FAA Issues Draft “Sample Size” Advisory Circular that Would Apply to PMA, STC, and RS-DER applications

The FAA has issued for comment a draft Advisory Circular focused on sample size for statistical analysis in PMA applications.

As many of you remember, there was a draft on statistical analysis issued several years ago. MARPA objected to that draft because it implied that every PMA application needed to be based on statistical analysis, which is typically incorrect where data can be directly measured.

This draft seems to once again imply that every “comparative test and analysis” application needs to use statistical analysis and thus every such application needs to meet a minimum sample size. The draft appears to apply to PMA, STC, and repair or alteration data approvals for turbine engine parts and APU parts.

Statistical analysis can be very useful for identifying features that cannot be directly measured (which is why it is a very frequent feature in FDA new drug applications, where the direct effect on the body typically has to be extrapolated from measurement of symptoms). Statistical analysis may be inappropriate where features can be directly measured, such as direct dimensional analysis (but statistical analysis may appropriate in some cases, like supporting an effort to identify dimensions that cannot be measured, or to prove compliance for proposed dimensions that vary from measured dimensions).

The AC includes a sample calculation in which the PMA application would be required to obtain 44 OEM parts, and compare them against a population of PMA replacement parts that was at least 44 parts deep. The formulas in this draft AC appear to drive a requirement for larger sample sizes than are typical for use in the industry, and they also seem to suggest that these sample sizes could apply to all “comparative test and analysis” applications, even when the features being reproduced can be directly measured (as opposed to features that must be extrapolated through statistical analysis).

Please read this FAA draft and let MARPA know your thoughts. If members feel strongly (positively negatively, or merely with suggestions about how to improve the draft), then MARPA would love to know your thoughts so we can support your ideas in the trade association’s own comments. Comments are due to the FAA [9-AVS-TechWriters@faa.gov] by September 12, 2025, so we would appreciate receiving your comments by September 5, 2025.