The next time you are asked for the hydroplaning speed...

[tcasblue]

...you might want to give the same answer as is applicable to so many questions. That is....It Depends. Why, because there is no simple technically correct answer. Life just isn't that simple.

Reading from an" accident report....

https://reports.aviation-safety.net/...135_ZS-SJW.pdf

"For the aircraft in question, the critical hydroplaning speeds with the required tyre pressure of 145 psi (pounds per square inch) are shown below:
Critical speed 1 = 8,6 x 145 = 103 kt
Critical speed 2 = 7,7 x 145 = 93 kt

Both critical speed formulas were used as the manufacturer does not clearly indicate that one has preference over the other, or is more pertinent to the aircraft type. There might be variation from one aircraft model to another as tyre sizes and pressures differ from model to model, with different tyres having to carry different loads The IIC obtained expert opinion from the author of a study into hydroplaning of modern aircraft tyres, and was informed that the critical hydroplaning speed is strongly influenced by the type of tyres fitted. The study indicated that the critical hydroplaning speed for the bias-type tyre fitted to the accident aircraft could have been as low as 6,8 x P (where “P” is the tyre pressure in psi).

According to the tyre manufacturer, the accident aircraft had been fitted with the bias-ply type tyre.

The critical speed may therefore have been as low as 6,8 x 145 = 82 kt
The tyre damage indicated that hydroplaning had indeed occurred. At what speed it happened, however, could not be determined with certainty. Using the formulae developed by NASA and the NRL, it is evident that there is a 20-knot window, which indicates that the critical hydroplaning speed for the type of tyres fitted to the accident was between 82 and 103 kt."

[DaveReidUK]

Originally Posted by tcasblue

For the aircraft in question, the critical hydroplaning speeds with the required tyre pressure of 145 psi (pounds per square inch) are shown below:
Critical speed 1 = 8,6 x 145 = 103 kt
Critical speed 2 = 7,7 x 145 = 93 kt

I think your quote might have lost something in translation.

Presumably it's
Critical speed 1 = 8,6 x √145 = 103 kt
Critical speed 2 = 7,7 x √145 = 93 kt

[Paul Lupp]

... and the runway surface type? Surely the amount of standing water etc also plays a part

[john_tullamarine]

Fella called Horne (he of the formula) started this stuff off 60 years ago - worth a read of the initial reports, such as https://ntrs.nasa.gov/archive/nasa/c...9640000612.pdf

[EXDAC]

not just pressure!

Originally Posted by john_tullamarine

Fella called Horne (he of the formula) started this stuff off 60 years ago - worth a read of the initial reports, such as https://ntrs.nasa.gov/archive/nasa/c...9640000612.pdf

"It should be pointed out that the hydroplaning speeds obtained from equations (3) and (4) are valid for smooth and closed pattern tread tires which do not allow escape paths for water, and for rib tread tires on fluid covered runways where the fluid depth exceeds the groove depths in the tread of these tires. Little quantitative data are yet available on the hydroplaning speeds for rib tread tires on fluid covered runways where the fluid depth is less than the groove depth of the tread."

So many seem to assume that hydroplaning speed is independent of tire tread design yet designers of road vehicle tires continue to develop tread designs optimized for water displacement.

[john_tullamarine]

Lots of stuff to read on road research into aquaplaning. See, for instance, https://scholar.google.com.au/schola...=1&oi=scholart

[tcasblue]

Originally Posted by DaveReidUK

I think your quote might have lost something in translation.

Presumably it's
Critical speed 1 = 8,6 x √145 = 103 kt
Critical speed 2 = 7,7 x √145 = 93 kt

Thanks,

I think the main point is that if you are under the impression that a simple formula will give you an accurate speed for your next landing, it won’t as there are many variables.

Not surprising really.

[tcasblue]

Perhaps I should have realized the obvious when it comes to reverted rubber hydroplaning but for some reason it seems like new info to me. Water is required for this kind of hydroplaning. To quote the same report...…..

"The scrub markings on the runway surface in the latter stages of the landing rollout indicate that substantial heat was generated between the tyre footprint and the runway surface, which can only occur when water is present on the surface. Tyres skidding on a thin layer of water heat up the water so that scrub markings form on the surface, and these could be associated with hydroplaning. "

Actually, I might disagree. The only time I saw flat spots on the tires on an aircraft I was crewing was a King Air where we had a frozen brake(s) and landed on a runway with 100% hard compact snow. But I suppose the snow melted then created the flat spot. Or maybe it was straight scrubbing of the rubber. Who knows, after all hydro does mean water.

The report with a link in my first post is actually quite interesting about hydroplaning and runway texture properties.

[tcasblue]

Do you know the ply of your tires? Apparently, it makes a significant difference in hydroplaning speed. The formulas mentioned above were developed by NASA on tests of bias ply tires. Radial ply and H-type tires have lower hydroplaning speeds. The formula may not be applicable to you.

“Recent studies into the critical hydroplaning speed were conducted by the National Aerospace Laboratory (NLR) in the Netherlands. “These studies were considered necessary as the data that was compiled by NASA was ageing and need to be revised to conform to modern new tyre types that were introduced for civil aircraft. The following three tyre types were considered;

bias-ply; radial belted tyre, and; the type-H tyres.

It was concluded from the analysis that the radial-belted and type-H tyres had a significant lower hydroplaning speed than the bias-ply tyre. This was caused by the difference in tyre footprint characteristics of these tyres.”

Below, a link to tire construction....

https://www.aviationhunt.com/aircraf...-vs-bias-tyre/