A Structural Perspective on U.S. Army Airworthiness Impacts Resulting from Changes in Rotorcraft Component Fatigue Strength, Loads, and Usage

This paper provides insight into the methods used by U.S. Army Aviation Engineering Directorate personnel to assess airworthiness impacts due to changes in loads, usage, or strength, and resulting effects on calculated safe-life retirement times. Statistical analysis of system reliability for overflight of the safe-life time for components with assumed failure distributions forms the mathematical basis for the method. Topics include failure rate, baseline failure rate, percent change in system level risk, immeasurable risk, and mathematical relationships between each. Results include figures to aid in visualization and tables to enable the reader to check an implementation of the method for specific examples. The paper presents the results of analysis used to derive Weibull slope parameters based on recently developed fatigue reliability methods and available mission loads spectra. Finally, the paper presents a new method to establish life factors required to convert retirement intervals with a delta failure rate (for an airworthiness impact) into retirement intervals with immeasurable risk. Modified life factors address special cases of incompatible confidence and analysis uncertainty.