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CrossWind Component Formula

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CrossWind Component Formula

Old 28th Jul 2016, 19:30
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Question CrossWind Component Formula

Hey guys ! I`m trying to find the formula to calculate the crosswind component like this tool does. I could get something for just crosswind but i can`t seem to figure out how to apply gusting wind to the formula. Thanks !
richardstevens is offline  
Old 28th Jul 2016, 21:29
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Looks like the calculator is simply adding half the gust factor to the steady wind, and then calculating the crosswind component of the result. This means that the crosswind component at touchdown could be greater than you think.

The basic calculation of crosswind component is: Wind Speed x Cosine of the angle between the wind and the runway.

On this basis a good rule of thumb is: 15 degrees = 1/4 of the wind, 30 degrees =1/2 the wind, 45 degrees = 3/4 of the wind, and anything over 60 degrees = all of the wind as a crosswind component.


MJ

Last edited by Mach Jump; 28th Jul 2016 at 21:48.
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Old 28th Jul 2016, 22:47
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Then there is the "clock" method - basically the same as MJ uses but with a few more options - take the crosswind angle and imagine it on a clock face, so 15 degrees is 1/4 way round, 20 degrees 1/3 etc, so 40 degrees is 2/3 off, so a 20 knot wind I would call 21 for convenience giving me a 14kt crosswind component - for gusts just take the maximum gust and do the same if you need it.
At least with this you can apply it in the air using your head rather than fiddling with formula or a tablet.

N.b. Head/tailwind component can also be worked out by subtracting angle off from 90 and applying the same clock method.
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Old 28th Jul 2016, 23:54
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Same numbers, but I do 1/6s rather than 1/4s, one sixth of the wind speed per ten degrees.


Gusts? - if there's significant gusts other than straight down the runway I'm probably not going to take off, so don't have to be able to do sums about them in my head. Point being of course that the gusts don't come from the same direction as the steady wind. Which I would expect any tool that did these sums for me to get right, wouldn't I.
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Old 29th Jul 2016, 07:50
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One that used to be in the Australian ATC documents.

15 30 45 60 75 (degrees off)

30 50 70 90 All (percentage which is crosswind)
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Old 29th Jul 2016, 09:46
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Take a look at this: Crosswind Calculations made Easy.pdf

Using a DI, VOR, ... to visualise the cross, and head, winds is really simple - especially if you're working hard on other things.

OC619
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Old 29th Jul 2016, 11:02
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Richard: remember, that in the Northern Hemisphere, when a wind gusts it tends to veer ie. increase its direction in a clockwise direction. This means that with a crosswind from the left then the gust will veer and make the crosswind component less. The converse is true from the right! By the way, this is not an exact science either in the measuring or reporting.
To visualise this easily - imagine a crosswind that is at exactly 90 degrees to the runway. This results in only drift but no change in groundspeed. Now imagine the effect of a veering gust.
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Old 29th Jul 2016, 11:37
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Originally Posted by Mach Jump View Post
Looks like the calculator is simply adding half the gust factor to the steady wind, and then calculating the crosswind component of the result. This means that the crosswind component at touchdown could be greater than you think.

The basic calculation of crosswind component is: Wind Speed x Cosine of the angle between the wind and the runway.

On this basis a good rule of thumb is: 15 degrees = 1/4 of the wind, 30 degrees =1/2 the wind, 45 degrees = 3/4 of the wind, and anything over 60 degrees = all of the wind as a crosswind component.


MJ
I'm no expert but I've a feeling using Cosine will give you the head/tail wind component and Sine will give you X-Wind. Happy to be corrected though!
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Old 29th Jul 2016, 13:20
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SOHCAHTOA - just go back to school maths.

Sin = Opposite over Hypotenuse

So, Sin (angle) x Hypotenuse = Opposite

So, sin (angle between runway and wind) x total wind = crosswind component.


Quick check
=========
sin(0)=0, so no angle, no crosswind component.
sin(90)=1, so 90 degrees angle, it's all crosswind
sin(45)=0.70 (the clock code would use 75%, near enough)
sin(30)=0.5 (the clock code would use 50%, spot on).



And as just said above, Cos for head/tailwind components. Cos(0)=1, so it's all headwind, cos(90)=0, so if it's all crosswind there's no headwind.



Gusting winds are still just winds - apply the same factor, you get correct answers.

G
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Old 29th Jul 2016, 20:10
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a bit off topic BUT often wondered why do you get given wind direction and speed when on final - why not just give cross wind component and direction (such as 10 knots from the left). It would be easy for the FISO or Controller to calculate and save all this faffing about with rules of thumb..
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Old 29th Jul 2016, 20:30
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Ooops.

Sorry, guys. Yes, you are right.


MJ
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Old 29th Jul 2016, 21:22
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Originally Posted by avonflyer View Post
a bit off topic BUT often wondered why do you get given wind direction and speed when on final - why not just give cross wind component and direction (such as 10 knots from the left). It would be easy for the FISO or Controller to calculate and save all this faffing about with rules of thumb..
It should be just as easy for the pilot then. If the wind is 30 degrees off runway heading, half of it is crosswind. If it's 60 degrees or more, you count all of it. Rocket science it aint.
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Old 29th Jul 2016, 21:53
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sure - I haven't got religion about the idea but, my guess would be that for a lot of low hours pilots the information on final becomes noise in the background whilst concentrating on the approach. They have in mind their cross wind limit is "X" and all they want to hear is a number lower than that. It no great thing and we get by but, we could make life just a mini tad easier..
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Old 29th Jul 2016, 23:49
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At quieter airfields, it's not a bad idea - although I can't see it actually happening.

At busier airfields, I think that controller workload would preclude it.

G
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Old 30th Jul 2016, 02:27
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ATC has a requirement to advise crosswind to aircraft when it reaches certain limits depending on aircraft type.

On final at uncontrolled airports, no formula will help when all you have is a windsock which is horizontal at anything above 15 knots. Best to learn what can and can't be done from the available cues, crab angle, control input and GPS ground speed and don't be too proud to go around.
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Old 30th Jul 2016, 17:42
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That's a new one on me Fuji - is this purely a big airport / air transport thing? I can't recall ever being advised of crosswind at high values at any civil airfield, and only a couple of times at military airfields.

G
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Old 30th Jul 2016, 18:19
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I was given crosswind at Wellesbourne a couple of months ago - 3 or 4 calls (no response required) on short final I recall, but then it was at 90 deg ish across and other runway not available due to market.
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Old 30th Jul 2016, 19:25
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For military airfields, Benson certainly do it in high crosswinds.
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