Crosswind Calcualtions

[socrates]

Hoping that some-one may be able to help out here.

I am trying to recall a method that was demonstrated to me in my dim and distant past for calculating the X wind on the ground. I know of 2 'mental' methods that I was taught; the first being associated with utilising the clock code, ie:

W/V 120/30
R/W 18
X wind: 15Kts based on the fact that 60 degrees equates to the 6 on the clock face and as that is then 1/2 [of the hour] the wind componant. This also works for 30 degrees being 1/4 of the wind componant.

The second method related to calculating the cross wind when airborne and refers to percentages, ie, 90% of the head wind componant etc.

I would be most greatful if some-one can point out the correct method of these 2 systems as I am getting myself a little confused and want to give a clear explanation to a colleague.

I do know how to use the wizz wheels for such calculations but really wanted to impart a little extra information just to help a particular situation.

Thanks in advance...

[This message has been edited by socrates (edited 28 June 2001).]

[john_tullamarine]

If it's important to know (and then it probably is important to know with some sort of reasonable accuracy), why not use the prayer wheel ? ... either the picture side or the trig functions according to your preference and the sophistication of your device.

Most of us still carry one in the top pocket when flying .....

Maybe it's just a case that I find it harder to remember the shortcuts than to do some things the "long" way ....

[juswonnafly]

Socrates, you are close but confused!

Assume that max x wind = wind strength

60 degs or more (clock face) = MAX

30 degs (half clock face) = x-wind of half max

45 degs (three quarters clock face) = x-wind of 3/4 of max

and so on...geddit?

A useful tip is to put r/w hdg on VOR rose, then strike an imaginary line vertically from the wind direction. Calibrate the horizontal scale from zero (centre) to max (outside edge of face).....this will give you the x-wind.

Does this help?

JWF

[john_tullamarine]

I'm feeling bad about being flippant before.

The clock trick, which I hadn't seen before, is fine up to about 45 degrees off the nose, with an error less than around 5 percent which is pretty reasonable.

If we start with 12 o'clock being wind on the nose (ie no crosswind) then each hour represents 5 degrees off the nose. Thus

1 o'clock = 5 degrees, multiplier = 1/12
2 o'clock = 10 degrees, multiplier = 2/12

and so on up to 45 degrees off the nose.

Beyond 45 degrees you can use a additional linear pro-rata figure to maintain an error better than around 5 percent.

For example, at 60 degrees, the multiplier becomes

= 9/12 + (60-45)/45 x 0.25
= 0.75 + 0.25/3
= 0.83

I still think that it is not worth the effort when the prayer wheel can be jostled quicker and easier for better data .....

[RVR800]

W/V 120/30

R/W 18

X wind: is nearly 30 kts !

Think about it ..

If the wind is 180/30 then xwind is zero
If it is 60 degrees from the left it is max

The clock rule is degrees off divided by 60

i.e.

10/60 20/60 30/60 40/60 etc

In this case its is 60/60 = 1 so full xwind!

If its 45 its 3/4
If its 30 its 1/2
If its 10 its 1/6
If its 60 its the full monty

Its easy...

Remember x wind issued by ATC at take-off is magnetic and so is you DI reading so no problems here

The aim in a commercial check ride is to give the examiner an estimate of the component and orientate ailerons

Know your xwind limits !

- - - - - - - - - - - - - - - - - - -

Working out the component in the air
is different and requires the TAS

60/TAS is the additional factor

If its 90kts You factor the above by 2/3
If its 120 kts you factor by 1/2

So a 30 kt wind will have max drift 15 degrees if TAS = 120 kts

[Yogi-Bear]

Phew! I just use the cosine of the angle off.
30 deg = .5
45 deg = .7
60 deg = .9
90 deg = 1.0
The little grey cells can just about manage that on the approach. ;)

[Huw Jorgen]

Probably just a typo but I presume you mean sine ?

[Yogi-Bear]

Those little grey cells again! http://www.pprune.org/ubb/NonCGI/redface.gif SOHCAHTOA :)

[This message has been edited by Yogi-Bear (edited 28 June 2001).]

[eyeinthesky]

Yogi:

At last someone has outlined the simplest method of mental calculation which I have always used. Any more accurate than that and perhaps you shouldn't be operating if the crosswind is that critical.

It also works for mental calculation of degrees of drift when airborne when you apply it to the maximum drift (which you calculated before departing:60 divided by TAS multiplied by windspeed in knots). The remainder (i.e. 1 minus the relevant value calculated from the clock code) when applied to the windspeed in knots will give you head or tailwind component.

For example:

Track 360 True, TAS 120 kts
W/V 300/25 (true). Max drift 12.5 degrees

Drift from the Clock Code:

60 degrees off = 0.9 x 12.5 = +/- 11 degrees: Heading: 349 T

Headwind: 0.1 x 25 = 2.5 kts HWC : g/s = 117.5 kts

haven't checked it on the whizziwheel, but seems to work in the air.



------------------
"Take-off is optional, Landing is mandatory"

[socrates]

Thankyou all very much for your contributions.


The answers I was looking for seem to have beem answered, any more comments are also welcome.

Juswonnafly, the response you gave seems to be the same method I was taught and also that referred to by Yogi, which is the 'quick' [mental] method I was shown for en-route nav and holds etc.

Once more many thanks for those replies and it is good to see that for once a thread has not degenerated into bickering, just useful and constructive comments!!

http://www.stopstart.fsnet.co.uk/smilie/cool5.gif

Now all I need is all these TS to clear and I can get on with flying...

Socrates

[foxmoth]

Yogi & Eye in t Sky - same method as the clock code, - clock code is just a cheat to give approx. sine rule, yes 60 degrees should be .9, not full x-wind, but it is close enough for practical use.

[Shed Driver]

foxmoth, thank goodness you said that! I am often amazed at how some people state things on this site which are just a repetition of others postings! There are many ways to do things in aviation, often the same technique with a different moniker. Its even more worrying that this is the instructors forum...

[Rambi]

Isn't it a shame that an aviator has to ask the question that started this topic?
http://www.pprune.org/ubb/NonCGI/confused.gif
The answer to this question should either be ready knowledge (it gets taught in every flightschool) or be found by simple calculation!!

Cheers.

[john_tullamarine]

Rambi,

Actually, it is a shame that someone sees the need to make fun of a well-intentioned question.

I for one,

(a) am an engineer .. so the sums are trivial but, you will see, my observations above are nowhere near as useful as some of the others

(b) have been flying for 35 years in aircraft ranging from rag and string to transport including considerable ground school instruction work and I have never heard of this approach .. so, just maybe, not everyone has the benefit of the lessons which someone gave you at an earlier stage in your career.

I would much rather see these sorts of questions keep coming up .... I am quite happy to learn from the better knowledge and experience of others ....

[Yogi-Bear]

If people hadn't asked questions, again and again, we would still be living in caves. http://www.pprune.org/ubb/NonCGI/eek.gif