Abstract: In the design phase for an aircraft structure, the fleet safe life should be assessed by accounting for the variability both in the structure and in load spectra. A 90% load spectrum (severe spectrum) has been proposed for the durability analysis and testing in JSSG-2006, MIL-STD-1530C in USA and GJB67.6A-2008 in China. But there are some confused viewpoints on the severity of the severe spectrum. This paper discusses primarily the selection method of the severe load spectrum. Assuming the life and the load damage have the log-normal distributions, and using the probability to represent the load spectrum severity, the expression of the load spectrum severity is obtained. For certain structural and load spectra standard variations, the severe probability value is derived. The results show that the load spectrum severity is related to the load scatter and structural scatter, and that the 90% severe spectrum based on damage as given in this paper ensures approximately the safe requirement of fleet structures.