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Aviation Safety - Hydroplaning

            Tires are on an airplane for a few reasons.  They have the duties of providing low friction rolling for takeoff and landing.  They help decelerate the aircraft to a safe speed when landing.  They absorb shock during normal and hard landings.  They are the only connection to the ground.  It’s important to understand how critical tires are for aviation safety.  A relatively high number of accidents occur during the landing/takeoff phase of flight, the only time tires are used. 

            Tires can often hydroplane when landing or taking off.  Hydroplaning is a situation which drastically reduces the coefficient of friction of the tire(s).  Ideally, a tire skids slightly when braking, still allowing the wheel to turn.  This warms the tire slightly, which increases the coefficient of friction.  If there is too much tire skid, the rubber warms even more.  It will quickly pass the point of maximum coefficient of friction, then lower significantly because the rubber compounds break down with all this excess heat.  If we add water to our runway surface, we get a whole new picture.[1] 

            Viscous hydroplaning is loss of coefficient of friction due to a thin film of water, slightly separating parts of the tire patch from the runway.  Aircraft control is reduced significantly.  Dynamic hydroplaning is where the tire contact patch is raised off the runway because of high speed and deep water.  It is a function related directly to speed and tire pressure.  The function is different for spinning vs. stationary tires.  Another factor is tread depth.  Reverted rubber hydroplaning is where the heat of friction (decelerating) causes water to instantly boil underneath the tire.  The water vapor expands rapidly, pushing part of the tire off the ground.  This lowers the coefficient of friction and aircraft control is nearly impossible. 



[1] Richard Wood & Robert Sweginnis, Aircraft Accident Investigation, Casper, WY: Endeavor Books, 2002, p.228-230