need to narrow the field (the defect happens on Shift 1 but not Shift 2 — why?). Fault Tree for safety-critical or high-consequence failures requiring exhaustive analysis.

Root Cause Analysis Toolkit: 5-Why, Fishbone, Fault Tree

New to this topic?

We recommend reading these guides first to get the most out of this one:

A3 Problem Solving The 8 Wastes Deep Dive

RCA Methods

Why’s (Minimum)

Fishbone Categories

Root Cause Tests

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.

📊 5-Why Example CNC Tool Breakage

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.

📊 Is/Is-Not Example Fastener Torque Failures

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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