A320-addict
Posts: 249
Joined: Fri Apr 20, 2001 1:24 am

Crosswind Versus Landing Distance

Fri Dec 13, 2002 4:35 am

Hi everyone...

An interesting question came up today performing a Sim session.

'Does full crosswind increases or decreases the landing distance?!'


If the wind is only partially cross, we would have a headwind component, and this of course decreases the landing distance. But what if we have full crosswind? I personally think that the use of differential braking to keep the airplane straight (certainly at low speed) during the crosswind roll out increases the landing distance. But if we only use the rudder (high speed) with max braking and later on when the speed goes down nose wheel steering, would we have the same landing distance as using the first technique?! Theoretically I think it would be shorter...

We tried several times in the sim to simulate this, but we ended up, at almost the same distance uses the different techniques...

According to our instructors (they were puzzled as well  Big grin  Big grin ) that was because during the landing roll out we use a mix of both techniques, and that it is almost impossible to use one technique without the other...


hope my question is a bit clear?!

Personally I thinks this is a very interesting subject ....


AA
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 4:56 am

Theoretically the ground roll with a 90 degree crosswind would be shorter because your groundspeed at touchdown is less. Realistically, I don't think you'll see much difference unless it's a really strong wind.

If you're landing in a 90 degree crosswind at 65 knots, that's 65 knots airspeed, which isn't aligned with the runway, it would be, wild number for example sake here, 10 degrees off the runway heading, so you'd take cos(10)*65 to get your groundspeed at touchdown.

Of course your techniques on the ground will play into the actual roll as well.

 Big thumbs up
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avroarrow
Posts: 804
Joined: Wed Sep 19, 2001 10:40 am

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 5:04 am

Um, actually your groundspeed would be lowest with a full on headwind not a full 90 degree crosswind. (Not that I'm looking to argue, I could be having an "off" day.) But I agree that only an extreme condition would make a noticeable difference, and also that the particular skills of the pilot make a big difference on the amount of "float" you get near touchdown.
Give me a mile of road and I can take you a mile. Give me a mile of runway and I can show you the world.
 
A320-addict
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Joined: Fri Apr 20, 2001 1:24 am

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 5:06 am

Ralgha,


If we speak in terms of ground speed, isn't the ground speed less with headwind instead of crosswind?! I think so, so.... according to me this theory doesn't work...

Thx anyway

AA


He Rick or Skipper, any thoughts from your side?!
 
Rick767
Posts: 2613
Joined: Sun Jan 30, 2000 8:11 pm

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 5:57 am

I think what Ralgha is saying is that with a 90 degree crosswind your GS is less than in a no-wind situation because you have to fly with the nose into-wind slightly so that you track the centreline. This does make some of the crosswind a headwind component.

Realistically, it makes little difference. In a strong crosswind we are more likely to execute a firm touchdown nice and early, and on that basis alone I would argue that a shorter landing roll would result. In a no-wind situation it is easy to float and try to make a gentle touchdown, and a few hundred metres of pavement can easily flash by the windows whilst doing that.

So I would say compared with a no-wind situation the landing roll would be shorter with a full 90-degree crosswind. But with a 20 knot headwind compared with a 20 knot crosswind, the landing roll will clearly be shorter with the headwind due to the much lower GS.
I used to love the smell of Jet-A in the morning...
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 7:57 am

Um, actually I didn't say your groundspeed would be LOWEST with a crosswind, I said it would be LOWER. As Rick767 noted, I meant relative to no-wind, as is everything when talking about modifications to landing distance. Like I said, it would make little difference unless it were a very strong headwind, you can actually figure out how much of a difference it will make.

Assume you have a touchdown speed of 60 knots and a 20 knot crosswind directly perpendicular to the runway heading. Because touchdown speed is all we care about, I'll use 60 knots of airspeed through the entire calculation. Let's call runway heading X and wind heading Y. You need a crab angle that will yield a velocity vector with a magnitude of 60, and a Y component of magnitude of 20.

crab = crab angle
V = Air velocity vector
X = X magnitude of V
Y = Y magnitude of V

First find the crab angle:
Y = sin(crab)*|V|
20 = sin(crab)*60
crab = arcsin(1/3) ~= 19

So our crab angle is about 19 degrees into the wind.

Now to find our groundspeed.
X = cos(crab)*|V|
X = cos(19)*60
X = 56.7

So our 20 knot direct crosswind reduces the touchdown groundspeed by 3.3 knots.

This works for slips too, since a slip will result in a velocity vector with the same angle to the runway heading as the crab angle.
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
 
Guest

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 10:20 am

Well, let me add this to the conversation...
It is frequently desirable to add a bit of "extra" airspeed in the case of gusting crosswinds. This extra speed would have the effect of adding to the landing distance.
Jetguy
 
avroarrow
Posts: 804
Joined: Wed Sep 19, 2001 10:40 am

RE: Crosswind Versus Landing Distance

Fri Dec 13, 2002 12:38 pm

Sorry Ralgha, for some reason I had assumed a comparison to a headwind, while you obviously had not..ah the perils of typing a discussion as opposed to just speaking, no intonation possible and no way to quickly include all of your thoughts  Smile/happy/getting dizzy
Give me a mile of road and I can take you a mile. Give me a mile of runway and I can show you the world.
 
A320-addict
Posts: 249
Joined: Fri Apr 20, 2001 1:24 am

RE: Crosswind Versus Landing Distance

Sun Dec 15, 2002 2:38 am

I agree that in rather windy (gusty) conditions a firm touchdown is really preferred. But if we would speak only theoretically and consider the same touchdown for both cases (no wind-full crosswind), what would be the effect on the landing distance?!

Ralgha,


Thx I did not consider that indeed with crosswind the aircraft's nose is pointed into the wind, which result in a lower groundspeed because until the moment of touchdown (flair), you still have a headwind component!


AA

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