Whenever you move a tire over a wet surface, there is always a risk of hydroplaning. That squeezing of water under the tread generates water pressure that, when excessively built up, can lift it off the pavement.
The resultant loss of surface contact means that the tire can’t develop friction with the surface. Traction is lost; therefore, so are accelerative and braking forces. Have this happen to you on a wet runway, and you’ll find that your airplane has just become a sled.
Hydroplaning can occur under various levels of water depth. When a layer of water that’s barely a fraction of a centimeter deep is present, it can still be enough to lubricate the surface. Known as viscous hydroplaning, even that thin film can prevent the tire from penetrating the water, forcing it to ride over top of it instead. Most often, lower speeds are involved, as are smooth surfaces that may be coated in debris, such as accumulated rubber from previous landings.
Dynamic hydroplaning involves deeper water, and results in complete loss of tire contact with the tarmac. Associated with higher speeds, hydroplaning of this nature occurs when the water layer builds up to such an extent that it resists displacement. A wedge of water forms at the leading edge of the tire. When the speed is such that the upward force from the water pressure equals the weight of the aircraft, the aircraft starts to surf over the surface of the runway.
The moment at which dynamic hydroplaning occurs is a function of water depth, tire tread depth, tire pressure, and speed. For hydroplaning to occur, the depth of water on the runway must exceed the depth of the tire’s tread. The grooves that make up the tread on a tire are designed to disperse water out from underneath the tire. If the tire encounters more water than its tread can push aside, it will skate on the sheet of water over which it’s moving.
Tire inflation pressure and aircraft speed have a lot to do with hydroplaning. As an aircraft’s speed is increased, the time that the tire’s contact patch is in contact with the ground is reduced. The result is a reduced coefficient of friction. Add a surface with a layer of water, and that coefficient of friction is reduced even more. Thus, braking power on wet surfaces decreases as speed increases.
The minimum speed, in knots, for dynamic hydroplaning to occur to a rotating tire is calculated as 9 times the square root of the tire pressure. For example, the hydroplaning speed of an aircraft with a tire pressure of 50 psi would be 64 knots.
The build-up of water under a non-rotating tire, such as one locked under the force of braking or motionless at the culmination of a flight, will cause it to hydroplane at a lower speed than will a rotating tire. For that non-rotating tire, the hydroplaning speed is calculated by multiplying the tire pressure’s square root by 7.7. Thus, at 50 psi, the non-rotating tire’s hydroplaning speed is 54 knots.
A third type of hydroplaning occurs when a locked tire skids on a wet runway. The excessive build up of heat due to friction raises the tire temperature causing the tire’s rubber to shred. That shredding can contribute to the trapping of water beneath the tire. As the water’s resultant temperature rises, it creates a cushion of steam over which the aircraft is capable of hydroplaning. This is known as reverted- rubber hydroplaning.
To minimize the risk of hydroplaning, tire tread depths should be monitored, and tires should always be properly inflated. Pilots should be vigilant of crosswind effects too. When hydroplaning conditions exist, it takes a 10 knot crosswind 7 seconds to force an aircraft off the side of a 200 foot wide runway.
Most major airports have runways that don’t allow pooling of water to occur. But, most of general aviation uses other than major airports. Therefore, it’s always best to be mindful: runways contaminated by water can be as slick as water on ice.