Wet Runway

[G-ALAN]

Dunno if this belongs in Tech Log but anyway my question is this. On a wet runway the aircraft will have a longer landing roll right, why is that? My instructor said it's because of the aquaplaning effect however logic dictates to me that the drag from the water would slow the aircraft down and therefore give it a shorter landing roll or at least cancel out any distance added by the aquaplaning effect. I asked my instructor this and he just said because it doesn't perhaps he thought I was being a smart ass but anyway I'm curious and need answers.
Cheers

[100% N1]

A wet runway will have a much lower coefficient of friction between the ground and the tyres than a dry runway will. This means that you can't apply as much brake pressure when on a wet runway or you'll lock the brakes and start skidding.

[NigelOnDraft]

G-ALAN...

Here's a way to check it out.

On a nice dry day, accelerate your car to 70mph. Next to a marker post hit the brakes - HARD. Try to avoid locking the wheels - or preferably, use a car with anti-lock brakes. Now measure how long (in yards) it took to stop.

Now move your post to this many yards from a brick wall. Wait for a nice wet day (plenty of those at the moment). Accelerate your car towards the brick wall and hold a stready 70mph. When you get to the post, brake as hard as you can - again, try to avoid locking the wheels.

Let us know how you get on!

NoD

[G-ALAN]

Thanks 100% that makes sense

NoD: Just tried it and had a great big bloody crash, it doesn't matter though because I used your car since mine doesn't have ABS, I hope you don't mind

[FlyingForFun]

Presumably, then, the landing distance for an aircraft without brakes (let's say a Tiger Moth so we've got an example that we all know) won't be similarly affected by the wet surface? And G-ALAN's initial assertion that the water would caused increase drag and stop the aircraft in a shorter distance - would that assertion hold in this case?

(Let's ignore the fact that no one in their right mind would want to fly a Tiger Moth in the rain!)

FFF
-----------

[SP]

Landing distance without brake on a wet runway should still be greater than the landing distance without brake on the same dry runway. As mentĦoned previously the friction coefficient on a wet runway will be less than on a dry runway resulting in a lesser friction force hence longer distance to stop.
Cheers

[Midland Maniac]

FFF,

I certainly think that if you land in water the size of the Atlantic, I am sure it would stop a Tiger Moth. The amount of standing water you get on rumways in this country certainly won't stop you any quicker, although we do have grooved runways now which to improve braking in the wet. According to JAR-OPS a 'grooved' WET runway, is classed as a DRY RUNWAY for performance calculations!!!

[None]

"According to JAR-OPS a 'grooved' WET runway, is classed as a DRY RUNWAY for performance calculations!!!"

Same with the FAA as long as the depth of the water is less than 0.1 in. or 2.5mm.

[dogcharlietree]

FFF. Re Tiger Moth. It may in fact stop in a shorter distance in the wet. Take dirt that is baked rock hard by the sun. Tail-skid friction on this surface is like that of a sealed surface.
However, after a good rain, the soil is a lot softer, so on landing, Pull the control column right back and the tail-skid digs into the soft soil. Just like a carrier landing

[error_401]

The reference your instructor made to aquaplaning is not quite correct while 100%N1 gave the best answer.

When it comes to water on runways what counts is friction coefficient.

A dry runway obviously has the best friction coeficient.
When measuring friction when rain starts usually you see a degradation in friction because of the mixing of water with dust and other deposits on the surface to form a slippery, muddy mixture. Depending on how much deposits you have, it can go up to viscous aquaplaning a term describing a situation where you have almost NIL friction coefficient. (As slippery as ice or snow) Not being absolutely correct in terms of aquaplaning it describes a situation where you lock the wheels when you just think of applying pressure to the brakes. This also applies to wet grass runways...

With continuing rain the friction coefficient usually rises again when all the mess has been washed away and can reach relatively high values again. - Still stay on the safe side and make calculations conservatively. (I love the illustration of NigelOnDraft )

Grooved runways have a high friction coefficient even when wet but that is more a question of texture. A coarse surface brakes better if you have rubber tires. Ask the motorcylce guys about abrasion of different road surfaces...

Aquaplaning is: A situation where you have so much speed AND ENOUGH WATER as to lift the tire completely off the ground. At the onset of aquaplaning you may experience partial aquaplaning (may be the case on a take-off). In a total aquaplaning situation (usually on landing) your friction coefficient is NIL! It may even be the case that wheels do not start turning at all! The problem with real aquaplaning is that it may persist even below the onset speeds.
Again - braking is equal to black ice which means nearly non-existent!
Once you slow down below a certain speed (which can not be determined and may be as low as taxi speed) you will brake normally as if on a wet runway.
Factors are mainly Water depth and speed. To make it a bit more relative - you need quite a lot of water on the surface to get aquaplaning. (Usually referenced as: Standing water)

As for drag: Aircraft tires have a nearly ideal form to reduce drag in water. The footprint looks like a boat and the tire too. So drag is not really helping when it comes to braking unless you put your plane down in knee deep water. Usually those who tried, got rid of their landing gear. If that helped in drag I don't know

More questions on aquaplaning - just ask.

RE: Tiger Moth

Add a plow - that adds braking force on hard baked dirt strips

[G-ALAN]

Thanx for replies. Thanx error_401 t'wos the sort of reply I was looking for.