by the FAA. Monitor and control the test so that the stress level at the critical location is held constant. One method to do this is to calibrate tip deflection against critical location strain measurements, then monitor and control tip deflection during the test to control stress level. If the calibration curve was determined at room temperature, correct it for the actual test temperature. The number of stress levels tested should be sufficient to support the technique used to compare the fatigue capability of the proposed PMA and type design blades.
e. When conducting the fatigue test, the target test duration for each test blade should be between 105 and 107 cycles. Of the 30 blades, test a minimum of 25 until a crack develops. The remaining blades may be run-outs (uncracked at 107 cycles). The test blade failures should be distributed over the cycle range. If an unexpected crack location is encountered and the crack site stresses cannot be determined because the alternate site lacked a strain gage, applicants may use the analytical model described in paragraph 9.a. to determine the stress level. To utilize the analytical model, the actual blade that cracked at an unexpected location should be measured and the dimensional measurements used to analyze that particular blade. An alternate approach would be to conduct further fatigue testing with strain gages placed at the alternate crack location(s). Applicants must thoroughly analyze unexpected failure locations and discuss them with the FAA.
f. Perform post-test metallurgical examinations to ensure the fatigue crack initiation location and crack formation mechanism on the proposed PMA blade and the type design blade are the same. In addition, verify that the fatigue crack was not initiated by a material defect such as a nick, scratch, pore, void or unacceptable microstructure. Material defects detected in a PMA blade would disqualify the test data and require further investigation to determine root cause. If defects are detected, applicants must review this result with the FAA.
g. Based on the acquired fatigue test data and using standard regression analysis (see reference in paragraph 3.d.), statistical techniques, and linear extrapolation, compute the minimum (-3 standard deviations) fatigue strength at 108 cycles for the PMA and type design blades. The test stress level at the crack site should be used to represent the high cycle fatigue capability of each test blade.
h. The minimum fatigue strength at 108 cycles of the proposed PMA blade must be at least equal to that of the type design blade. In addition, applicants should assess the failure locations and failure mechanisms and show them to be the same. The crack location should be repeatable and must be consistent between the proposed PMA and type design blades. If more than one crack site is detected in the original blade fatigue tests, the PMA blades may not crack at the secondary site at a higher rate than the original blades unless it can be explained.
10. Maintaining Compliance to § 33.83. PMA applicants must develop a design specification to support a quality control program to ensure that the production PMA blades’ natural frequencies and high-cycle fatigue capability continue to be consistent with the PMA blades used to gain FAA approval. The program may involve sampling plans and statistical process control techniques. It may also involve the measurement of a combination of or a subset of dimensions, material metallurgical properties, natural frequencies, HCF properties, and other part characteristics to ensure the production process is stable and repeatable.
Francis A. Favara Manager, Engine and Propeller Directorate Aircraft Certification Service 14