Landing in wet - is it really that different?

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JonP01
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Landing in wet - is it really that different?

Post by JonP01 »

As a lifelong motor racing enthusiast and hardcore race simmer, I obviously know a lot about the dynamics of tyres when attached to cars running around on tarmac - whether wet or dry. Obviously in motor racing, depending on the era and the category, mechanical grip may or may not be the dominant factor. Aerodynamics has certainly played an increasing role in the sport but it has never really been about aircraft flying around upside down :wink: For the most part anyway.

So it was with great surprise that I saw a lot of comments relating to a YouTube video I recently saw. It showed a turboprop making a seriously hard landing onto a wet runway. My first impression was the pilot just got caught out by the wind - afterall, I think any pilot who says there is not at least certain amount of "luck" in making a perfect butter landing in gusty, stormy and unpredictable conditions - is probably kidding themselves to some extent.

But then I saw some comments that the pilot did the right thing. By basically forcing the aircraft onto the runway rather than with the traditional roll out, flare and just letting the lift ebb away for a smooth touchdown. The reasoning was that you need the tyres to "bite" into the runway immediately that make contact with it so as to avoid skidding around on the wet surface.

I thought a bit more about all of this but I am none the wiser. I would have thought (admittedly as a non-pilot) that landing an aircraft embraces a different set of principals to wet weather adhesion in motorsport. After all, when you touch that runway, you have only just passed the point where the amount of lift provided by the wings is sufficient to lift the weight of the aircraft. That is why it lands!! But the thing is - you have only JUST passed that point. By literally walking pace. At that point, despite the deceleration, you will still have a very high degree of rudder authority for starters when you have touched the ground - even with all wheels. And other control surfaces will still operate to combat crosswinds. I mean, for instance, even student pilots are taught to hold the control surfaces in particular configurations when taxiing in wind - at speeds far below V1, for instance.

So I can understand the idea behind the theory but I have to wonder how relevant it is. Basically what I am saying is that you still have control of the aircraft in an aerodynamic sense. It is only when you have dramatically slowed down that mechanical grip becomes the over-riding factor. And by then the aircraft weight would have well and truly settled on the tyres. I can certainly some point to this if there is a fear of a severe cross-wind gust but even then I wonder about the wisdom of all but smashing an aircraft down rather than just landing the normal way.

What is the truth here? Do real world pilots use a dramatically different landing technique in the wet?

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CAPFlyer
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Re: Landing in wet - is it really that different?

Post by CAPFlyer »

What you're forgetting is hydroplaning/aquaplaning. A landing airplane is more like a boat than a car. Remember, the wheels are Not spinning prior to touchdown. If you were to "ease" onto the runway in that condition, you almost guarantee that the tires will hydroplane, thus you have no traction and no directional stability as you slow. Yes, you may have rudder authority right on landing, but as you slow down, that authority goes away and now you have to rely on the mechanical traction - that doesn't exist if you're hydroplaning. So think of a high speed boat moving along the water (and skipping off the top of it) instead of a canoe that is plowing through it at slow speed. The directional stability about the same too - the high speed boat is on edge and could pretty easily loose control if one thing got upset while the canoe is very hard to loose control of.

Anyway, here's some resources on the concerns of hydroplaning on landing -

https://www.faasafety.gov/gslac/ALC/cou ... eview=true

https://www.boldmethod.com/blog/lists/2 ... vent-them/

https://www.youtube.com/watch?v=3A88r6dgI10
Image

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AKar
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Re: Landing in wet - is it really that different?

Post by AKar »

In case of standing water (or risk thereof), it is rather common procedure to land a bit firmer. This is to avoid hydroplaning which can result in poor and/or uneven braking action.

Also, some airplanes rely on wheel spin-up for air-ground logic sequencing, and delayed wheel rotation can result in less than optimal aircraft behavior on touchdown.

-Esa

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Oracle427
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Re: Landing in wet - is it really that different?

Post by Oracle427 »

Yes, for sure. When in motion, the wing is taking weight off the wheels and the contact patch of most landing gear is very small to begin with.

A wet surface is very slippery on landing until you are nearly completely stopped. Standing water is nasty stuff as hydroplaning is all but guaranteed.

Aerodynamic effectiveness of controls diminishes rapidly as airspeed bleeds off and also depends on engine type on position and control surface positions relative to engine thrust and free airflow. You also have to consider other systems such as Beta, reverse, spoilers, anti-skid, etc. that more sophisticated aircraft possess that all simplify the pilot's workload.

Recently while flying a C170 on wet surfaces, in direct crosswind conditions of about 15 knots I observed that I would not maintain alignment with the runway at all, nor could I stop side drift with nearly full rudder and full aileron into the wind on takeoff! This means that I had full power and full thrust and airflow over that rudder and still could not maintain directional control until I accelerated to the point where the tail came up off the ground and the rudder became much more aerodynamically effective. I was actually initially crabbing, then sideslipping down the runway with the wheels "on the ground" until I lifted off. I did this three times before knocking it off.

On each landing, I could not easily side drift either and had to keep jockeying the throttle to keep the nose pointed where I wanted it to try and correct for drift all the while holding full aileron into the wind. Turning was difficult without weathervaning even at a walking pace.

All of this was an a wet asphalt surface with no standing water, but shortly after a rain and it was still misty. The aircraft has Alaskan Bushwheels which are not known for having good traction to begin with. :)

The plane would have had excellent control in the same windy conditions on that same runway had it been dry.

The other pilot/owner of the plane switched with me and I watched the wheels on each landing and noted that even without applying brakes, the wheels generally were not turning very much. I would say half the time they were just sliding around and barely rotating, if at all. If any brakes were applied, the wheels instantly stopped rotating.

Wet surfaces definitely affects how you takeoff and land.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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DHenriques_
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Re: Landing in wet - is it really that different?

Post by DHenriques_ »

Had a show back in the early seventies requiring me to stage from a small grass strip in Pa. I arrived about an hour after a storm had gone through. The field was short. I dragged it in low and slow (a Mustang) and dropped it in as a stall landing. The grass was wet. I realized immediately I had no braking ability due to hydroplaning. It took a lot of effort to keep things under control but I managed it. Fortunately the wide gear played well and final settled in for me. I had excessive speed and no runway left so I powered it around to the left side allowing a wide ground loop.
Yes, I can tell you for a fact that landing on a wet surface DOES have it's problems. :-))
Dudley Henriques

JonP01
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Re: Landing in wet - is it really that different?

Post by JonP01 »

Thanks so much for the replies everyone. They were very informative and have obviously settled this conundrum for me. I will change the way I land in the sim in the wet now! I have obviously been getting away with it. Maybe in a sim, but not in real life!

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