In general, the presence of a fluid on a runway decreases the available braking force that can be generated when skidding at constant total vertical load. This is perceived to arise because part of the load on the tyre is supported by the contaminating fluid. Because decelerating forces are developed in sliding by shearing action, the area of the footprint within which a frictional force can be generated is therefore smaller than on a dry runway.

The interaction between tyre, water, and ground is a complex example of a multi-phase flow. It is therefore not readily described in a simple, algebraic, engineering format. This Technical Memorandum presents a description of the simplified view of the way in which the footprint may be divided into three zones. In addition, an easily used model for the pressures in the zones is given. Method for determining the length and width of a tyre footprint are also provided.