A model of atmospheric turbulence has been developed by W.G. Tank, which was used to predict the rate of turbulence induced unstarts for example routes of the High Speed Civil Transport, a proposed supersonic commercial airliner. This paper focuses on the validation of this model. We used it to predict the exceedance rate of vertical accelerations of 747s on a trans-Atlantic route, and compared this prediction with observed exceedance rates. These data were obtained from two large scale experiments using 747s in commercial service.