Six Nines Reliable Army Fatigue Critical Component Using Operational Loads Data

This paper describes the work performed to determine a 0.999999, 6 nines, reliable fatigue critical component life using field monitored loads. The Tie Bar of the MH-47 is substantiated by Centrifugal Force (CF), which is a direct function of rotor speed, Nr, which is a monitored parameter in the Structural Usage Monitoring System (SUMS). Six nines of reliability has been the Army target for component reliability and it is generally assumed that legacy safe-life methods are near this level of reliability. With monitored loads it is possible to develop a statistical model for loads and determine an actual reliability value. This paper presents multiple methods for the Army's first attempt at establishing a retirement time using an absolute component reliability. Reliability is gained using a reduction of the Endurance Limit and mean and standard deviations of binned loads across multiple aircraft. Most notably fatigue lives can vary widely if the independent variable reliability contributions are assigned arbitrarily.