Surface topography of a magnetic disk is one of the key parameters to determine safe fly height in a hard disk drive. In order to quantify the flying capability of the disk surface, glide avalanche is a useful parameter because it is the parameter which indicates the height of the surface topography from the mean surface below which a head can not fly without contacts. The avalanche is regarded as the floor of the disk surface intrinsically defined by the surface finish during disk production. The glide height is set at higher level than the avalanche and usually limited by isolated defects or contamination on the surface. Because of this direct couple between glide avalanche and the disk surface finish, the measurement and prediction of the glide avalanche are very important. In this paper, we discuss the importance of fly height characterization of a glide head and the relationship between disk surface topography and its avalanche. We propose a simple yet versatile model based on a composite roughness, which can be applied to a polished surface and to a textured contact start/stop zone including laser bumps.