using CRP 1 to calculate average wind from spot wind chart??

[liam548]

How do I use the wind side of the computer to interpelate between say 4 different wind speeds and directions from F214?

I understand there is a way of drawing on the speeds then turning the slide until it gives you the average heading and average speed for the wind from the 4 spot winds.

The instructions for this are not in the booklet.

Thanks

Liam

[Rhyspiper]

From ATPL groundschool I was under the impression that you add the speeds together and divide by 4. No need for CRP-1

[GIZZAJOB]

yep thats it no need for crap/1/5

[liam548]

Thats what I have been doing up to now for speed and yes it will work for speed but wont it be wrong for average wind directions?

[GIZZAJOB]

No , I generally only ever use the two spot wind boxes rather than four and a quick interpolation is usually reasonably good . The wind after all is never quite what it seems so you are not gonna lose too much accuracy doing it that way .

[liam548]

thanks for your replies.

I am right in thinking that averaging 4 wind directions by dding up and dividing by 4 will give a completely incorrect answer arnt I?

I just thought that sometimes 2 boxes just wont cover the larger routes...

[GIZZAJOB]

Really depends how far you are flying if you are going from lands end to norfolk then maybe you need to look at different boxes but a qxc flight two should be adequate .
YOu will probably find that the answrr wouldnt be too far removed in any case . try it and have alook
I only ever used two for cpl and never had much of an issue . Just remember it is never so accurate that you can rely on it totally . Once airborne you can assess how accurate it is .

[LH2]

Technically, you could do bilinear interpolation of the four corners, or use a 1/distance squared weighted average of any number of spot readings near your point of interest. Two interpolations in each case, for either the rho and Phi (if using a polar reference system), or the U and V components (if using a rectangular system).

Or in practice, you eyeball it.

[Whopity]

using CRP 1 to calculate average wind from spot wind chart??An average wind, is the mean wind measured over a period of time; you can only interpolate spot winds (Met mans guess) from a spot wind chart!

[jollyrog]

Or, you just look at the AIRMET forecast for your part of the country which will have already worked out something reasonable for you.

[IO540]

I think LH2 has the right answer, but the real Q is what is the exam paper looking for?

To be practical, the Winds Aloft forecasts are almost never right. Errors of 30-50% in the wind speed are pretty common, especially at the low levels (say 2000ft) at which GA pilots are asked to demonstrate their dead reckoning prowess.

Averaging the direction (heading) figure is also the right thing to do, if the variation is small, say 30 degrees. The problem is if one spot shows 090 and the other one shows 270. The average is then 180 :) What this tells you is that there is a frontal system lying between the two, or something like that, and you need to apply different winds to different segments of the route.

[BEagle]

Ah - an analogue problem which digi-yoof do seem to find so very hard.....

Just interpolate between 2 values at the same latitude and obtain a figure, then the other 2 at the other latitude. Then interpolate between your 2 results to obtain the 'middle of the box' value.

Or just look at the figures and use the TLAR technique.

[nickyjsmith]

Yeah, That Looks About Right.

[liam548]

I think LH2 has the right answer, but the real Q is what is the exam paper looking for?

To be practical, the Winds Aloft forecasts are almost never right. Errors of 30-50% in the wind speed are pretty common, especially at the low levels (say 2000ft) at which GA pilots are asked to demonstrate their dead reckoning prowess.

Averaging the direction (heading) figure is also the right thing to do, if the variation is small, say 30 degrees. The problem is if one spot shows 090 and the other one shows 270. The average is then 180 :) What this tells you is that there is a frontal system lying between the two, or something like that, and you need to apply different winds to different segments of the route.


thanks for all your replies, most helpful. I have passed the exam bt wanted to know the "real world" way of doing it which is going to actually help my flying and navigation

[liam548]

An average wind, is the mean wind measured over a period of time; you can only interpolate spot winds (Met mans guess) from a spot wind chart!

yes I meant using the spot wind chart but inputting the figures into a CRP 1 somehow to get the average. But the other replies show that this does not work and there are better methods out there.