An investigation of the siphon break phenomena during defueling of an aircraft wing tannk was performed using Computational Fluid Dynamics. A computational model was developed of a wing tank with a particular siphon break line design for which test data are available. A two-phase fuel-air model was solved with the volume-of-Fluid method. The results indicate the model is able to predict the siphon break event dependence on defueling rate and the resultant undershoot height, measured from the fuel pool level to the siphon break line end, as well as the liquid mass flow rates inside the main pipe. It can thus be used to improve the design to reduce defuel times or assess performance for this kind of passive fuel system.