Competing Failure Risk Assessment

A Full Scale Fatigue Test (FSFT) is traditionally required by the United States Navy (USN) to be completed prior to or during the early stages of inducting aircraft into service. In addition, USN aircraft are generally equipped with a usage and loads monitoring system to compute fatigue damage accumulation and identify usage severity/changes over their entire operating life. But many times, structural fatigue cracking does happen in the fleet aircraft earlier than anticipated. Infant/early fatigue failures typically occur due to material defect, manufacturing process, unanticipated load/load-path/load distribution, "abnormal" fatigue behavior or maintenance induced, while the normal fatigue or wear out failures would usually take place in the late stage of the aircraft service life as designed. These infant mortality and long life failures should be considered separately as competing failure modes. Due to the fleet operational needs coupled with program budget and schedule constraints, sometime it is necessary to induct aircraft into service even before the FSFT has initiated. Thus, there is a need to develop a better methodology to predict reliability for structural integrity during the fleet service life, and for determining maintenance requirements. Using historical and current service fleet fatigue cracking data from different aircraft types/models/series, a Weibull distribution is developed to predict risk and failure/hazard rate. Group structural failure scale, shape probability density functions and their values then can be used to ascertain the optimum time for conducting a Service Life Assessment Program (SLAP).