1. Take your tire pressure in kg / sq cm 2. Divide this by the specific gravity of the contaminant 3. Take the square root of this value 4. Mulitply this by 34 5. The result is the speed of aquaplaning in kts.

Source: Airbus "Getting to Grips with Aircraft Performance" Section 5.5.2.4, Page 82.

What we're talking about is "Dynamic Hydroplaning". The above formula does not apply to the other two types (Viscous and Reverted Rubber Hydroplaning).

Some may argue that the specific gravity factor considers Viscous Hydroplaning...but, I believe it does not.

First, the specific gravity of water is around 1. It varies, slightly, based on temperature and degree of contamination (how pure it is).

Second, did you take the square root the tire pressure? Note, the formula I cited is based upon tire pressure expressed in Kg/cm*2 (not pounds per square inch).

I've read several places that the principle of 9 X the square root of the tire inflation pressure (in lbs/in*2) has been re-evaluated. Some engineers consider a more accurate formula as 7 X the square root of the tire inflation pressure (in lbs/in*2).

Then, there's the theory (I've read this somewhere, but I honestly can't remember where,) that the formula is incorrect if, upon touchdown, there is no initial wheel spin-up, i.e. a real greaser of a landing on a runway that is contaminated with water after a heavy rain storm. In this case, you may never realize any braking action.

Interesting stuff.

In any event, it's important to note that the coefficient of friction (mu) is very low with standing water. In fact, I've seen figures published where the mu with standing water is not much higher than that of wet ice.

To some degree, tire wear plays a role, too. How much tread is on the tire has some bearing...but not as much as you might think.

Years ago, Professional Pilot magazine had a really good series of articles on braking, hydroplaning, coefficient of friction, etc. You may be able to retrieve these somehow via the internet.

In ProPilot treatise, the author discussed how a pilot in a typical runway over-run accident, many times, experiences all three types of hydroplaning. Initially, the pilot is faced with dynamic hydroplaning. As the landing roll continues, reverted rubber hydroplaning comes into play. Finally, as the aircraft skids through the touchdown zone of the other end of the runway, viscous hydroplaning is experienced.

In countries that are lesser developed (a nice way to put it), the runways are not crowned, not grooved, and not regularly pressure-sprayed (to remove rubber and other contaminants). Attempting to land during or immediately after a heavy rain puts a pilot into the 'test pilot' category. Whether or not he'll get stopped...well, all bets are off.

The subject has been covered in Flight Testing. The thread provides links to references which associate SG with aquaplaning speed and also indicates that a simple formula might not apply to all aircraft. In particular “modern aircraft tires have lower hydroplaning speeds than those predicted by the well-known and commonly accepted equation … because of the differences in the footprint dimensions of the newer tires as compared to the older bias-ply tires.” The diagram in the linked presentation indicates that the hydroplaning boundary ranges from 9*SQR x tire pressure to 6*SQR x tire pressure (psi).

I would like to understand...How do you find this formula? And can you demonstrate this formula?? I'm french and I'm doing research on this topic...Can you help me???

Then, there's the theory (I've read this somewhere, but I honestly can't remember where,) that the formula is incorrect if, upon touchdown, there is no initial wheel spin-up, i.e. a real greaser of a landing on a runway that is contaminated with water after a heavy rain storm. In this case, you may never realize any braking action.

Which is why we have learned that you should make a positive landing (i.e. not a greaser) when there is a possibility for aqua planing.

The 2001 report NLR-TP-2001-216 Safety aspects of aircraft performance on wet and contaminated runways gives additional and updated information related to the NASA/Horne's equation. Have a look at chapter 2.3 Hydroplaning in this report and you will see that there are differences related to "spin-up", "spin-down" and radial tyres.

Table 1 in the report shows Typical take-off, landing and hydroplaning speeds of commercial jet and turboprop aircraft

In the May 2010 NLR/EUROCONTROL report by G.W.H van Es, - A study of runway excursions from an European Perspective - following message is found:

Modern aircraft tires like radial tires can have lower aquaplaning speeds than the older cross-ply tire designs. This fact is not very well known to the pilot community.

The NASA doc above is the definitive source, as it is based on original research.

The important things to take from it:

The formula only applies if the tyre can be supported by the fluid - that is that the depth of the water above the surface of the runway is greater than the tyre tread depth.

The speed required to support the weight of the aircraft is (in almost every normal case) greater than the touch down speed of that aircraft.

... so the pilot "clued up" on aquaplaning understands that:

it is worth noting the tread depth on the walk-around.

That you don't need to worry about "aquaplaning" if the runway isn't flooded

that regardless of the calculated speed, you always need to worry about "slippery" conditions if it is wet (or contaminated with rubber, or oil or slush etc.) and,

that, if asked by a pedant, it is only the main wheel pressure you need to calculate - as the nose wheel doesn't (in almost every case) have any braking, and at the speeds in question, steering authority is still with the rudder.