SAFETY FACTORS AND RISK IN USAGE MONITORING AND FATIGUE SUBSTANTIATION OF HELICOPTER COMPONENTS

A study has been made of the errors and variability in the fatigue substantiation process, and of the variation in manoeuvre loads and usage during normal operation of a UK military helicopter. Both S-N curve behaviour and service loading have been investigated. Three components were studied. One was a location on the lift frame, the others were a main rotor blade linkage and a rotor pitch change link. Service loads were measured using strain gauged components and a data recording system. Usage was monitored by manual identification and recording of manoeuvres throughout the helicopter flight. It was found that usage variability is very great, with coefficients of variation in excess of 100% for the majority of manoeuvres. It was found that usage measured in service was significantly more benign for all types of mission, than that assumed in design. Mechanical test results showed significant errors in damage accumulation and mean stress models for life prediction. Monte Carlo simulations demonstrate that calculated probability of failure is dominated by material and modelling errors; usage and manoeuvre load variability playing a minor role.