The fleet life reliability analysis under the 90% severe load spectrum

The fatigue life scatter of an aircraft fleet includes the variabilities of the material, structural property and the load spectrum. With more and more knowledge on the load variation, the 90% severe durability load spectrum has been proposed in JSSG-2006 of USA and used in the service life assessment program for several aircraft types. However, what is the life reliability of a fleet under this 90% severe spectrum and how severe the load spectrum should be are still perplexing. This paper would discuss the fleet life reliability under the severe load spectrum. Assuming both the fatigue life under a specified load spectrum and the load spectrum damage of a fleet follow the log-normal distribution and the standard deviations are σ_S and σ_L respectively, the fleet life was deduced to follow a log-normal distribution, the log mean life is unchanged and the variation is the sum of σS2 and σL2. The expression of the fleet life reliability P was also deduced and it is influenced by the σ_S and σ_L. The typical values of P were calculated for the cited σ_S and σ_L. To ensure the fleet service safety under the 90% load spectrum, the requirement on the severity degree, P_L, which is also the load reliability, was discussed. It showed that the severity of the severe spectrum is not a constant. If the 90% damage spectrum was used, the standard deviation of load spectrum damage should be controlled less than 0.14 at least.