The FAA Engine and Propeller Directorate has issued a new draft Advisory Circular that could have a significant effect on companies seeking PMAs, STCs, or approval of repair or alteration of turbine engine and APU parts. Draft AC 33-Geometry: Geometry and Dimensional Considerations for Comparative Test and Analysis for Turbine Engine and Auxiliary Power Unit (APU) Replacement, Redesign, and Repaired Parts is directed at applicants who use comparative techniques to reproduce dimensional characteristics of parts during the reverse engineering process.
The AC is intended to provide guidance to help PMA (and other) applicants assess their reverse engineering methods to identify possible causes of dimensional inaccuracies, as well as helping applicants develop adequate criteria for determining dimensional similarity between the PMA design and the sample. To that end, the draft AC identifies several dimensional and geometric factors that the FAA expects should be assessed in seeking to ensure the integrity of dimensional characteristics for the purposes of showing similarity. These factors range from geometric relationships to precision and accuracy to influence on critical parts.
The background section of the draft AC states that because reverse engineering methods vary in their measurement techniques, interpretation and combination of data, and in dimensioning systems, they do not typically produce a design with the exact same dimensional properties as the type design part. It further explains that because dimensional differences may exist, functional assessments–which may include both test and supplemental analytical data–will be necessary to safeguard type design functional properties, as well as capabilities of interfacing and higher level assemblies. Such explanations help to illuminate the FAA’s expectations of an applicant in making showings of dimensional similarity. MARPA would like feedback from our members describing to what extent such test data is currently available and practical within the industry.
Although this AC, like all Advisory Circulars, is neither mandatory nor regulatory, such guidance documents do sometimes become ingrained and relied upon in considering applications. It is therefore important to review the draft and offer comments to the FAA to ensure the guidance reflects the realities of our industry. MARPA will be closely reviewing the guidance and offering comments on behalf of the PMA community. If you identify any particular issues within the guidance, please bring them to our attention so we can incorporate them into our comments. Email your concerns to firstname.lastname@example.org. We also recommend that members who identify concerns file their own comments on the guidance. Comments are due July 3, 2014 and should be submitted to Mark Bouyer of the Engine and Propeller Directorate, at email@example.com