Runway Surface Friction: Helps and Hindrances

Runway surface friction is expressed as the coefficient of friction; that is the ratio of the friction force (F) between two surfaces that make contact and the normal force (N) which exists between an object resting on the surface -- and the surface itself i.e. F/N. Many factors can affect this ratio such as the physical characteristics of two surfaces, the prevailing temperature at the point of contact, aircraft wheel and brake systems, and the speed of movement of the object (tire) over the surface.

One physical characteristic that plays a major role in F/N ratios is the texture of a runway surface, more specifically when a runway is wet. Macrotexture runway surfaces have a visible roughness that allows water to escape from beneath the tires preventing aquaplaning on water loomed runway strips. Microtexture runway surfaces have a fine scale roughness more detectable by touch than appearance and this surface allows aircraft tires to break through the residual water that remains after the bulk of water has dispersed. A runway that has a grooving texture is also vital in enhancing surface friction. Runway grooves aid more rapid water dispersal and allow for better tire traction. Normal grooving parameters are 6mm deep and 6mm wide spaced at 38mm apart.

Factors that hinder runway surface friction include rubber tire deposits and areas of runway that are painted with pilot markers that are inevitably more slippery than normal surface areas. Rubber deposits occur after every average aircraft landing. A thin layer of about 1.4 lb of rubber is left on the runway and can result in a loss of surface friction when accumulation is not removed periodically. In addition, painted markings create slippery surfaces that are remedied by adding small amounts of silica sand or glass beads to marker paint mix.


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