Downloaded from https://www.everyspec.com ADS-79D-HDBK should be the basis for development of the statistical distributions on fatigue strength. Flight load survey should be the basis for development of the statistical distributions on flight loads. A.6.3 Evaluation of reliability when loads are monitored and fatigue strengths are statistically modeled (individual component fatigue damage assessment based on loads monitoring) Loads monitor will be part of the FLM activities for understanding reliability of retired parts. The reliability analysis is based on statistical evaluation of fatigue strength when the component load spectrum is monitored.
The fatigue strength may be modeled as normal or log normal distributions. Bench fatigue test data should be the basis for development of the statistical distributions. The fatigue damage calculated using the baseline mean-3 sigma fatigue strength curve for a normal distributed strength would result in 0.99865 reliability when actual load spectrum is applied. Component is removed from aircraft when it reaches the minimum threshold of required reliability defined in this appendix.
A.6.4 Evaluation of reliability when usages are monitored and design damages applied (alternate individual component fatigue damage assessment based on regime recognition and design damage.) For legacy aircraft baseline fatigue substantiation may not have sufficient data in the bench fatigue tests or load survey tests that allow development of statistical distributions of critical parameters. If a detailed probabilistic analysis is not available for determination of component reliability, maximum accumulated damage should be tracked to no more than 0.5. Baseline retirement times are based on composite worst case design spectrum which is assumed to add one nine of reliability.
The adjustment of the accumulated damage is to ensure baseline reliability is maintained when component damages are accumulated using the actual flight maneuvers 7 . . Damage fractions greater than 0.5 can be used for retirement criteria if probabilistic based analyses demonstrate that baseline fleet risk levels are maintained. A.6.5 Evaluation of reliability for usage spectrum update For the case of updating usage spectrums for legacy aircraft, statistical analysis of the usage data is used to determine a statistical approximation of an updated composite worst case spectrum as discussed in section A.5.1.
Consider use of a ―mean plus two sigma‖ spectrum 8 to avoid the need for additional fatigue strength working curve reductions or probability analysis. As an alternative a ―mean plus sigma‖ spectrum may be applied with appropriate probability analysis. Use of a mean spectrum is not appropriate. 7 Benton ,Robert E., Jr., ―Further Advances in a Recently Developed Cumulative-Damage Reliability Method‖, American Helicopter Society 66 th Annual Forum Proceedings , Phoenix, AZ, May 11-13, 2010. 8 Note that the rule of thumb that ―a factor of 2 in life is worth a factor of 10 in reliability‖ is verified via a probabilistic analysis example in J. Zhao and D. O.
Adams, ―Achieving Six-Nine‘s Reliability Using an Advanced Fatigue Reliability Assessment Model‖, Presented at the American Helicopter Society 66 th Annual Forum, Phoenix, AZ, May 11-13, 2010. 40