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Wet runway ops

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Wet runway ops

Old 13th Oct 2013, 12:53
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Wet runway ops

Hi guys,

I have a question regarding the take-off on a wet runway. I know the ASDA will be longer due to a lower friction on the runway. That is because the energy is applied on the wheels and they could skid over the runway.

But how does a wet runway affect the start of the take-off? Since the energy is not applied on the wheels but on the plane itself, the lower frictioncoefficient should have no influence on the wheels. Still it seems that the aircraft needs more runway for the first part (before V1)

I would say that the runway friction coefficient should only affect the braking (ASDA) part, can anyone clarify?

thanks!

Last edited by p0ngkid; 17th Oct 2013 at 09:13.
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Old 17th Oct 2013, 09:14
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Can anyone clarify this to me?
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Old 2nd Nov 2013, 14:36
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You know what it's like when your car hits a puddle? It slows down due to the retardation effect of the water on the surface.

Same with an aeroplane on a wet runway.

There is doubtless a more technical explanation that someone with waaaaay more knowledge than I can provide.
 
Old 2nd Nov 2013, 15:37
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As far as I know it is only the stopping distance is longer, hence a lower V1.
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Old 2nd Nov 2013, 16:46
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First ASDA is always the same! This is a fixed value, unless the runways gets longer when it is wet
Your max take-off mass gets lower for the same runway length.

In order to make the MTOM penalty less, the usual screen height of 35 feet is reduced to 15 feet only, hence a longer part of the runway may be used for the takeoff roll up until VR.

Which is by the way interesting, when people say it's safer to take wet runway performance calculations for a damp runway, sometimes you end up assuming and derating even more.

Last edited by cosmo kramer; 2nd Nov 2013 at 16:50.
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Old 2nd Nov 2013, 18:11
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During the acceleration phase the coefficient of friction is not relevant.

What matters is the fact that the wheels must push the water out of the way in order to move forward. This requires energy. The fact that some of the available energy is being used to push water aside, means that there is less energy available to accelerate the aircraft. So the distance required to accelerate to a given speed is greater.
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Old 2nd Nov 2013, 19:21
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...come on. We are talking less than 3 mm water for a runway to be wet. Do you really think that would have a measurable impact on acceleration?
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Old 2nd Nov 2013, 19:55
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Possibly what the OP might be thinking about is why TORR and TODR increases with a wet runway.

On a balanced field, V1 is lowered to counter the increased stopping distance on a wet runway (your braking is degraded, so you must initiate the stop at a lower speed in order to accelerate and stop in a given distance).

Now, it is given that you must be able to continue single-engine from V1 (actually Vef, but let's disregard that difference for the matter of this discussion). The distance needed to accelerate from V1 to V2 is increased because the difference is larger, because the V1 is lower.

Dispatch rules for reduced screen height, as well as balanced RTOW calculations will probably give you some variations to this, but the concept is still that a wet runway gives higher ASDR, which is counterd by a lower V1 giving again a shorter ASDR, but the lower V1 gives you a longer TORR/TODR.
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Old 2nd Nov 2013, 23:47
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There is also the matter of the surface condition factors that must be applied when operating from grass strips.

For dry grass the factor is 1.2, but for wet grass the factor 1.3.

These factors do not apply just to the ASDR. They also apply to the TORR and TODR. Neither the TORR nor the TODR include any deceleration phase, so the factor must be to take account for some decrease in acceleration.

If the water cannot reduce the acceleration rate, why is the factor for wet grass greater than that for dry grass?
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Old 3rd Nov 2013, 00:00
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Takeoff/Landing on Wet, Contaminated, and Slippery Runways.
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Old 3rd Nov 2013, 00:32
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Who operates FAR 25 aircrafts on grass strips?

But anyway, grass has to move out of the way, wet or dry. Wet grass is apparently heavier. But what does that have to do with a paved runway, where nothing has to move out of the way? Apples and bananas.

Are you seriously thinking that 0-3 mm water will have anything other than a very theoretical minuscule effect on acceleration?
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Old 3rd Nov 2013, 02:42
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Who operates FAR 25 aircrafts on grass strips?
Quite a few places around the world.
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Old 3rd Nov 2013, 09:00
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Cosmo,

you are reading far more into the original question than is actually there. To aid you I have copied the entire question below.

Hi guys,

I have a question regarding the take-off on a wet runway. I
know the ASDA will be longer due to a lower friction on the runway. That is
because the energy is applied on the wheels and they could skid over the
runway.

But how does a wet runway affect the start of the take-off? Since the energy is not applied on the wheels but on the plane itself, the lower frictioncoefficient should have no influence on the wheels. Still it seems that the aircraft needs more runway for the first part (before V1)

I would say that the runway friction coefficient should only affect the braking (ASDA) part, can anyone clarify?

thanks.
Now if you can manage to set aside your dismissive attitude for a moment you might help us by pointing out where exactly the OP includes either FAR 25 aircraft or paved surfaces.

You might also explain how your statement that

But anyway, grass has to move out of the way, wet or dry. Wet grass is apparently heavier.
Differs significantly from my original statement that

the wheels must push the water out of the way in order to move forward. This requires energy. The fact that some of the available energy is being used to push water aside, means that there is less energy available to accelerate the aircraft. So the distance required to accelerate to a given speed is greater.

In response to your question


Are you seriously thinking that 0-3 mm water will have anything other than a very theoretical minuscule effect on acceleration?
I can only say that it is not a matter of what I think. The people who decided that a factor of 1.3 must be applied to wet grass clearly thought that 3 mm of water would make a significant difference. I suspect that they knew far more about this subject than you or I do.

Last edited by keith williams; 3rd Nov 2013 at 09:55.
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Old 3rd Nov 2013, 15:26
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Now if you can manage to set aside your dismissive attitude for a moment you might help us by pointing out where exactly the OP includes either FAR 25 aircraft or paved surfaces.
Runways are paved, as opposed to grass strips.

How many Cessna 152 operate with V1? None.
How many FAR 25 aircraft operate with V1? All.

Still it seems that the aircraft needs more runway for the first part (before V1)
Now how many airliners operate on wet grass again?

If you want to discuss leisure flying, why not (although I believe the first P in PPRuNe stands for Professional). But then maybe start another thread?
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Old 3rd Nov 2013, 15:35
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Anyway Bfisk, nailed the answer to the question. So I guess there is nothing more to discuss.

Except for FAR 25 aircraft, wet runway does not have an impact on acceleration prior to V1. A contaminated runways does (e.g. standing water, more that 3 mm).
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Old 3rd Nov 2013, 17:29
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Runways are paved, as opposed to grass strips.
Well I'mlooking at the data for Alderney airport and it includes the following

RWY 03/21Grass
RWY 08/26Asphalt/Grass
RWY 14/32Grass

They are all called runways but they all include grass.

It looks like you have just made up the idea that all runways must be paved.


How many Cessna 152 operate with V1? None.


Not allFAR 23 aircrfat are Cessna 152s. FAR 23 and its (EU equivalents) also cover anumber of twin engine aircraft. These all have V1 values.

CS23.51 (theEU equivalent of FAR23) includes the following for commuter category light aircraft:

(1) V1 must beestablished in relation to VEF as follows:


Howmany FAR 25 aircraft operate with V1? All.

But that does not mean that no FAR 23 (or EU equivalent) aircraft have V1 values. (see the quote from CS23 above)


Now how many airliners operate on wet grass again?

I don't know, but there are certainly some.

[QUOTE]If you want to discuss leisure flying, why not (although I believe thefirst P in PPRuNe stands for Professional). But then maybe start another thread?[/QUOTE

Now youappear to be suggesting that people such as flying instructors, flight test examiners and air taxipilots are not professional pilots. I'm pretty sure that they would disagreewith you.

Come onCosmo! The simple fact is that you jumped to the conclusion that the questionwas exclusively concerned with airliners operating from paved surfaces. Your answer was correct for that situation, but ignored the other possibilities.


The bottom line is that wet paved surfaces will not affect accleration, but wet grass surfaces will do so. The OP asked why/how the wet surface affected acceleration, so we could reasonably assume that he/she wanted to discuss the situation in which this was true.


Last edited by keith williams; 3rd Nov 2013 at 17:34.
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Old 3rd Nov 2013, 18:59
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Where did the original poster ask about GRASS?
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Old 3rd Nov 2013, 19:19
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He/she did not refer specifically to grass.

But he/she did say the following:


Still it seems that the aircraft needs more runway for the first part (before V1)
That statement is true only when operating from grass. So if you had taken the time to read and think about the question, you might have spotted that fact.

But that would have required attention to detail and a little bit of patience...qualities which do not appear to be your strong points.
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Old 3rd Nov 2013, 19:38
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Very attentive of you. I am sure he appreciates your explanation about wet grass. Seeing he is from Holland, do you have values for wet tulips too?
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Old 3rd Nov 2013, 19:56
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Except for FAR 25 aircraft, wet runway does not have an impact on acceleration prior to V1. A contaminated runways does (e.g. standing water, more that 3 mm)
Summary of Falcon 20 Contamination Drag Results, 1996 to 1999. J.C.T. Martin. Transport Canada, Aircraft Certification, Ottawa, Ontario. PROCEEDINGS of the 2nd International Meeting on Aircraft Performance on Contaminated Runways

During takeoff on a contaminated runway, aircraft acceleration is reduced and takeoff distance is increased due to:
displacement of contaminant
impingement of contaminant on aircraft

Contamination drag is the sum of the displacement drag and the impingement drag.

Contamination drag is a function of Equivalent Water Depth (EWD). For the Falcon 20, DCONTAM is negligible below 0.1 inch EWD (approximately 3 mm). And then it rises and becomes an issue above 3mm.
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