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.
Ah, 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 some daydreaming to avoid getting too depressed!
Seriously, though, the "surfing" I'm talking about has to do with making surfboards out of your airplane's tires. As snow and ice problems disappear with temperatures warm enough to melt the stuff, a new problem appearswet runways and taxiways. 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 the surface and act like a surfboard. How does this happen? Basically there are three types of hydroplaning which can occur separately or together.
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 water as being pretty soft, and maybe it seems a bit unusual that the water truly can't get out of the way, but that's how it works. Think back to your days as a kid at the lake. Did you ever 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 its normal "footprint" and starts to ride up on a wedge or film of water. This is partial 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 the tire 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 to still have aerodynamic control, you should be okay. But, typically, there isn't a lot left in that speed range.
What speed range are we talking about? This will depend on tire pressure. It sounds too good to be true, but extensive testing done by NASA and tire manufacturers has shown that a 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 the square root of the tire pressure (P).
Let's see what this actually means to us light-plane drivers. My old Cessna 150 owner's manual says the main gear tires should be inflated to 21 PSI. Thus, V=9*sqrt(21) or 9*4.58 or about 41 knots. Since Vso (stall speed in the landing configuration) is shown as 41.5 knots (48mph), it is apparent that you will have to be right on the ragged edge at touchdown to completely avoid hydroplaning speed.
Several other typical general aviation machines fare about the same. A Cherokee Arrow is a bit heavier, and the mains are supposed to get 27 PSI. So 9*sqrt(27) yields 47 knots with a Vso of 55 knots. Hmmm. The book for the B Model Piper Aztec says 42 PSI for the main 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 will be 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 is sitting 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, another five for the crosswind and another five for the gusts, you will be right back where you started.
From a technique standpoint, the secret is to keep flying the airplane, and let aerodynamic 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 braking responses.
All of the above applies to tires that are the correct size, in good shape, and properly inflated, of course. As the tread wears down and/or the pressure drops, so does the hydroplaning speed. Those big, bald "tundra" tires you see on Alaskan Super Cubs will hydro at just about any speed!
This one can fool you, because it can happen even at very low speed. Essentially, it is sliding on some liquid other than water or in a situation where water has mixed with something. For example, an area of a ramp or runway could become contaminated with a number of substances.
The run-up area would be a good candidate here, because a lot of aircraft sit over the same spot for a few minutes. Although not leaking much oil individually, the collective effect of a drop or two from dozens of aircraft can produce a noticeable stain. Then along comes a little rain to lift this out of the pavement surface, and it gets slick in a real hurry.
Touchdown zones are even worse, because they get oil shaken loose by the landing impact and a lot of rubber dust, too. At some really busy airports, they have to periodically go out 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 rain will wash away most of this, so most occurrences are as the rain starts or after a light shower or heavy dew.
Reverted Rubber Hydroplaning
Here's a weird one that you'll probably never encounter. For this to happen, a very precise amount of water must be presentnot too much and not too little. The friction generated 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 lot nor does it last very long.
So, how do we combat this monster? Get the water off the runway, that's how! Needless to say, proper drainage should be considered in any runway or ramp construction, but many smaller 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 to encounter water at the lowest possible speed, certainly below the critical speed for your bird.
In many cases, even good drainage isn't good enough, because the water is falling and flowing 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 under the tire. The bad news is that it costs more. The decreased surface area causes the pavement to wear out faster and needs replacing sooner. In some cases, it has been possible to grind it all flat and re-groove it.