Why use a firm landing on wet runway

[getonit]

I have heard that when landing a jet on a wet runway you should use a firmer landing. Is that true and if so why? I can't find anything that explicilty says to do that, I have the formula for hydroplaning but nothing there that helps. Only thing I could think of (ie guess) is it might compress the tire more and get more surface area but I don't see how that would help.

 

[Andy Neill]

Remembering that the Force of Resistance for Friction (Fr) is equal to the coeeficient of friction (u) times the Normal Force (Fn) or the force pressing perpendicular toward the surface, I can see where a firm landing will minimize the time where lift is reducing Fn making the Fr (the force necessary to overcome the force that keeps your tires effective in keeping you on the runway) relatively low.

Fr = Fn * u

 

[avbug]

Also, it aint' gonna slide around so much.

 

[C172Heavy]

Although it seems if you smack too hard on the runway the frictional force would be high for a second and then when the plane started to rise up again the force would actually be much lower again for a while whether you bounce or not.

 

[avbug]

The goal isn't to bounce. It's to put the airplane down and keep it there.

As the mains touch, the nosewheel is lowered smoothly to the surface while deploying reverse thrust. As the nosewheel is lowered, angle of attack is reduced, settling the weight of the airplane on the wheels.

Another neat trick is to make all the passengers jump in the air just before touchdown, such that they land just as the mains touch.

At least three participating ICAO countries are experimenting with giant tracts of fly paper on the runway. This greatly increases the coefficient of friction, and to date, not a single case of a fly hydroplaning has been observed.

 

[bigD]

At least three participating ICAO countries are experimenting with giant tracts of fly paper on the runway. This greatly increases the coefficient of friction, and to date, not a single case of a fly hydroplaning has been observed.

This is SO impractical. Everyone knows that the solution is to cover the runway with uneaten cake donuts. By my math, it'll take just over 12 million cake donuts to cover a 9000'x150' runway.

 

[avbug]

Plain might work, but if it's got sprinkles, glaze, or powdered sugar, the coefficient of friction of donut (Dfc) goes down considerably, especially in the presence of rain, milk, or any heavy weight petroleum oil.

For gear up emergencies, or emergencies in which potential threat to life or property may exist due to fire, hazmat spill, or starvation, covering the runway in day-old donuts may be a very appropriate soloution.

 

[Brother Francis]

Donut etiquette

You get a bunch of pilots around 12m donuts and within 3 minutes there will be 1/4 donut left. (No one wants to be the pig that eats the last one so they break it in half, then someone breaks the half in half because they don't want to be the pig either. Pride overcomes greed with the remaining 1/4.)

Uneaten donuts; phffffyeahright Now I've heard it all.

As an aside:

Avbug- My hat is off to you. I have been in this business in various capacities for 25 years. I learn something from you every time you post.

 

[TurboS7]

You need to get the wheel spin up as fast as possible so that the anti-skid will be working properly and immediate braking can occur. This also insures operation of the auto-spoilers, and auto-brakes if they are installed.

 

[dogman]

A firm landing will also disssipate your kinetic energy faster.

 

[typhoonpilot]

TurboS7 has the right answer but not the "why?" that you asked for. The " Why ?" is that you want the tire to break through the film of water on the runway. A smooth landing can result in the tires remaining on top of the film of water on the runway. This would be most dangerous on an ungrooved runway.

Further to donut discussion, just think about the total lack of friction if the afformentioned cake donuts were replaced with those greasy KK donuts


Typhoonpilot

 

[avbug]

All joking aside...this is a serious and important subject.

Would the Dfc change if the donuts were served hot?

Remember that hydroplaning is a function of speed and tire pressure. Forcing the airplane onto the runway doesn't obviate the basic physics of what is happening.

A much more effective method is to use snow tires. These tend to break the surface tension much more effectively, allowing instant traction. So there you have it. Snow tires, or chains. Studs are optional.

 

[Sctt@NJA]

A non-donut reply

I think the reason is much simpler. Trying to land real smooth eats up runway. (holding the airplane off the runway getting your vertical speed nearly 0 so that you can barely tell you touchdown, meanwhile perfectly good runway going behind you.)

Anytime you have less than tons of runway (and a wet runway is like a shorter runway) you are better off just "putting it on the ground". It probably wont be the smoothest landing, but it will give you more runway to stop on.

 

[TurboS7]

The real probem is tire reverberation. The film of water can build up between the tire and the runway, the water heats up and acutally melts the rubber on the tire resulting on it becomeing unairworthy. I had a problem with a Lear one night landing in the PHX area. It was Christmas Eve and we were flying from Denver to PHX area landing at Falcon Field. It had been raining all night and I shot the ADF approach with a circle to land. The approach was perfect but just at I landed I realized that there was 6 inches of water on the runway. Water was everywhere so I shut down both engines so I wouldn't FOD them, no reversers on a LR-24. At the same time I extended the spoilers and got on the brakes. To my amazement the aircraft just kept going, finally I blew three tires which brought me to a stop. I borrowed a jack and moved some tires around and go the aircraft off the runway. Upon examination of the tires I found that they had totally reverberated right through all the plys. Upon further investigation I found that the mechanic who put the tires on had torqued the tires improperly preventing wheel spin. On the water the wheels didn't spin up the anti-skid didn't work hence my Learjet suddenly had skis. If the tires hadn't blown we would have gone right off the runway. Sometimes as a pilot you can't control all your destiny.

 

[typhoonpilot]

Sctt@NJA makes a good point, but again that is not the reason that a firm landing is recommended. TurboS7 has a good story about reverted rubber hydroplaning, 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 very good book to have in your aviation library is " Handling the Big Jets " by D.P. Davies. It is quite old but the information is timeless when researching answers for questions like this. The only place you can get a reasonably priced copy is through Transair Pilot Shop in the U.K. They have a website.

Typhoonpilot

 

[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.