Hydroplaning prevention technique

[punkalouver]

An article I am reading on hydroplaning in Business and commercial aviation states thar "some airlines under wet conditions instruct their pilots to land at a higher sink rate to get the tires spinning up".

Is this a policy at your airline or do you do this on your own to ensure wheel spinup?

[Old Smokey]

The 2 airlines and 2 aircraft manufacturers that I've worked for / done work with, never recommended 'Thumping it on', a composite of all of their recommendations was much more like -

Do not attempt for a smooth landing on a Wet Runway.

Use the normal sink rate, but flare just a little less than normal to ensure positive contact.

Regards,

Old Smokey

[flying_elvis]

Max Auto Brake and Arm the spoilers, fim touchdown. The objective is to get below hydroplaning speed which is something like 9*sq root of tire pressure. I think is about 110 or 120 kts typically.

Always remember...aileron into the wind and step on the centerline.

[Blip]

If I may say so...

It makes no sense to me selecting a high autobrake setting when landing on contaminated runways, especially when there is any crosswind involved (which is virtually all the time if the wind is not calm).

By selecting MAX autobrake, you are certain to activate the antiskid system which indicates that the tyres are right on the edge of their ability to apply force on the runway surface. By increasing the braking force of the tyres to a maximum, you have reduced the cornering capability to virtuallly zero!

Imagine you are in a car, hurtling down a two lane road (one lane each way) at a speed way above that which would cause aquaplaning, say 130 km/h (never mind 130 kts!). If you come across a section of the road which is contaminated with 13 mm of standing water (maximum allowed by Boeing) or slush or snow etc, I can imagine the thought would cross your mind " I really shouldn't be going so fast!". But to reduce speed, do you really think it is wise to jump on the brakes as hard as you can and put all your faith in the anti-skid system. No way!. As soon as you do that, you're going to loose all cornering capability, and slide off the side of the road and hit a tree, or drift into oncoming traffic.

The best thing you can do (to maintain directional control) is to take your foot of the brake and throttle, put it in neutral, and let drag slow you down. Once your'e below the aquaplaning speed, perhaps then you might consider applying light pressure on the brakes, but certainly not enough to risk activating the antiskid modulation.

If I'm landing a B737 on a dry runway with a touchdown speed of 135 kt, with autobrakes 1 set, the ground roll to a stop will be around 1950 metres (as per the performance manual). Initially with reverse thrust and spoilers extended, the deceleration is virtually wholy achieved without the use of wheel brakes! Only in the later part of the ground roll when the reverse thrust and spoiler become less affective do the wheel brakes start contributing to maintain a constant deceleration.

The point I would like to make here is that quite often there is enough runway length to select Autobrake 1 and pull up with a comfortable margin. Even if 1 doesn't do it for you, Autobrake 2 would. If this is the case, why would you select Max autobrake to land on the same runway when it is contaminated and risk sliding off the side of the runway?

Remember that if you do start to slide to one side of the runway, you will have to release the wheel brakes, reduce reverse thrust to idle, and then try to regain control and get back on to the centreline before applying braking again. How much runway is that going to cost you?

Wouldn't it be far better to keep the wheel braking to the minimum extent possible (thus maximising cornering capability and directional control), and decelerate at a rate that will still have you pulling up by the end of the runway, plus a reasonable margin as previously calculated by reference to the performance manual before comencing the approach?

(edited for the inevitable typo errors)

[4PW's]

That's well put together, Blip.

[tribo]

At the end of report in this link you will find the Boeing article "Landing on slippery runways" from 1992 dealing with the subject.

http://www.rvtv.nl/data/201201_1.pdf

Hydroplaning and NASA

3 short movies at this link:

http://lava.larc.nasa.gov/BROWSE/hydroplaning.html

[Blip]

Thanks for the links tribo.

I found the report from the Dutch transport safety board interesting, both for the circumstances contributing to the accident, and also the reference to the Boeing article.

I suppose the point I found most interesting about the accident and which has occured to me in the past is that the braking condition of the runway is very often not consistant along the entire length of the runway. In this case Runway 01L offered good/medium braking action at the start of the landing roll and virtually zero braking action toward the other end. So in those circumstances perhaps it's better to "make hay while the sun shines". In other words, if they had slowed to taxi speed before reaching the last third of the runway, we wouldn't be reading about them now.


Reading the Boeing article raised a question in my mind.

There is a graph on page 86 of the accident report (page 10 of the Boeing article) and on page 88 of the accident report (page 12 of the Boeing article) which shows the braking and cornering capability of the tyres when compared to the tyre slip.

What I don't undertand is why the braking capability is zero when the the tyre slip is zero, but at the same time the cornering capability is high. Wouldn't the braking capability be the same as the cornering capability. ie the tyres offer the same grip in all directions. How much slip is there when autobrakes 1 is selected on a dry runway? Virtually zero I would have thought. What about when the aircraft is stationary with the park brake set. No slip there either, however the tyres are capable of resisting quite a force before there is any slip and aircraft movement. Same thing when you park a car on the side of a hill. No slip but there is much braking and cornering resistance preventing the car from sliding down the hill. In fact the braking and cornering capability would be the same. That is to say, it would be just as hard to push the parked car forwards or backwards as it would be side to side.

Just thinking aloud.

P.S. Thanks for raising the question punkalouver.

To answer your question. Yes we are told not to attempt a smooth landing. Just put her on nice and positive to achieve a good wheel spin-up. If after the inital flare, the main wheels don't go on the runway, just a slight easing of the back pressure on the control column will lower the nose just enough (maybe half to one degree in pitch at a guess) to establish again a descent rate towards the runway. Sometimes this can be over done somewhat and before you know it, the passengers are telling themselves under their breaths that you can't fly (or land) an aeroplane!

[Captain Stable]

One inaccuracy, Blip - many don't say it under their breath! :uhoh: ;)