A critical analysis of the six-nines reliability methodology

Abstract

The current certification requirements of helicopter dynamic components demand that the probability of failure at the retirement life does not exceed one in a million, i.e. a reliability level of 0.999999 (six-nines reliability). It is therefore imperative that the component retirement time for fatigue critical rotorcraft dynamic components be determined using probabilistic rather than deterministic methodologies. In the helicopter industry a method known as ‘Top of Scatter-mean minus three standard deviation (()σμ3−TOS) reliability method is commonly used for determining the component retirement time over which the probability of failure is ≤. In this paper, the results of the 610−()σμ3−TOS reliability method for an example of constant amplitude loading have been compared with that of an exact analytical solution, Monte Carlo simulation and Second-Moment methods. Examples using Monte Carlo simulation and a numerical approximation are evaluated for variable amplitude loading, where Miner’s linear damage summation rule is assumed. Finally, this paper shows that the ()σμ3−TOS reliability method is non-conservative for both constant and variable amplitude loading.