Interpolating winds aloft HELP!!

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Nathan Reim

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Hey guys, I am a student pilot, I am working on my light sport license and I cant seem to find anything solid that explains how to interpolate winds aloft... I know its out there but even my study material lacks any answers. So I am planning a flight tomorrow I will being flying at 150 True Course from KLHZ to KGWW at an altitude of 5500 feet. Winds Aloft will change from now and when I take off tomorrow, I know ill have to do this again, that's alright as long as I can figure out how to do this.
Here is the info for both my flights as well as the current report for WAF. These are DIRECT flights
KLHZ - KGWW True Course 150° 9° magnetic variation =141° Alt- 5500
KGWW - KLHZ True Course 333° 9° magnetic variation = 342° Alt- 4500
How do I interpolate the winds at 5500 and 4500 effectively? By hand not using foreflight or anything else.

FT 3000 6000 9000

RDU 3210 3113+12 3215+03
 
Last edited:
Why does your magnetic variation flip between the two legs?
 
RotorDude said:
Why does your magnetic variation flip between the two legs?
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Does your Magnetic variation not flip if you are heading east or west? I though, say in this case i have a MV of 9 going east I though It was -9° or going in a westerly direction it was +9°. AM I wrong?
 
Nathan Reim said:
Does your Magnetic variation not flip if you are heading east or west? I though, say in this case i have a MV of 9 going east I though It was -9° or going in a westerly direction it was +9°. AM I wrong?
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Magnetic variation doesn't care where you are going. It only depends on where you are.
You should probably have a little discussion with your CFI about this.
 
RotorDude said:
Magnetic variation doesn't care where you are going. It only depends on where you are.
You should probably have a little discussion with your CFI about this.
Click to expand...


Aright, that's why I came here to ask. I have asked my CFI to help me understand this interpolation stuff but he's extremely busy so it's been a little hard to get anything other than a quick run through of it, and I haven't been able to catch on. Very glad I got the magnetic variation stuff cleared up. I had an instructer tell me that it differed depending on east or westerly heading. Either plus or minus depending on direction....i may have misunderstood him as well. So I'll fix that in the original post, thank you.
 
East is least (-), west is best (+) but it's the value of the Mag Var, not the direction you are heading.

True, Virgins, Make, Dull, Company

True course, +/- Mag Var = Mag Course +/- Deviation = Compass Heading.
 
Magnetic variation is explained in many pilot study books.
The FAA's PHAK dedicates chapter 7 to the instruments, with magnetic variation being discussed on page 7-23.
Chapter 15 also explains magnetic variation well on page 15-6. And then it goes onto wind correction on page 15-9.
Good luck!
 
Just if you have the 3k and 6k values you just split the difference. If winds are 320 at 20 for 6 and 300 at 10 for 3k you'd plan 310 at 15 for 4500. If your altitude is 3500 or 5500 you weight it proportionally.
 
Ok, Im now savvy with Magnetic variation, Now whats the best way to interpolate the winds aloft. What is the formula for that? I know you are basically just getting finding whats in between the 3000 and 6000 known waf data. So for the data in the original post, I got 13kt at 310 for 5500 feet. I just took the wind directions from 3k and 6k and divided them by 2, then added the two wind speeds and divided by 2. Is this sufficient?
 
BigBadLou said:
Unless you flunked math in high school, interpolation and extrapolation should be easy and without questions.
So what exactly are you asking? Whether your math is correct or whether using interpolation is correct?
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I'm asking wether or not simply dividing the known WAF data Is the correct way to interpolate wind direction and speed. Basically I need a step by step example, I think I'm doing this correctly but on the last two flight plans I've made my CFI said I did it wrong, and then I have about 7 active duty F15 pilots saying I'm correct and then telling me that no matter what it's going to change, it's only accurate for about as long as it takes to interpolate the data if it's done within five minutes of the WAF update, and that if you use that data and fly a significant distance you will end up way off target. It seems like the only accurate way to get this info is in the air at the specified alt. It's seems like an extra step for flight planning that doesn't really need to be done. But if your gps fails you need to know how to do it.
 
Example:

3000 300 at 30
6000 350 at 90

You want to figure out 4500. Well for 4500 that one you do split it down the middle because 4500 is 50% from 3000 to 6000. 325 at 60

For 5500 it's not half way so you can't just split it. 5500 is 5/6ths of the way between 3000 and 6000.
So you have to weight it 5/6th of the way towards 6000.
Take the difference in heading and speed between 3000 and 6000.
50 and 60 respectively.
50*(5/6) = 41.6. 300+41.6 = 341.6 heading. Round to 342.
60(*5/6) = 50. 30+50 = 80.
342 at 80.
 
Flight planning is just that, planning. And yeah, if your altitude will be between two WA forecasts levels, then you split the difference by whatever percent (1/2, quarter, 1/3, whatever) you are between the numbers given. The other saying goes something like, "no plan survives contact with the enemy". Once you get there, the actual numbers will be different. You can use your plan to get you started, but when you get into reality, you use your chart, eyeballs, and landmarks to follow your course, and your compass will end up showing whatever it needs to show.
 
EdFred said:
Example:

3000 300 at 30
6000 350 at 90

You want to figure out 4500. Well for 4500 that one you do split it down the middle because 4500 is 50% from 3000 to 6000. 325 at 60

For 5500 it's not half way so you can't just split it. 5500 is 5/6ths of the way between 3000 and 6000.
So you have to weight it 5/6th of the way towards 6000.
Take the difference in heading and speed between 3000 and 6000.
50 and 60 respectively.
50*(5/6) = 41.6. 300+41.6 = 341.6 heading. Round to 342.
60(*5/6) = 50. 30+50 = 80.
342 at 80.
Click to expand...


Perfect this is what I needed to see.
 
Nathan Reim said:
I'm asking wether or not simply dividing the known WAF data Is the correct way to interpolate wind direction and speed. Basically I need a step by step example, I think I'm doing this correctly but on the last two flight plans I've made my CFI said I did it wrong, and then I have about 7 active duty F15 pilots saying I'm correct and then telling me that no matter what it's going to change, it's only accurate for about as long as it takes to interpolate the data if it's done within five minutes of the WAF update, and that if you use that data and fly a significant distance you will end up way off target. It seems like the only accurate way to get this info is in the air at the specified alt. It's seems like an extra step for flight planning that doesn't really need to be done. But if your gps fails you need to know how to do it.
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Agreed, the winds WILL change by the time you get up there, no question about that.
Like others said, it is just a forecast.
And like you said, you still need to calculate the expected wind. So use linear interpolation for the angle AND speed (separately) and you're good. Just like you did. Simple math. No need to overthink it.
But then when planning fuel, expect the winds to be higher anyway. Your CFI will thank you and your own life will thank you. Winds are never what is forecast. Expecting a higher headwind and lower tailwind is good practice to give yourself some safety margin.
If your CFI still says you're doing it wrong, as how he means it should be done right. Only one way to find out.
Now go fly! :)
 
And remember that the winds are rarely if ever linear. I was flying approach last week. The wind at 2200 was something above 30 kts. At 1100 there was a bit of turbulence and the wind dropped to 15 kts. It was that abrupt a change. This is not uncommon. So for flight planning (especially right now, working on your cert) interpolate. In the real world, be aware of the real world...
 
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