Automotive Cpk Requirements

How is process capability actually enforced across APQP, PPAP, Safe Launch and serial production in an automotive supply chain?

Common automotive practice uses 1.33 for established production and 1.67 for some initial or launch studies, but the controlling value is always the applicable customer-specific requirement, drawing, Control Plan, or internal standard. Do not treat these values as universal pass/fail rules.

Verify the drawing, customer-specific requirements, sourcing documents, Control Plan, and approved study method. Common reference values—not universal rules—include:

• 1.33 for some established-production studies.
• 1.67 for some initial, launch, or machine-capability studies.
• Higher targets or alternate controls for designated characteristics.
• Approved non-normal or attribute methods when normal capability is unsuitable.

Automotive capability is not a single number. It is a contractual chain running through the Customer-Specific Requirements (CSR), the drawing, the Control Plan, the PFMEA, and the PPAP submission. Common practice references 1.33 for established production and 1.67 for initial or launch studies on Significant / Critical Characteristics, but the controlling value is always the CSR and the classification on the drawing. No IATF, AIAG or VDA document mandates a universal Cpk figure.

• VDA 6.3 and IATF 16949 internal audits routinely check Cpk against the Control Plan.
• OEM scorecards often use Cpk/Ppk differently; the governing document decides.

1. Extract capability requirement from CSR and drawing — not from a generic 1.33 assumption.
2. Align PFMEA Special Characteristics with the Control Plan reaction plan.
3. For PPAP, use consecutive-run data (typically ≥ 300 parts, one shift, one setup) and report Ppk.
4. For Run@Rate, verify the process holds capability at contractual cycle time, not lab conditions.
5. During Safe Launch, apply the customer multiplier (containment inspection) until exit gates are met.
6. In serial production, monitor Cpk per subgroup plan; escalate per the Control Plan reaction column.
7. Feed customer scorecard PPM back into PFMEA occurrence and Control Plan detection.

• Universal 1.33/1.67 assumption applied without reading the CSR.
• Tool wear compensation used during the PPAP run but not documented.
• Special Characteristic on the drawing not flagged in the Control Plan.
• Reaction plan references SPC action but no responsible role or containment trigger.
• Safe Launch exit criteria closed on calendar days, not on demonstrated Cpk.
• Ppk reported where the customer requires Cpk (or vice versa).

• Many PPAP capability studies pass only because they were run under lab-optimized conditions and never repeated at real cycle time.
• Ongoing Cpk usually decays after PPAP because tool-compensation rules were never formalized in the Control Plan.
• Customer scorecard PPM and internal Cpk almost never match — investigate before assuming the customer is wrong.
• A "Cpk 1.67 PPAP" that becomes "Cpk 1.10 serial" is almost always a reaction plan problem, not a capability problem.

• Do not claim capability compliance from lab or engineering-run data — the CSR requires production conditions.
• Do not pool multiple cavities, spindles, or fixtures into a single Ppk for PPAP.
• Do not use Cpk from an unstable process — stability first, capability second.

• Initial (Ppk) vs Ongoing (Cpk): PPAP uses Ppk on a consecutive run; serial monitoring uses Cpk on rational subgroups.
• Capability vs PPM: Customer scorecards report PPM; internal Cpk must be reconciled with it, not defended against it.
• Capability vs Control Plan: Every capability number is only as good as the reaction plan that keeps it there.

Use the free Process Capability Calculator to compute Cp, Cpk, Pp, Ppk, sigma level, and predicted PPM from your measurement data — no sign-up required.

• Never re-run a failed PPAP study without recording the original data — it is the audit trail.
• Never negotiate down a Significant Characteristic classification without customer engineering sign-off.
• Never close an 8D from a customer complaint on a passing sample alone — require a fresh capability study.

• PPAP study planning — PPAP capability evidence begins with study planning.

• ✓Customer-Specific Requirement extracted verbatim, not assumed
• ✓Drawing classification (Critical / Significant / Standard) matches Control Plan
• ✓PPAP study uses production tooling, production cycle time, production operator
• ✓Ppk vs Cpk convention matches customer request
• ✓Reaction plan has owner, trigger, containment and escalation path
• ✓Safe Launch exit gates defined on capability evidence, not calendar
• ✓Scorecard PPM reconciled with internal Cpk each period

• Process condition: statistical stability is required.
• Distribution assumption: use a distribution model justified for the data.
• Confirm process stability and measurement-system adequacy before interpreting a capability index.
• Use a justified distribution model or non-normal method when the normal model is unsuitable.
• Numerical targets shown on industry pages are common examples, not universal requirements. The contract, drawing, customer-specific requirement, Control Plan, and validation protocol take precedence.