Aquaplaning speed

[Bear Cub]

Does anybody out there fly any type of a/c that has a real time read out of the current tyre pressures?

I approached Ronaldsway recently in heavy rain, "runway wet, wet, wet" (the radar man said) - and the phrase "nine times square root of tyre pressure" sprang to mind.

Now if only I had taken a pressure gauge to the tyres on my walk round....

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[Wee Weasley Welshman]

Moved to Tech Log.

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[SLT]

Cub, you are absolutely correct re. the 9x sqaure root of tyre pressure etc., but I know of no a/c with the readout you mention. I would imagine that if such an animal exists, it would be some brand new all singing all dancing job that most of us have never seen. I know that the Airbus doesn't have it, and I don't think the Boeing does either.
I would imagine that you best policy (as I'm sure has already occurred) is to use the tyre pressure that the book says your tyres should be inflated to - I'm sure they won't be that far off. Personally, I would work it out for whichever set of tyres (mains or nose) has the lowest pressure and use that speed - thereby ensuring you maintain directional control.
I hear the rain in the IOM has been quite something - no change there then!!

Hope this Helps

SLT

[This message has been edited by SLT (edited 04 October 2000).]

[Silky]

Dont forget the nose wheel..... square root of the pressure by 7

[Royan]

The A340 has this option and it shows the tire press. For practical purposes you should have in mind the aquaplaning speed ,and it is calculated as follows;
7.7x squqre root of the main tire pressure. when you depart your tire pressure is correct hopefully because it is part of the checks done by maintenance and the pilot just has to satisfy himself during walkaround that it is o.k otherwise he can have it checked . So for example the aquaplaning speed for the L1011 is 112 knots. I hope this will be an answer to the inquiry , the tire press you can get from your Manual .

[Moonbeam Purple]

As far as I know it's also an option for the B744(Tirepressure indication).

Moonbeam

[This message has been edited by Moonbeam Purple (edited 04 October 2000).]

[mutt]

Don't have the books here, but i thought that tire pressure was displayed for both the B744 and B777.

Mutt

[Bear Cub]

Wonder if we could get Boeing to talk to Piper - the snag was that I was in a Piper Seneca V...it just made me wonder, that's all.

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[CCA]

TPIS - Tire Pressure Indicating System.

It's an option on the 744 and I think standard on the 777 as CX chooses to have the bare minimum on their 400's, (No Auto-start, No taxi light just turn-offs, No PVD, No Headrests, No Full face Oxy, No Therapeutic Oxy, No lower Weather radar they have a torch, No Elec Driven Hyd pumps on 2 & 3, No TPIS, No Doors Auto/Man Indication even though the system is fitted when the A/C is built the wires just aren't hooked up, Plus the bare minimum on the PFD, ND and EICAS).


[This message has been edited by CCA (edited 05 October 2000).]

[Oz_Pilot]

We've had 7.7 and 9 times sqrt tyre pressure... one of them is dynamic and one is static (i.e. wheel not turning) but which is which??

[Bear Cub]

Oz....I believe the Americans use 8 and a bit.

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[Cornish Jack]

We use 9sqrt P, so, for the 47-400 the speed comes out at 127.27922061357855439215198517887 kts - approximately :): :):

[reynoldsno1]

It's 9 times for dynamic aquaplaning for all you Romans (or hydroplaning if you are of Greek descent)...
To be strictly correct you should also multiply the result by the specific gravity of the contaminant, so if someone salts the runway....

[Bear Cub]

OK...so how about adding the runway specific gravity to the ATIS?

This could be the start of a job creation scheme - get some person to go out and take three readings every 30 minutes (they won't block the runways for long).

....but it still comes back to knowing what the tyre pressures are - and are they affected by regional QNH?

How about more job creations...as well as waiting on stand for the refueller, you could all wait for the tyre pressure man to come along with his pocket pressure gauge from Halfords and take the readings.

Another little placard to put on the pedestal with the "cabin secure" card - just for tyre pressures.

Wonder how long it would take on a 747 with..what, is it 18 or 20 wheels?

P.S. .... yes, I'm teasing.

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[Tor]

Could someone explain why aquaplaning speed is related to tyre pressure only?

The mass of the A/C must go into the equotation aswell - right?

Tor

[jtr]

As weight increases, the tyre bulges thereby increasing surface area touching the runway/water, and thus maintaining the same lb/sq in. irrespective of weight.

Sounds a bit sus to me, but that's what I was told by someone who should know.

[Tor]

Ok - sounds reasonable.

So this speed, if you are above it the A/C will aquaplane and if below not? Or how does it work?

Tor

[jtr]

I think you've got it. I read an accident report/inquiry, and this NASA guy was spouting them (the formulae) in the courtroom, he had spent all his life investigating this sort of stuff, and used them as if they were the law.

[tailwinds]

tyre pressures are required to be checked as part of the daily. this is in the tech log on our company aircraft and can be referred to. question is , how does the temperature exposure at cold and high altitudes affect tyre pressure on a long flight (cold soak). so without a gauge indication, tpis, the calculations are all out the window. if you are not happy, maybe use a low pass and afterburners to dry out the runway. :) or find a dry place that has a good pub.

[alosaurus]

Where does tread/water depth come into all this?Can 1mm of water 4 mm tread depth have the same Vp as 3mm and bald tyres?

[Checkboard]

The closest I have seen to an original source, and the numbers that seem to be quoted by most pilots:

Aquaplaning can occur when running in water and in cetain depths and densities of slush. Present indications are that simple dynamic aquaplaning is unlikely to occur in water depths much below 0.2 in. although under some conditions the minimum depth may be as low as 0.1 in. Once aquaplaning has commensed it can be sustained over areas where the water depth is less than that required to initiate aquplaning and to lower speeds that that required for initiation.
A simple formula has been derived from data obtained during trials which shows the relationship between the minimum speed at which aquaplaning may commence and the aircraft's tyre pressure. This states that the minimum initiating aquaplaning speed in knots is approximately nine times the square root of the tyre pressure in pounds per square inch. Speed in this context is of course in context of true ground speed and not indicated airspeed, although it not be precisely calculated...

HANDLING THE BIG JETS D.P. Davies
Page 188
(Typing errors are mine)

There was an old thread on this topic that had a link to a Dunlop site that was unteresting, but was lost in a server upgrade (sorry!)

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Tech Log forum moderator

[OverRun]

Checkboard,

Fully agree with your post and J P Davies. I've spent the last couple of days since this thread re-activated trying to internalise this. For a while there, I got lost in images of a military or business jet aircraft with small high pressure tyres slicing through standing water, while the big low pressure tyres of a widebody aquaplaned on top. That didn't help me internalise it very well.

Finally gave up, pulled out the books and built a spreadsheet to run some numbers. I used the L1101 and the BAC 1-11 (since I've got the data for both). Both run similar tyre pressures of 175/174 psi respectively (about 1200 kPa). At MTOW, the large main gear tyre on the L1101 has a footprint of 279 square inches, and on the BAC 1-11 the main gear tyre has a footprint of 136 square inches. Both have the calculated contact pressure of 175/174 psi (the same as the tyre pressure). Both should encounter dynamic aquaplaning at 119 knots and static aquaplaning at 102 knots.

Take the load off both planes, and the weight/tyre decreases. The radius of contact (and we assume here a circular contact area aka the footprint) decreases, and the footprint of the tyre decreases. Calculate the new ground contact pressure, and it remains the same at 174/175 psi. For the BAC 1-11 at 50,000 lbs weight, the contact area is down to 68.25 square inches, and the contract pressure is 174 psi. L1011 at 200,000 lbs has a footprint of 135.71 square inches and a contact pressure of 175 psi. In all the weight changes, the aquaplaning speed doesn't change.

One can intuitively visualise the onset of aquaplaning. The water is trying to push the tyre upwards off the runway, and this is being resisted by the weight on the tyre pushing down, together with the area (or footprint) of the tyre. These two are brought together as a single expression in terms of the contact pressure. The contact pressure is the same as the tyre pressure. So the aquaplaning speed is therefore represented by a function of the tyre pressure.

[Bear Cub]

Wel....I'm glad I asked.

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[reynoldsno1]

The discussion so far has focussed on dynmic aquaplaning...

There is, of course, viscous aquaplaning and reverted rubber aquaplaning. The former normally occurs on very smooth surfaces at low speeds. The latter can result from locked wheels superheating water and causing the tyre rubber to melt and trap steam beneath the tyre.