The FAA has proposed a new mechanism for distinguishing major changes in type design from minor changes to type design. The proposed advisory circular is available online. Information on submitting comments is also available online.
The fundamental difference between major changes in type design from minor changes to type design is described in 14 C.F.R. 21.93(a), which states:
(a) In addition to changes in type design specified in paragraph (b) of this section, changes in type design are classified as minor and major. A “minor change” is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are “major changes” (except as provided in paragraph (b) of this section).
Notice that the existing language of 21.93 does NOT impose a burden to identify whether a proposed change is a major or minor change to type design. This is important to notice because the proposed AC would impose such a burden (and claims tha the burden is already imposed by 21.93, despite the fact that there is no such burden imposed by 21.93. While affirmative classification of minor changes can aid in compliance, it is not a requirement of the regulations.
There has been considerable debate over the years about the difference between major and minor, and about the scope of the word “appreciable.” In fact, the final report of the ARAC major-minor committee admitted that the current regulatory language does not adequately distinguish major from minor in a manner that provides the public with objective standards.
The FAA proposal does not define the term “appreciable” nor attack the major-minor issue from any traditional angle – instead it provides a mechanism by which would-be STC applicants may confirm that their proposal represents a major change to type design by relying on system effects analysis.
The first problem with the proposal is that it appears to impose a new burden to perform an analysis and affirmatively determine that a minor change to type design is, in fact, minor. This is a potential problem for the PMA community because seemingly minor PMAs will now need to be analyzed using system effects analysis, despite the fact that the PMA part may be obviously minor in its affect on “the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of” a product. The current laguage of the proposed AD would seem to require a systems analysis (or other analysis previously found “acceptable” to the FAA) that confirms the fact that a small, non-complex and non-safety-sensitive PMA part may clearly have no appreciable affect on weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of a product. This appears to be wasteful of resources in such cases, where a change is obviously minor.
An example of an obviously minor change is the replacement of an original part in an aircraft with a replacement part that was manufactured under a licensing agreement PMA. Neither the PMA applicant nor the installers of such a PMA part should be required to perform a formal analysis in cases where it is obvious that the change is only minor and has no appreciable effect on safety or airworthiness of the product.
We will continue the analysis of this proposal with a review of the substantive merits of the proposed system.
There are several ways to comments on the proposed Advisory Circular:
Email comments to:
Deliver comments by mail or hand to:
Sarbhpreet S Sawhney
Aircarft Certification Service
Aircraft Engineering Division
Certification Procedures Branch – AIR-110
950 L’Enfant Plaza, SW 5th Floor
Washington, DC 20024
Fax comments to: