we investigate the asymptotic behavior of the free path of a variable density random flight model in an external field as the initial velocity of the particle goes to infinity. the random flight models we study arise naturally as the boltzmann-grad limit of a random lorentz gas in the presence of an external field. by analyzing the time duration of the free path, we obtain exact forms for the asymptotic mean and variance of the free path in terms of the external field and the density of scatterers. as a consequence, we obtain a diffusion approximation for the joint process of the particle observed at reflection times and the amount of time spent in free flight. this is based on joint work with doug rizzolo and eric wayman.