Hydroplaning is what happens when you land your airplane (or drive your car) quickly over a water-covered surface, and instead of your tire cutting through the water and contacting the asphalt (or concrete) below, it rides on top of the water with little traction.
Actually, in a little airplane with a long runway, this is really no big deal. For the first part of the rollout, it’s no different than landing on ice, which is really no big deal.
However, with a heavier, faster airplane and a short runway (and a faster than normal approach), this can often result in running off the end of the runway. They had to put grooves in runway 25 at CYOW, the four-bars were running off the end so much – typically in an aircraft with no thrust reversers (just think of all the money they saved!) - after a period of brief heavy rain which left standing water, and as I said, a faster than normal approach. Oops.
NASA did some research in the 1980’s and came up with Horne’s formula for predicting the theoretical speed above which a tire will hydroplane:
V (knots) = 9 x square_root(tire pressure in PSI)
which once again is amazingly simple, and once again includes a square root.
Let’s look at some examples:
30 PSI: 9 x sqrt(30) = 49 knots
50 PSI: 9 x sqrt(50) = 64 knots
100 PSI: 9 x sqrt(100) = 90 knots
150 PSI: 9 x sqrt(150) = 110 knots
200 PSI: 9 x sqrt(200) = 127 knots
Weirdly, there is no accounting in the formula for either the aspect ratio of the tire, or the tread. One thing I should mention is that the tread on your light aircraft tire (e.g. Goodyear Flight Custom III, which has a couple of axial grooves) doesn’t really help much with traction in water or snow. In fact, a perfectly bald smooth tire is airworthy. The tread cuts are more used as wear indicators. When the tire is bald, order a new one!
It is worth mentioning that according to Goodyear, a FC III tire with cord showing is legally serviceable. This surprises most pilots and mechanics. Go look up their documentation.
I should mention that recent research has shown that the constant factor of “9” in Horne’s formula has been shown to be somewhat optimistic with newer aircraft tires, where a constant of 7 or even 6 gives more accurate results.