Abstract: The exhaust pipe of hydraulic tank in an aircraft hydraulic system fractured. The causes of fracture were analyzed by means of macro observation, metallographic examination, hardness test and micro observation, combined with the pipe layout and service conditions of the exhaust pipe. The results show that the fracture mode of exhaust pipe was bidirectional bending fatigue fracture, because it bore the abnormal load along the transverse direction, and the abnormal load may been related to the fixing mode of hydraulic oil tank.