Abstract: After the aircraft was used for a period of time, cracks were found in the skin and strengthening stem of the lubricating oil heat dissipation outlet of the lower cover of the engine. The reasons for the cracks were analyzed by means of macro and micro morphology examination, electrical conductivity test, metallographic structure analysis, chemical composition determination, scanning electron microscope observation, and microhardness test. The results show that the crack of the strengthening stem is fatigue crack, which was mainly formed under cyclic loading of the cracked skin and the joint. The crack of the skin is fatigue crack caused by stress concentration of rivet hole.