Slip sliding away

But after the storm blows through, the fresh clear air behind the weather system invites us to go up and enjoy the day. But beware, the rain will have left something behind: wet runways. Those surfaces can be a hazard.

Just like cars on wet roads, aircraft on wet runways are subject to the laws of physics. A slick surface can be rather precarious to any object moving over it quickly.

Hydroplaning occurs when a vehicle’s tires lose contact with the surface they are rolling on. A thin film of water separates the tires from the ground, causing a loss of traction, which leads to diminished braking capability and directional control.

In aviation, we talk about three different types of hydroplaning. Dynamic hydroplaning happens when there is standing water on the runway, and the aircraft tire is completely supported by or “riding on” the water. The water layer builds a resistance, and wedges underneath the tire, lifting it up off the runway surface.

Viscous hydroplaning is when the tire can’t penetrate a very thin layer of water and rolls on top of it. This can occur with less than one one-thousandth of an inch of moisture and at much lower speeds than dynamic hydroplaning, for example, when an aircraft lands on a smooth surface such as asphalt. It can feel like sliding on wet ice. The increase in stopping distance is impossible to predict accurately, but it has been estimated to be as much as 700 percent.

The third kind is called reverted rubber hydroplaning. This is when a hot tire basically heats up the water on a wet runway surface after a long skid, almost to the point of boiling it, and the steam prevents the tire from contacting the runway directly.

The effects of hydroplaning are greater when the speed is higher, when tires are worn down or underinflated, in a significant crosswind, or when landing on a slick, ungrooved runway.

The FAA’s Pilot’s Handbook of Aeronautical Knowledge, Chapter 11, shows us a quick and simple way to calculate the minimum speed, in knots, of when your aircraft will likely begin to hydroplane: multiply the square root of the main gear tire pressure in pounds per square inch by nine. For example, if the main gear tire pressure is 36 psi, the aircraft will begin to hydroplane at 54 knots (9 times 6).

The Airplane Flying Handbook’s Chapter 9 also has important information about the phenomenon, including ways to mitigate it.

A pilot’s best bet in those kinds of conditions includes touching down as close to the approach end of the runway as possible. That way, you won’t need to brake hard (or at all), allowing yourself the maximum landing distance available to roll out. And this is the time when you should place the wheels solidly on the pavement (to increase immediate grip), rather than trying to “grease it in” softly. Steering capability is increased the sooner you lower the nose or tailwheel.

If the runway is slippery, and you foresee the possibility you might slide across the wet surface, it might be a good decision to divert to a more suitable airport. A healthy crosswind could complicate matters even more, increasing the danger the aircraft could be blown off the surface.

If you are someone who only ventures out when the weather is perfect, hydroplaning is a hidden hazard that you don’t deal with often. If possible, go and practice landings on wet surfaces with an instructor so that the next time a monsoon hits your airport, you can be confident in your abilities to enjoy the calm after the storms.

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