AC 33-12 Figure 15. Effects of Residual Stress from Destructive Evaluation.
Spring Back from Residual Stress After Sectioning Section A Section B Section C In this example, as the distance from the turbine blade root increases (Sections C to B to A), the magnitude of the spring back from residual stress increases Figure 16. Features Independent of Residual Stress during Destructive Evaluation.
Section A Examples of Dimensions Unaffected by Residual Stress for Sections A, B and C Section B Section C 25
AC 33-12
5.19 Dimensional Validation.
5.19.1 I n accordance with 14 CFR 21.303(a)(4), test reports and computations for the replacement part must show that the engine continues to comply with its airworthiness requirements, up to the certificated performance requirements, when the replacement part is built anywhere within the limits of its approved design. Similarity in dimensional variation between the type design and replacement parts depends on the extent to which they are dimensioned similarly. Unless there is a licensing agreement, there is a potential for the replacement part drawing to have differences in dimensioning criteria or overlooking design attributes. These differences can increase the amount of variation in the replacement part.
5.19.2 The FAA procedures f or approving replacement parts allow applicants to develop data from part-to-part comparative tests using the sample sizes prescribed in available advisory material. When sample sizes are prescribed by the guidance, they are based on the condition that the sample population meets certain prerequisites and that the test methods and test criteria used for the comparisons are consistent with the guidance. For example, a prerequisite for using a prescribed sample size could include a condition that the replacement part is designed to be within measurements taken from the type design part.
5.19.3 The sample of replacement parts used for compliance testing will not likely represent the full range of dimensional variation in the reverse-engineered design. Moreover, when applicants select test samples from a single manufacturing lot, the physical properties of the test samples could be untypically uniform. Therefore, functional test results could be misleading if the dimensional variations in the replacement parts represent a small range, or a bias, within the potential tolerance band, and engine compliance is sensitive to the effects of the dimensional variation in the part. The conformity data should show if the sample of replacement parts have any dimensional characteristics that could limit the applicability of the compliance test results to the specific samples being tested and not to production parts that exhibit typical process variation.
5.19.4 If applicants develop a compliance plan assuming the assessments would verify dimensional similarity, but the results demonstrate differences in the replacement part, they should revise the compliance plan. The revised compliance plan should include any necessary assessments to account for the effects of the differences on engine compliance. Alternatively, applicants could modify the replacement part design to align its functional performance with the type design part. This additional activity may require more specimens and tests than were proposed in the original certification plan, and tests involving higher-level assemblies using a range of engine operational boundary conditions or a new project application and methods of compliance. Though not covered in this AC, differences resulting from the replacement part manufacturing processes and materials should also be resolved similarly.
5.20 Maintenance Manuals.
5.20.1 TC holders develop maintenance manuals fo r their engines. Th e serviceability and return-to-service instructions for individual parts are based on the engine configuration, production processes, and quality systems that flow into their manuals. For example, the tests, inspections, and limits in the manuals account for aspects of the type design that might have changed since it was new and not for features that continue to conform to the type design drawing. Therefore, TC holder maintenance manuals normally do not instruct operators to check all the dimensional characteristics and qualities that affect the airworthiness of the part. They also do not typically implement tests and inspections to account for differences that an applicant might introduce to a replacement part by reverse engineering.
5.20.2 Applicants that base their replacement part designs solely on the information available in TC holder maintenance manuals may have introduced dimensional differences into new parts and repairs and subsequently introduced differences that could affect engine compliance. Therefore, the TC holder maintenance procedures do not provide sufficient information to substantiate reverse-engineered designs or new repairs. Thus, maintenance manuals do not apply to the replacement part until after the replacement part is found to be equivalent to the type design part through suitable comparative assessments or is shown to be compliant with applicable airworthiness standards using general analysis (direct compliance).
5.20.3 The test procedures in maintenance manua ls prescribed by appendix A to Part 33 are meant to verify the characteristics of the part that are affected by the repair. The tests do not substantiate reverse-engineered parts or repairs that substantially remanufacture type design parts. For example, some airflow tests are intended to check for damage or blockage that might have occurred during the repair. Though the test results may show that at room temperature, ambient air will flow similarly between two air-cooled turbine part designs; it is not likely the tests specified in the manual will provide any useful insight into how the relative cooling in each design compares in an engine environment. Also, these tests will not reveal which features on the part have a substantial influence on the engine system.
6. SUGGESTIONS FOR IMPROVING THIS AC.
If you have suggestions for improving this AC, you may use the Advisory Circular Feedback Form at the end of this AC.
DANIEL J.
Digitally signed by DANIEL J. ELGAS ELGAS Date: 2025.09.08 11:49:57 -04'00' Daniel J. Elgas Aviation Safety Director, Policy & Standards Division, Aircraft Certification Service 27
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