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Fatigue crack growth behavior in aluminium alloy 7050-T7451 under afighter aircraft wing root bending moment spectrum was investigated. The crack growthdata were measured by quantitative fractography for three groups of specimens withdifferent stress concentration geometrical features. Based on the analysis of themeasured spectrum crack growth data using linear elastic fracture mechanics, it wasfound that the concept of geometry factors in the stress intensity factor could notcollapse the crack growth rate data derived from each stress concentration feature,particularly near the small crack growth region. In order to investigate the possiblereasons for this, finite element analysis was used to determine notch plastic zone sizesfor each stress concentration geometry. As a consequence, an alternative crack growthdriving force by considering both notch elastic-plastic stress field and gross net-sectionstress field was proposed and used to interpret the fatigue crack growth data underspectrum loading.