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.
January 14, 1998
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:
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
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
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
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
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.
As far as the airplane is concerned, sound, properly inflated tires are the first line
of defense. Then, as mentioned earlier, avoid the water when possible, and keep
"flying" the airplane.