We previously wrote in this space about a new Draft Policy Statement issued by the FAA concerning the vibration surveys and engine surveys required under section 33.83 of the Federal Aviation Regulations. The draft guidance attempts to more narrowly address the FAA’s concerns about full engine test for type certificate applicants.
MARPA plans to provide comments on this Draft Policy Statement to the FAA and has sent a draft of our comments to the MARPA Technical Committee for review. If any of our members wish to review our draft comments to provide their feedback we would love to hear from you. Please email Ryan Aggergaard at email@example.com if there are particular issues in the draft statement you believe should be addressed so that we can incorporate our members’ concerns.
MARPA also encourages our members to file their own comments on the Draft Policy Statement. Comments are due to the FAA by November 21, 2014. Comments should be emailed to Dorina Mihail at firstname.lastname@example.org. Comments can also be mailed to her at:
Federal Aviation Administration
Engine and Propeller Directorate
Standards Staff, ANW-111
12 New England Executive Park
Burlington, MA 01803
MARPA looks forward to your comments.
The FAA has released two new advisory circulars that may affect the PMA community. Both advisory circulars are issued by the Transport Aircraft Directorate and apply to Part 25 aircraft (and parts thereof).
We would be interested in hearing from any MARPA member who is affected by one of these new advisory circulars.
The FAA has issued a new advisory circular for statistical analysis. This AC only applies to engine and APU parts.
The new advisory circular uses statistical analysis to arrive at correct sample sizes. This sample size formula is introduced for persons who are trying to correlate two populations of parts. MARPA had pointed out to the FAA that typically a PMA applicant does not correlate two different populations of parts – instead they derive the reasonable tolerances on one population of parts, and then design and produce within those tolerances. The FAA would like PMA applicants to arrive at their appropriate sample size, test the parts being reverse-engineered, and then produce an equivalent number of pre-PMA parts to test for the same properties (and then correlate the two populations). This is contrary to current FAA regulatory guidance, which requires the design to be approved and the requires the production quality system to ensure that parts are all produced within the approved design parameters. In essence, ACO engineers will now take control of the quality assurance system through the design process.
One of the problems with the AC is that it relied on statistical analysis for clinical trial sample size as the basis for assessing statistical analysis of reverse-engineering sample size. Clinical trials for pharmaceuticals typically rely on populations of hundreds or even thousands of people. Trying to test hundreds or thousands of parts in order to reverse-engineer them is simply not realistic. Furthermore, the degree of part-to-part difference under modern quality assurance systems does not support such large sample sizes.
For those cases where the equations in the advisory circular give a lower number for the appropriate sample size, the FAA has also established minimum sample sizes. The AC sets some minimum limits for the number of parts that must be tested in order to derive certain values (remember that you need that number of PC parts and also that number of reverse-engineered parts to meet the AC’s requirements):
Minimum Number of Parts to be Sampled
- For basic material properties that are more dependent on alloy constituency than on part manufacture process – 10 approved parts from three separate lots with at least three parts per lot
- For properties affected by how the material is processed during part manufacture such as high-cycle fatigue, low-cycle fatigue, creep, tensile strength, crack growth, etc. – 30 approved parts
- For fatigue testing – at least 25 tested parts that are run until they crack
- For parts exhibiting complex geometry or complex manufacturing variables, additional specimens may be required
- For parts considered to have a high degree of criticality, greater sample sizes may be required
No statistical basis is offered for these minimum sample sizes.
The new advisory circular provides guidance for statistical analysis of sample-size despite the fact that no regulation actually requires such a broad-based sample. The advisory circular appears to potentially change the regulations by increasing the burden on applicants. To the extent that this is true, it is inappropriate.
Our concern is that despite warnings that this is non-mandatory guidance, this guidance may be used as if it were a regulation, with offices refusing to accept PMA applications that are otherwise valid, but that failed to use this AC as a basis for identifying sample size.
If you find that this AC effectively changes the application obligations imposed on you as a PMA applicant, STC applicant, or other FAA-approval applicant, then please contact MARPA so we can raise this concern with the appropriate personnel at the FAA.
AC 33-10 is known by its full title: Statistical Analysis Considerations for Comparative Test and Analysis Based Compliance Findings for Turbine Engine and Auxiliary Power Unit Replacement, Redesign and Repaired Parts.
Some of you may be wondering where to find the FAA’s policy memo on design approval holder restrictions on ICA availability. Older MARPA links to the original position of that guidance are no longer valid because it has been moved. But it is still available if you know where to look!
Many MARPA members look for this policy memo because it clarifies that anti-competitive language in ICAs (restricting use of PMAs or third party repairs) is unacceptable to the FAA:
While not exhaustive, the FAA finds the following practices of using restrictive language in the ICA or through restrictive access or use agreements unacceptable under the provisions of 14 CFR §21.50(b) and related ICA airworthiness requirements:
1) Requiring the owner/operator to only install DAH-produced or authorized replacement parts, articles, appliances, or materials.
2) Requiring that alterations or repairs must be provided or otherwise authorized by the DAH.
3) Requiring the use of only maintenance providers or other persons authorized by the DAH to implement the ICA.
4) Establishing, or attempting to establish, any restriction on the owner/operator to disclose or provide the ICA to persons authorized by the FAA to implement the ICA.
MARPA had a very good meeting today with Mark Bouyer and Ann Azevedo of the FAA’s Engine and Propeller Directorate (EPD).
The focus of the meeting was status on EPD policy that may affect PMA manufacturers.
Azevedo explained that she has responded to the comments on the Statistics Advisory Circular (AC), and hopes to have the final draft of that guidance available to the public by September. The Statistics AC is meant to address FAA concerns that have been recognized in practice, such as misusing statistical methods to show equivalence, and underestimating the appropriate sample sizes.
Bouyer expects the Materials AC to go out for public comment this month. The Materials AC will identify the essential data that is necessary when a PMA applicant is trying to replace the material used in the type design. MARPA members should watch for this one, and be prepared to offer their comments.
The FAA had published the Geometry AC for comment. This AC is meant to enhance awareness of how reverse engineering can introduce dimensional differences in replacement parts. MARPA Board members have expressed that the draft of this proposed guidance appeared to be very helpful to the industy.
The Burner Rig AC, which was also previously out for comment, is expected to be issued by September. It is expected to provide a method for establishing functional equivalence for certain degradation modes in parts such as oxidation, hot corrosion, erosion, etc. The AC is expected to identify existing technology as a means of compliance.
Finally, the FAA is internally reviewing AC 33.8 with a plan to update and clarify the AC. The updates are intended to make the AC easier to use.
The FAA has been very active and diligent in preparing guidance. In the immediate future (before the Conference), MARPA members should expect to see the release version of the Statistics AC and the Burner Rig AC, and they should expect to see the Materials AC go out for comment.
MARPA recently filed comments on the FAA Engine and Propeller Directorate’s draft Advisory Circular 33-Geometry, discussing geometry and dimensional considerations for comparative test and analysis for turbine engine and APU replacement parts. In a previous blog post we observed that AC identified a number dimensional and geometric factors that the FAA expects to be assessed in ensuring the integrity of dimensional characteristics for the purposes of showing similarity.
We requested feedback from our members describing to what extent the FAA’s expectations were reasonable and practicable, and identifying any issues with the proposed guidance on which MARPA should comment. We received several very helpful responses from our members that helped us shape our comments to the proposed AC. Among the issues members identified were:
Feedback from our members is both helpful and valuable to our comments, as it helps us to identify issues that directly affect members’ businesses, and helps us to better focus our resources on those matters that are important to the PMA community. The result is more detailed and on-point responses to the FAA to better help shape the guidance material that will ultimately be issued.
We greatly appreciate the feedback we received from our members on this Advisory Circular, and we hope that our members will continue to answers our requests for responses as additional guidance and rulemaking documents are issued. Together we can work with the FAA to develop the best possible guidance for our industry.
The FAA is currently seeking comments on its Draft Advisory Circular Engine Overtorque Test, Calibration Test, Endurance Test, and Teardown Inspection for Turbine Engine Certification (§§ 33.84, 33.85, 33.87, 33.93).
As the title suggests, the AC offers guidance on compliance with the engine overtorque, calibration, and endurance tests, and teardown inspection called out in Part 33 of the Federal Aviation Regulations. Although the guidance is directed at engine manufacturers, foreign regulatory authorities, applicants for engine type design approval, and FAA designees, it also notes that parts manufacturer approvals “may require running certain endurance testing for compliance with § 33.87″ and refers to AC 33.87-2 for guidance on showing compliance by comparative test methods.
MARPA would like to know to what extent members anticipate this AC might effect them, and whether we should submit comments. If you plan on submitting comments, or have already done so, we would would like your feedback so that we can incorporate member concerns into our comments.
Comments on the Draft AC are due next week, so if you have feedback for us please submit them to email@example.com soon!
The FAA is proposing to issue a revision “B” to its Advisory Circular 33.74/92-1B (Turbine Engine Continued Rotation And Rotor Locking). This AC provides guidance on how to demonstrate compliance with the continued rotation and rotor locking requirements of 14 C.F.R. §§ 33.74 and 33.92.
Marc Bouthillier of the Engine and Propeller Directorate has said that this revision features some updated references, and clarifications of guidance. He has said that the FAA’s compliance guidance IS NOT intended to be changed in this revision.
We’d appreciate your insights about whether you think there are any substantive changes. We’d also appreciate hearing from PMA manufacturers that produce rotating parts, whether you find that the requirements of Advisory Circular 33.74/92-1A (the current version) have been applied to PMA applicants.
In addition, the AC has always included in this AC some requirements that appear to go beyond the strict requirements of the regulations. Many of these are requirements that seem to have been endorsed as sound practices by engine OEMs. We would be interested in hearing whether the PMA community finds that any of the extra-regulatory requirements of the AC are imposing a burden on PMA manufacturers.
Comments on this proposal are due to the FAA by September 12, 2014 so we would appreciate it if you could provide your comments to us at MARPA no later than the end of August. You can also file comments directly with the FAA by emailing them to firstname.lastname@example.org
Today, the FAA announced the issue of AC 21.3o3-4. The title of this AC will be “Application for Parts Manufacturer Approval Via Tests and Computations or Identicality.”
As of this time, the AC is not yet available to the public. When it is available to the public, it is expected to be posted online at this location:
Note: the issue date published on the FAA’s website is March 21, 2014; however the AC is not yet available through the FAA’s online advisory circulars, nor through the Regulatory and Guidance Library (RGL). The note on the FAA’s website states “This document‘s content is not currently available.”
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 email@example.com. 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 firstname.lastname@example.org