Don’t “Hang Ten” in Your Airplane

As winter grudgingly gives way to spring and icy runways become wet ones, a new hazard emerges: hydroplaning. It might be great for surfboards, but it's no fun at all when it happens to an airplane on the runway. Bruce Edston explains the three different forms of hydroplaning and offers some prevention strategies.


AirmanshipAh, yes, I can see it now: Frankie and Annette on the beach in some island paradise.Reality check! Considering the crummy weather we expect in the winter, maybe we need somedaydreaming to avoid getting too depressed!

AVEMCOSeriously, though, the "surfing" I’m talking about has to do with makingsurfboards out of your airplane’s tires. As snow and ice problems disappear withtemperatures warm enough to melt the stuff, a new problem appears—wet runways andtaxiways. Under the right circumstances, water can be as big a problem as ice or snow.

We are speaking of hydroplaning, of course, where the tires lose contact with thesurface and act like a surfboard. How does this happen? Basically there are three types ofhydroplaning which can occur separately or together.

Dynamic Hydroplaning

This is the most common form, and it’s because of fluid density pressures. Essentially,this just means the water can’t get out of the way fast enough. We normally think of wateras being pretty soft, and maybe it seems a bit unusual that the water truly can’t get outof the way, but that’s how it works. Think back to your days as a kid at the lake. Did youever do a belly flop off the diving board? How soft did the water feel?

As a result of the inertia of the water, the tire does not make full contact over itsnormal "footprint" and starts to ride up on a wedge or film of water. This ispartial hydroplaning and, obviously, steering and braking effectiveness will suffer.

Increase the speed a bit more, and the at-rest momentum of the water is such that thetire does not make contact with the surface at all. This is called total hydroplaning.Steering and braking effectiveness are now non-existent. If you are going fast enough tostill have aerodynamic control, you should be okay. But, typically, there isn’t a lot leftin that speed range.

What speed range are we talking about? This will depend on tire pressure. It sounds toogood to be true, but extensive testing done by NASA and tire manufacturers has shown thata very simple equation is surprisingly accurate. This equation is expressed as follows:

V = 9 * SQRT(P)

This means that the total hydroplaning velocity (V) in knots is equal to nine times thesquare root of the tire pressure (P).

Let’s see what this actually means to us light-plane drivers. My old Cessna 150 owner’smanual says the main gear tires should be inflated to 21 PSI. Thus, V=9*sqrt(21) or 9*4.58or about 41 knots. Since Vso (stall speed in the landing configuration) is shown as 41.5knots (48mph), it is apparent that you will have to be right on the ragged edge attouchdown to completely avoid hydroplaning speed.

Several other typical general aviation machines fare about the same. A Cherokee Arrowis a bit heavier, and the mains are supposed to get 27 PSI. So 9*sqrt(27) yields 47 knotswith a Vso of 55 knots. Hmmm. The book for the B Model Piper Aztec says 42 PSI for themain tires, which produces a 58-knot hydroplaning speed, and Vso is listed as 54 knots.Since normal approach speed is 1.3 x Vso, it can be seen that in almost all cases you willbe touching down well above the minimum hydroplaning speed.

Of course, if you can land into the wind, this speed problem will largely disappear,since it is ground speed with which we are really concerned. After all, the water issitting on the ground and what counts here is how fast we hit the water.

You could still blow it, though, if you add five knots for the spouse and kids, anotherfive for the crosswind and another five for the gusts, you will be right back where youstarted.

From a technique standpoint, the secret is to keep flying the airplane, and letaerodynamic drag do its thing until you are below the magic speed for your airplane. Then,with the tire in full contact with the runway, you can expect normal steering and brakingresponses.

All of the above applies to tires that are the correct size, in good shape, andproperly inflated, of course. As the tread wears down and/or the pressure drops, so doesthe hydroplaning speed. Those big, bald "tundra" tires you see on Alaskan SuperCubs will hydro at just about any speed!

Viscous Hydroplaning

This one can fool you, because it can happen even at very low speed. Essentially, it issliding on some liquid other than water or in a situation where water has mixed withsomething. For example, an area of a ramp or runway could become contaminated with anumber of substances.

The run-up area would be a good candidate here, because a lot of aircraft sit over thesame spot for a few minutes. Although not leaking much oil individually, the collectiveeffect of a drop or two from dozens of aircraft can produce a noticeable stain. Then alongcomes a little rain to lift this out of the pavement surface, and it gets slick in a realhurry.

Touchdown zones are even worse, because they get oil shaken loose by the landing impactand a lot of rubber dust, too. At some really busy airports, they have to periodically goout and grind this stuff back down to the pavement.

Once again, just add water for a nice, slippery mess! In most cases, a good hard rainwill wash away most of this, so most occurrences are as the rain starts or after a lightshower or heavy dew.

Reverted Rubber Hydroplaning

Here’s a weird one that you’ll probably never encounter. For this to happen, a veryprecise amount of water must be present—not too much and not too little. The frictiongenerated between the skidding tire and the pavement does two things: It melts the rubber,which forms a seal around the footprint of the tire, and turns water to steam, which,sealed in by the melted rubber, supports the tire.

Obviously, this requires a very delicate balance of forces, so it does not happen a lotnor does it last very long.

Hydroplaning Prevention

So, how do we combat this monster? Get the water off the runway, that’s how! Needlessto say, proper drainage should be considered in any runway or ramp construction, but manysmaller airports (with correspondingly smaller budgets) just can’t do much about it.Hopefully, the smaller airport won’t be so small that you can’t land beyond the water.

If outright avoidance isn’t possible, then you really have to try your utmost toencounter water at the lowest possible speed, certainly below the critical speed for yourbird.

In many cases, even good drainage isn’t good enough, because the water is falling andflowing as fast as possible, but the flow itself is deep enough to cause hydroplaning.Thus, most all-weather, airliner-type runways have to resort to another strategy: grooves.

The good news is that runway grooving provides a path for water to escape from underthe tire. The bad news is that it costs more. The decreased surface area causes thepavement to wear out faster and needs replacing sooner. In some cases, it has beenpossible to grind it all flat and re-groove it.

AVEMCOAs far as the airplane is concerned, sound, properly inflated tires are the first lineof defense. Then, as mentioned earlier, avoid the water when possible, and keep"flying" the airplane.