Hydroplaning can occur when landing on a runway surface contaminated with standing water, slush, and/or wet snow. It can seriously affect ground controllability and braking.
The three basic types of hydroplaning are dynamic hydroplaning, reverted rubber hydroplaning, and viscous hydroplaning. Any one of the three can render an airplane partially or totally uncontrollable anytime during the landing roll.
Dynamic hydroplaning is a relatively high-speed phenomenon that occurs when there is a film of water on the runway that is at least one-tenth inch deep.
As the speed of the airplane and the depth of the water increase, the water layer builds up an increasing resistance to displacement, resulting in the formation of a wedge of water beneath the tire.
When the water pressure equals the weight of the airplane, the tire is lifted off the runway surface and stops rotating. Directional control and braking is lost.
Dynamic hydroplaning is often affected by tire inflation pressure.
Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced.
It can happen on takeoff as well as landing.
Reverted Rubber Hydroplaning
Reverted rubber (steam) hydroplaning occurs during heavy braking that results in a prolonged locked-wheel skid. Only a thin film of water on the runway is required to facilitate this type of hydroplaning.
The tire skidding generates enough heat to cause the rubber in contact with the runway to revert to its original uncured state (think ‘melting’). The reverted rubber acts as a seal between the tire and the runway, and delays water exit from the tire footprint area. The water heats and is converted to steam which supports the tire off the runway.
Reverted rubber hydroplaning damaged tire.
Close-up of reverted rubber tire damage.
Reverted rubber hydroplaning frequently follows dynamic hydroplaning, during which time the pilot may have the brakes locked in an attempt to slow the airplane. Eventually the airplane slows enough to where the tires make contact with the runway surface and the airplane begins to skid.
The remedy for this type of hydroplane is for the pilot to release the brakes and allow the wheels to spin up and apply moderate braking.
Reverted rubber hydroplaning is insidious in that the pilot may not know when it begins, and it can persist to very slow groundspeeds (20 knots or less).
Viscous hydroplaning is due to the viscous properties of water. A thin film of fluid no more than one thousandth of an inch in depth is all that is needed.
The tire cannot penetrate the fluid and the tire rolls on top of the film. This can occur at a much lower speed than dynamic hydroplane, but requires a smooth or smooth acting surface such as asphalt or a touchdown area coated with the accumulated rubber of past landings. Such a surface can have the same friction coefficient as wet ice.
And although this kind of hydroplaning may seem as ‘vicious’ as a mad dog when you encounter it, please remember it is pronounced like “vis-kus”.
It is best to land on a grooved runway if available.
Touchdown speed should be as slow as safely possible.
After the nosewheel is lowered to the runway, moderate braking should be applied.
If hydroplaning is suspected, the nose should be raised and aerodynamic drag used to decelerate to a point where the brakes do become effective.
Proper braking technique is essential. The brakes should be applied firmly until reaching a point just short of a skid. At the first sign of a skid, the pilot should release brake pressure and allow the wheels to spin up.
Directional control should be maintained as far as possible with the rudder.
Remember: In a crosswind, if hydroplaning should occur, the crosswind will cause the airplane to simultaneously weathervane into the wind as well as slide downwind.
This blog post is based on content in the FAA’s “Airplane Flying Handbook”. It is an ABRIDGED version of the Chapter 8, “Approaches and Landings” within that book.