PPAP capability requirements explained
What the AIAG PPAP manual asks for in the initial process study: Ppk 1.67 or Cpk 1.33, stability, special characteristics and what to do when a characteristic falls short.
Updated 2026-10-24
What the initial process study is
Element 11 of the AIAG Production Part Approval Process (PPAP) is the initial process study. It shows, before series production starts, that the process can produce the special characteristics within specification. It is based on a significant production run, typically at least 100 readings in 25 or more subgroups.
Acceptance criteria (AIAG PPAP 4th edition)
| Index | Interpretation |
|---|---|
| > 1.67 | Meets the acceptance criteria |
| 1.33 to 1.67 | May be acceptable, contact the customer and review the study |
| < 1.33 | Does not meet the acceptance criteria, contact the customer |
For stable processes, Ppk is used in the initial study; Cpk can be reported once stability is demonstrated over time. Customer-specific requirements take precedence: many customers ask for Cpk ≥ 1.33 and Ppk ≥ 1.67, or apply the same rules to all characteristics on the drawing.
Before you calculate
- Prove the gauge first with an MSA. PPAP needs it anyway (element 8).
- Check stability on a control chart. Unstable processes need special handling: the index alone is not accepted.
- Check normality. For naturally skewed characteristics use a distribution fit and the percentile method.
Doing it efficiently
Measure all characteristics of the run in one sheet and paste it into the capability summary: you get Cp, Cpk, Pp and Ppk for every characteristic in one table and see at once which ones need a closer look. Open those in the process capability study for control charts, distribution fit and a PDF report for the PPAP binder.
When a characteristic falls short
- Agree a corrective action plan with the customer, often with 100 % inspection as interim containment.
- Record the special characteristic and its control in the control plan.
- Repeat the study after the improvement.