In Advisory Circular 33-X the Federal Aviation Administration (FAA) describes comparative test methods to be used in support of Parts Manufacturer Approval (PMA) applications for turbine engine and auxiliary power unit (APU) parts. The tests include experimental modal analysis, natural frequency measurements, and high cycle fatigue testing at operating temperature. These test methods are intended to show engine compliance with §33.83 (Vibration Test) and §33.87 (Endurance Test) of Title 14 of the Code of Federal Regulations (14CFR part 33), and APU compliance with paragraphs 5.10 (Vibration) and 6.3 (Endurance Test) of Appendix 1 of TSO-C77b, Gas Turbine Auxiliary Power Units, is still valid with the PMA part installed.
The vibration tests (modal analysis and natural frequency measurements) are designed to show that the natural frequencies of the PMA blade fall within the range of the original equipment manufacturer (OEM) blade. This is accomplished by performing an experimental modal analysis (Modal Testing) on sets of a few PMA blades and a few OEM blades. Modal Assurance Criterion (MAC) is used to correlate mode shapes and identify corresponding natural frequencies. The natural frequencies of 50 PMA blades and 50 OEM blades are then measured and compared to determine whether the range of PMA blade frequencies falls within the range of the OEM blades.
The high cycle fatigue test is designed to show that the minimum fatigue strength at 108 cycles of the PMA blade must be at least equal to the type design. Prior to testing an analytical model of the blade should be developed and used to predict the failure locations. The model should accurately predict the maximum strain amplitude and location as well as the strain distribution for the mode selected for high cycle fatigue testing. The predicted strain and crack locations should be correlated with the experimental results. The model should also correlate with the measured natural frequencies and mode shapes.