Why use a firm landing on wet runway

[TurboS7]

I agree with you 100%(about me missing the point too), another reason we have tread on the tires is to break through that film and give the water somewhere to go.

 

[DoinTime]

but again the correct answer to the original question is: a firm touchdown is necessary to break through the water just in case hydroplaning conditions are prevalent and a smooth touchdown would be enough to induce hydroplaning

A firmer than normal touchdown will only give you maybe 1-2 seconds of improved hydroplaning characteristics as the g load is dispersed through the shock absorbers. Once the all of the aircraft's kinetic energy has been directed down the runway instead of into it you are just as screwed as you were if you would have performed a silky smooth landing. There is nothing subjective about hydroplaning. It occurs when hydrostatic pressure meets or exceeds the weight that the aircraft is exerting on the ground.

Abbreviating your flare and landing hard should put your wheels on the ground closer to the threshold and give you more runway to utilize the wheel brakes and reversers. "Punching" through the film on the surface of the water is an old wives tale that should have died the first time along with bell bottom pants. Insects have a difficult time breaking through surface tension. Aircraft weighing tens of thousands of pounds do not.

 

[wil]

I love Flightinfo.com! Not only a great discussion of braking coefficients but donuts as well! Now, what if the runway in question was subjected to the beloved Krispy Kreme "glazing" waterfall? Would the tire tread be good enough to disperse the sugary goodness? Would wheel spinup for A/S be hampered? And, what is the surface tension of a freshly glazed runway? uuuuum freshly glazed runway!

 

[typhoonpilot]

When I was at WestAir years ago, my roommate was a former Ag pilot. He used to tell me how he would set the wheel brakes and go water skiing on a lake with the airplane. It seemed like a pretty far fetched story, but as long as the wheels didn't try to start rolling I figured it was possible if the pilot was a very good stick.

Lets translate that to this question. If the wheels don't break though the surface tension because of a very smooth touchdown then you will be water skiing in a jet. That wouldn't be much fun. DoinTime says it is an old wives tale about breaking through the surface tension. I guess he is smarter than the former Chief Test Pilot of the British Civil Aviation Authority.

DoinTime also mentions 1 to 2 seconds of improved hydroplaning characteristics. At 130 knots two seconds is close to 500 feet of runway. Not to mention what TurboS7 has pointed out about early activation of spoilers and autobrakes which could easily make the difference in stopping before the end of the runway or trickling off the end at 60 knots. I think I would like to have those two seconds right at the beginning instead of wishing I had them when approaching the end of the runway.

Typhoonpilot

 

[GCD]

There's a good article in the April edition of Airline Pilot Careers magazine which addresses a lot of this.

It will also be reproduced in the Climb And Maitain's June edition.

PS The hydroplaning is not a wive's tale. An FO of mine was so proud of the landing he made on a wet runway. You couldn't even feel the wheels touch. He was so proud of himself. When we got in the gate, I said, "Let's go see how much that landing cost the company."

We went to the mains to inspect the tires. All of the main tires had reverted rubber burns on them. Lesson learned on the company's dime.

 

[A Squared]

Typhoon pilot,

It is possible to waterski an airplane on the water. I know people who have done exactly that. It can be used to land on an otherwise too short sandbar in a river. You can put the airplane’s wheels on the water and start slowing the airplane down, so that you are at or below what otherwise would be the stalling speed as you roll up onto land. A few years ago, Mountain Pilot Magazine (now defunct) ran an article complete with photographs of this technique. Incidentally, I can assure you that you do not need to have your brakes locked to do this. Don’t ask how I know this.

Getting back the subject of the thread, this is a result of hydrodynamic planing, not surface tension. Hydrodynamic planing is what keeps a water skier up, or a power boat or jet ski "on the step" and on top of the water. Hydrodynamic planing results when the force required to displace the water exceeds the weight of the waterskier or boat, or airplane. It has nothing to do with surface tension.

Surface tension is a weak and fragile phenomenon. It is what allows a water strider insect to walk on top of the water or enables you to fill a glass slightly above the rim without spilling, or if you are very, very careful it enables you to leave a common pin suspended on top of water. Pour water into a glass until it is slightly above the rim. Touch the water with your finger and it will spill out, because the surface tension has been disrupted.

I’m sure that you have seen a water skier, and have seen his wake and the roostertail that he throws up. It’s pretty obvious that the waterskier has thoroughly disrupted the surface tension of the water, so it’s not much of a factor.

Now, it is possible, I suppose that in a thin film, surface tension (not hydrodynamic planing) has some effect on braking. I don’t know, I’m skeptical though. The fact that someone carries a title such as "Chief Test Pilot of the British Civil Aviation Authority" does not prove anything. It’s a bit like those idiots who try to prevail in an argument by claiming they have more flight time than someone else .... ummmso what? Does he have a rational explanation of the phenomenon which is solidly based on scientific fact? If so, let’s see the explanation, if not, well repeating someone’s job title is a pretty unpersuasive argument. Any explanation of surface tension which involves waterskiers is factually incorrect and serves only to illustrate a flawed understanding of surface tension and hydrodynamic planing.

 

[TurboS7]

Unless you have waterskiied with a Super Cub you aren't a real Alaskan pilot. My instructor at school grew up in Palmer, he showed us pictues of him doing that in a Cub. I guess when you are a teenager up there that is what you do for kicks.

 

[typhoonpilot]

Now that we have established that water skiing an airplane is possible and done fairly often in Alaska lets go back to the original debate. My point about water skiing an airplane was a corallary to what would happen if you touch down on a wet runway too smoothly. So let's ask the question, " what would happen to your Piper Cub if you touch down on the lake too hard ? "

I may not be able to explain the physics of that, but I think we can all picture it pretty well. Can anyone say, "cartwheel" ?


Typhoonpilot