Methodology for Fatigue Substantiation of Alternate Sources and Degraded Modes on Helicopter Dynamic Components

The Ad Hoc Committee on Fatigue Methodology of the Rotorcraft Advisory Group of the Aerospace Industries Association is considering methods for fatigue substantiation of new Alternate Sources/Alternate Processes/Alternate Materials on flight-critical helicopter dynamic components, and also for new substantiations which consider the effects of Degraded Modes (i.e., defects). This paper describes this methodology, the basis for it, and examples of its application from Boeing, McDonnell Douglas, and Sikorsky fatigue substantiations. For "Alternate" parts, the key requirement is to conduct an engineering evaluation of every potentially significant structural change due to supplier (any level), manufacturing process, or material. The majority of these changes can be evaluated by analysis, experience with similar cases, and/or coupon testing. In a few cases, full-scale testing, typically two specimens, would be appropriate. The inclusion of the effects of degrading conditions in fatigue substantiations is a logical enhancement of the safe-life method, which currently is based only on testing of conforming, new parts. The recommended methodology consists of an evaluation, for each component, of which degrading conditions can realistically occur and the potential strength reduction for each. Experience with similar components, analysis of degrading conditions, and/or coupon testing is used to reduce the number of conditions which require full-scale testing to a manageable number, if any. Results of the testing can be used to define any required corrective actions, such as periodic inspections, reduced retirement time, or design/process/inspection changes.