On Sept. 12, ARSA joined five other aviation trade associations – led by the Modification and Replacement Parts Association (MARPA) – in commenting on FAA Draft Advisory Circular (AC) 33-13, “Sample Size Considerations for Comparative Test and Analysis for Turbine Aircraft Engine and APU, PMA, and Third-Party Repair Parts.”

As reported when ARSA learned of the draft AC on Aug. 6, the AC attempts to provide “practical approaches to define sample sizes” necessary for statistical analysis demonstrating equivalency by a PMA applicant to an approved article. The guidance’s implication that such analysis is necessary for all applications would impose an unnecessary burden on the industry. Statistical comparison is inappropriate where features can be directly measured, such as direct dimensional analysis – it may be a tool available to support equivalency during an application but need not be imposed as one.

The comments directed the FAA to focus on specific safety concerns rather than impose blanket expectations for statistical measurement. Through an extensive issue background, the group’s submission demonstrated the “Min-Max Measured” paradigm as the long accepted methodology for both industry use and FAA oversight. The paradigm is defined in FAA Order 8110.42B:

“Variations in the sample measurements and accepted engineering practices determine the tolerances in part dimensions. The resulting tolerances for the PMA part should not exceed the minimum and maximum dimensions measured on the sampled approved parts. Exceeding these limits requires substantiation.”

Establishing this foundation, the comments then list a series of specifically objectionable elements in the draft AC before requesting the FAA’s withdrawal of the guidance entirely.

“We believe that issuing this AC is not necessary and would only add confusion by adding guidance that disagrees with existing guidance in AC 21.303-4 and AC 33-8,” the comments said. “We believe that the FAA should work with specific applicants on closing compliance / understanding gaps. The FAA could also develop specific guidance for showing compliance to particular regulations, like AC 33.83-1 and AC 33.87-2.”

In addition to ARSA and MARPA, the comments were supported by the Aviation Suppliers Association, the Commemorative Air Force, the International Air Transport Association, and the National Air Carrier Association.

To read the complete draft AC, click here.

To read the complete comments, click here.

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