Ask a quality engineer what keeps a process stable and they'll likely say "SPC" or "standard work." But the document that ties everything together — the one that tells you what to control, how to measure it, and what to do when it drifts — is the Control Plan.
In AIAG's APQP framework, the Control Plan is one of the five core documents. In Japanese manufacturing practice, its equivalent — the kanri keikaku (管理計画) — is treated not as a submission requirement but as the source of truth for day-to-day quality discipline. Understanding what a real Control Plan contains, and how to build one that works, is a fundamental skill for any quality professional.
What a Control Plan Actually Is
A Control Plan is a structured document that describes the methods used to control a product or process during production. It captures every characteristic that matters — dimensions, material properties, functional parameters — and specifies how each one is monitored, measured, and responded to.
製造工程における品質特性の管理方法を定めた文書。測定方法・頻度・管理限界・対応処置を工程ごとに明記し、品質の安定を担保する。
Unlike a work instruction, which tells an operator how to perform a task, a Control Plan tells the quality system what to watch and when to act. The two documents complement each other: the work instruction defines the process; the Control Plan defines the surveillance of that process.
The Three Phases: Prototype, Pre-Launch, Production
In APQP practice, Control Plans evolve across three phases. The Prototype Control Plan is created early in development to guide initial builds — it tends to be broad, covering all characteristics with manual measurement. The Pre-Launch Control Plan is used during trial runs and pilot production, with tighter measurement frequency and initial SPC data being gathered. The Production Control Plan is the final, approved version used once regular production begins.
Many organizations skip straight to the production version and treat the earlier phases as paperwork exercises. Japanese manufacturers who build quality in from the start use each phase's Control Plan as a genuine tool for learning — adjusting characteristics, measurement methods, and reaction plans based on what they discover in earlier builds.
What Every Row Must Include
A Control Plan is organized by process step. Each row represents one characteristic at one operation. The essential columns are:
Part / Process Number — links the row to the process flow diagram and FMEA. Process Name / Operation Description — identifies what is being performed. Characteristic — the specific feature being controlled (dimension, parameter, visual attribute). Classification — whether the characteristic is Special (SC), Safety (S), or Regular. Specification / Tolerance — the acceptance criteria. Measurement System — what gauge or method is used. Sample Size and Frequency — how many pieces and how often. Control Method — SPC chart, attribute chart, periodic inspection, or 100% check. Reaction Plan — what the operator or inspector does when the result is out of control.
"A Control Plan without a Reaction Plan is just a measurement schedule. The reaction plan is what transforms data into quality control." — Common axiom on Japanese shop floors
Special Characteristics: The Heart of the Control Plan
Not all characteristics are equal. Special Characteristics — those whose deviation from specification would cause safety risk, non-compliance, or significant customer dissatisfaction — require heightened attention. In the AIAG system, these are marked with symbols: a diamond (◆) for Safety characteristics and a circle (●) for Significant characteristics. Customer-specific systems may use different symbols, but the intent is the same.
不適合が安全・法規・機能に重大な影響を与える品質特性。管理計画書において優先的に管理され、SPC適用・全数検査・承認済み変更管理などの特別な管理策が求められる。
For Special Characteristics, the Control Plan typically requires 100% measurement or SPC monitoring — not periodic sampling. Process changes affecting these characteristics require customer notification or approval. On the factory floor, Special Characteristics are often highlighted in red on work instructions and tagged with visual aids so no operator can miss them.
Linking the Control Plan to FMEA and the Process Flow
The Control Plan does not exist in isolation. It is the third member of a triad: Process Flow Diagram → FMEA → Control Plan. Each identified failure mode in the FMEA should trace to a control measure in the Control Plan. If the FMEA says "risk of incorrect torque application," the Control Plan should specify how torque is verified, how often, and what happens when it is outside specification.
When this linkage breaks down — when the FMEA is updated but the Control Plan is not — gaps open up between identified risks and actual production controls. This is one of the most common audit findings in automotive quality systems. Building a numbering convention that connects all three documents (process step 10 in the flow = row 10 in the FMEA = row 10 in the Control Plan) prevents this drift.
Keeping the Control Plan Alive
The most common failure of Control Plans is becoming a static document. They are created for PPAP submission, approved, and then never updated as the process evolves. Two years later, the Control Plan specifies a gauge that has been replaced, a frequency that was doubled after a customer complaint, and a reaction plan that no operator has ever read.
In high-performing plants, the Control Plan is reviewed whenever the process changes — new tooling, new material lot, new operator qualification requirement. It is also reviewed when a nonconformance escapes to the customer, as the escape itself indicates a gap in the control strategy. Treat the Control Plan as a living document that reflects the current state of your process, not the state it was in when you launched.