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.




Dec 2014