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Abstract: The scatter of the variable load spectra is considered separately from the scatter of the aircraft structure to assess the safe-life of a fleet. Suppose the fatigue life under the specified load spectrum follows the log-normal distribution with a constant coefficient of variation, and the load spectra damage follows the log-normal distribution with a constant standard deviation, the theoretical distribution function of fatigue life of the fleet that accounts for the variability of the structure and the load spectrum is derived with the conditional probability model. Monte-Carlo simulation shows that the distribution of the fleet fatigue life can be described approximately by a log-normal distribution function, and the log expectation is the log mean life under variable load spectra and the variation equals the sum of the structural variation and the load spectra variation. The scatter factor method is then adopted to assess the safe-life. The scatter factor of the individual aircraft with definite load spectrum, as well as the scatter factor of the in-service fleet that takes the actual load scatter into consideration are listed. [Copyright &y& Elsevier]

Highlights: [•] Variability of load spectrum of a fleet is analyzed. [•] Fatigue tests are conducted to test the variability of load spectrum. [•] Load spectrum damage follows a lognormal distribution. [ABSTRACT FROM AUTHOR]

Abstract: 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 σS and σL 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 and . The expression of the fleet life reliability P was also deduced and it is influenced by the σS and σL . The typical values of P were calculated for the cited σS and σL . To ensure the fleet service safety under the 90% load spectrum, the requirement on the severity degree, PL , 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. [ABSTRACT FROM AUTHOR]