Key Characteristic Control Plan Template
Key characteristics are the features customers choose because variation in them affects form, fit, function, performance or service life. A key characteristic control plan makes sure someone measures them in the right way, at the right frequency, and knows what to do when they drift.
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Key Characteristic Control Plan Template
Key Characteristic Control Plan
Company: Acme Precision Aerospace Inc. Part number: PN-48213 Drawing revision: C Control plan revision: A Date: 2026-10-20 Customer: Tier 1 aerostructures customer Reference: AS9100D clause 8.5.1, SAE AS9103 (variation management of key characteristics) and customer designation of key characteristics.
Key characteristic control
| KC # | Characteristic and tolerance | Operation | Measurement method | Sample plan | Cpk target |
|---|---|---|---|---|---|
| KC-1 | Bore diameter 25.000 +0.013 / -0.000 mm | OP30 finish bore | Air gage, master ring verified each shift | 100% of parts, SPC chart every 5th part | 1.67 |
| KC-2 | Position of 4 holes, 0.05 mm to datum A, B | OP40 drill and ream | CMM per program, fixture ID recorded | First 3 parts per lot then 1 in 5 | 1.33 |
Measurement system
| KC # | Gage ID | Resolution | Calibration due | GR&R result | Notes |
|---|---|---|---|---|---|
| KC-1 | |||||
| KC-2 |
Reaction plan
Stop production, quarantine parts since the last good check, notify quality, contain and sort, investigate root cause, and restart only with quality approval after a verified first piece.
If a sample is out of tolerance, stop and check the last good measurement. Quarantine all parts produced since. If the Cpk from the latest study falls below the target, treat the characteristic as not capable and move to 100% inspection until the process is corrected.
Capability review
Initial capability: collect at least 30 consecutive parts in stable conditions. Compute Cpk as the lesser of (USL minus mean) divided by three standard deviations and (mean minus LSL) divided by three standard deviations. Record the result with date, machine, and operator. Re-verify after any tool change, program change, fixture change or process change.
Approvals
A customer designates key characteristics and can set the required capability and sample plan, which override the defaults here. This template is a starting point and does not copy any customer or standards form. Sampling and Cpk targets are examples; use those in your contract or procedure.
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How to use the template
Start from the customer's list of designated key characteristics and the drawing. Enter the part information, then describe up to two key characteristics in the fields and add more rows in the exported document if you have more. For each one, record the operation that produces it, the measurement method and gage, the sample plan, and the capability target. The reaction plan is common to all but you can write one per characteristic. Export to Word to add the rest of your routing detail and put the document under revision control.
Standards and method behind it
SAE AS9103 describes variation management of key characteristics: identify them, plan how to control them, collect data, analyze capability and take action. AS9100D clause 8.5.1 requires controlled conditions for production including monitoring and measurement. The usual capability index is Cpk, the smaller of (USL minus mean) over three sigma and (mean minus LSL) over three sigma. A common target in aerospace is 1.33 for ordinary and 1.67 for critical characteristics, but the customer sets the requirement.
- Cpk = min((USL - mean)/(3 sigma), (mean - LSL)/(3 sigma))
- Use at least 30 consecutive parts for an initial study
- Measurement system should be capable before the process is judged
Sampling and reaction plans that hold up
Choose sampling by risk and capability. A characteristic with Cpk well above target may be sampled lightly, while a poorly capable one needs 100 percent inspection until improved. The reaction plan must say who stops the process, what happens to suspect parts and who can restart. Keep it short enough that an operator on night shift can follow it. Test it: ask an operator what they would do when a point goes out of control.
Common mistakes
Control plans often list every dimension instead of the key characteristics, which dilutes attention. Others set a Cpk target but never calculate it. Another gap is ignoring the measurement system, so apparent drift is really gage noise. Finally, plans are written once and not revised after a tool change, a customer revision or an escape. Treat the plan as a living record tied to the drawing revision.
Frequently Asked Questions
What is the difference between a key characteristic and a critical dimension?
A key characteristic is designated by the customer or design authority because variation affects fit, function or life, and it triggers variation management. A critical dimension is an informal term. Always use the customer designation when there is one.
What Cpk should I target?
Follow the customer requirement first. Where none exists, 1.33 is a common minimum for capability and 1.67 is used for more critical features. Choose the target in your procedure and apply it consistently.
Is this the same as a PPAP control plan?
No. PPAP is an automotive process with its own control plan format. This template follows the intent of aerospace key characteristic management and can be adapted to a customer's own form.
How many parts do I need to calculate Cpk?
At least 30 consecutive parts from a stable process is the usual guideline. Smaller samples give wide uncertainty. For low volume parts, use the customer's guidance or a Ppk study with documented limitations.
We can connect inspection results from your CMM and ERP to a live capability view so Cpk is calculated for you instead of in spreadsheets.
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