Fatigue crack propagation on aircraft external fuel tank horizontal fin is one of the common phenomenon of structural failure. In the past, it was reported that the primary factor of the fatigue crack is the oscillatory vertical load acting on the horizontal fin. Furthermore, the previous studies have revealed that the most of oscillatory vertical load on the horizontal fin was due to the influence of vortex air flow induced by opening the speed brake. The main goal of this study is to evaluate

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... the behaviour of the horizontal fin of external fuel tank related to oscillatory vertical load generated by speed brake. To achieve this objective, structural analysis based on Finite Element Method has conducted by utilizing the dynamic load history of speed brake. Especially, this research examined the effect of dynamic behaviour on fatigue life with a fatigue analysis in both time and frequency domain.