Abstract: In the design phase of an aircraft structure, the dispersion of structural and load spectra must be taken into account to evaluate the safe-life of a fleet. Thus it is critical to determine the load spectrum scatter factor. The distributions of 73 F/A-18C aircrafts' design lives and 74 F/A-18 aircrafts' damage rates are analyzed, and the results show that the load spectrum damage of the fighter aircraft follows lognormal distribution. The definition of load spectrum scatter factor and two methods to estimate the load spectrum scatter factor are proposed. The formula for calculating load spectrum scatter factor is presented and the calculated values are compared with empirical values.