novice
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Why Is Wind Measured In Degrees True?

Mon Jan 28, 2013 10:17 pm

Why is wind measured in degrees true when heading are generally always measured in degrees magnetic?
 
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Web500sjc
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RE: Why Is Wind Measured In Degrees True?

Mon Jan 28, 2013 10:44 pm

Ill take a stab...

the maps pilots plan their flights on are sectioned off by true cardinal directions (north, south, east, west), and since this is done it is easier for pilots to take their plotters and find the true course, and is therefore easier to use wind measured in degrees based off of true north.

Magnetic heading is used because for most of the history of flying, the quickest and most reliable way of finding ones heading was a magnetic compass.

Just to clarify Course is the direction one goes over the ground, and Heading is the way one points in the air or water.
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Mir
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RE: Why Is Wind Measured In Degrees True?

Mon Jan 28, 2013 11:15 pm

Quoting web500sjc (Reply 1):
the maps pilots plan their flights on are sectioned off by true cardinal directions (north, south, east, west), and since this is done it is easier for pilots to take their plotters and find the true course, and is therefore easier to use wind measured in degrees based off of true north.

It's not really easier, actually. Not harder either, just a different process. With the way things are now, you have to take your true course, apply the wind correction, then convert to magnetic. If the winds were in magnetic, you'd just covert your true course to magnetic first before applying the wind correction. Same result, just different order. Why they decided to do it that way I don't know, though it does make it easier to look at a map of the country and see which way the winds are really blowing in relation to the ground - in some areas the magnetic variation can lead to the numbers being 15-20 degrees off of the true values.

It should be noted, though, that this is only for winds aloft - METARs still report winds in magnetic values.

-Mir
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novice
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RE: Why Is Wind Measured In Degrees True?

Mon Jan 28, 2013 11:19 pm

Thanks for the reply web500sjc
I see the logic in what you're saying, though the maps that i plan from have variation lines running through them so therefore enables you to see magnetic heading from them, so i can't see the advantage of having wind in degrees true?
 
novice
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RE: Why Is Wind Measured In Degrees True?

Mon Jan 28, 2013 11:23 pm

Ya i see just a different order thanks Mir   
 
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Web500sjc
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 12:58 am

Quoting Novice (Reply 3):
I see the logic in what you're saying, though the maps that i plan from have variation lines running through them so therefore enables you to see magnetic heading from them, so i can't see the advantage of having wind in degrees true?

the variation lines do not point north, they just place a line upon which the difference between true north and magnetic north is known, and ascribe the known difference at that line.


Another point I had not brought forward, the magnetic north pole moves in relation to the true north pole, and so the maps would periodically change orientation so that magnetic north would still be at the top.
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Starlionblue
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 1:03 am

Quoting Mir (Reply 2):
It should be noted, though, that this is only for winds aloft - METARs still report winds in magnetic values.

The way an instructor explained it to me is that (in the US), the rule of thumb is that "printed" products measure in true, while "voice" stuff measures in magnetic. The METAR is related to the ATIS, thus "voice".

So the winds aloft forecast is in degrees true, while the wind check from the tower is magnetic.

Another way to look at it is that it makes sense to have METAR/ATIS in magnetic since runway headings are magnetic. You use this number when close to the airport only. However for route planning true is more practical since maps use true directions, and as Mir says variation can be quite large.

If everyone still navigated by compass, having magnetic wind directions in the forecast would make sense. But most people don't anymore except as a last resort.


By the way with GPS becoming the preferred method even in general aviation, will we someday see runways with true headings?

[Edited 2013-01-28 17:04:33]
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novice
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 3:09 am

Quoting Starlionblue (Reply 6):
If everyone still navigated by compass, having magnetic wind directions in the forecast would make sense. But most people don't anymore except as a last resort.

Though most GA traffic navigate using the DI and that uses magnetic heading
 
SAAFNAV
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 4:31 am

It comes from back then when people still plotted.

All your plotting is done using a protractor on a plotting sheet. You align that with the longitude lines, which all point to True North.
Then, using a Nav Log, you work out everything, from left to right, True Track, Wind Vector, Drift, Magnetic Variation, Deviation, and then you get Magnetic Heading.

If you had to plot using Magnetic Headings, everytime you put your protractor down, you would first have to find out the variation at your position, add or subtract from 360, align to Magnetic North and then plot your position or whatever. Obviously, this could lead to a lot of confusion, so it's far easier to just make on addition or subtraction afterwards.

Magnetic Winds are spoken on ATIS Recordings and by ATC for runways, since you land on magnetic. A wrong calculation in the critical phase could lead to unexpected crosswind components.

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Starlionblue
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 9:36 am

Quoting Novice (Reply 7):
Quoting Starlionblue (Reply 6):
If everyone still navigated by compass, having magnetic wind directions in the forecast would make sense. But most people don't anymore except as a last resort.

Though most GA traffic navigate using the DI and that uses magnetic heading

I can only speak from my own experience, but if I needed to go anywhere beyond 10 minutes I flew with the GPS with pilotage as backup, DI as backup to that and whiskey compass as final backup. The DI was mostly useful for keeping track of things over a short distance (like holding a heading during a LOC approach). If (when) I turned the autopilot on I set it to follow the GPS.

In the G1000 planes I used the GPS 99.9% of the time, except when I had to do VOR/ILS/LOC approaches.
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glen
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 10:27 am

Quoting Mir (Reply 2):
It should be noted, though, that this is only for winds aloft - METARs still report winds in magnetic values.
Quoting Starlionblue (Reply 6):
The METAR is related to the ATIS, thus "voice".

That's not quite correct. Printed METAR (e.g. at the met office) are also in true values. Only ATC towers, ATIS and airport advisory service (real time reprts to flight crew) report wind as magnetic. See also this documentation (the only "official" publication I found in short time):
www.uscg.mil/auxiliary/missions/auxair/metar_taf.pdf
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Starlionblue
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 10:51 am

Quoting glen (Reply 10):
That's not quite correct. Printed METAR (e.g. at the met office) are also in true values. Only ATC towers, ATIS and airport advisory service (real time reprts to flight crew) report wind as magnetic. See also this documentation (the only "official" publication I found in short time):
www.uscg.mil/auxiliary/missions/auxa...f.pdf

So there is a difference between ATIS and METAR values. Very interesting! Thx for info.

Also strengthens the "printed" vs "voice" rule.
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SAAFNAV
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RE: Why Is Wind Measured In Degrees True?

Tue Jan 29, 2013 1:36 pm

Quoting glen (Reply 10):

That's not quite correct. Printed METAR (e.g. at the met office) are also in true values. Only ATC towers, ATIS and airport advisory service (real time reprts to flight crew) report wind as magnetic. See also this documentation (the only "official" publication I found in short time):
www.uscg.mil/auxiliary/missions/auxair/metar_taf.pdf

True, forgot to point that out.
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peterpuck
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RE: Why Is Wind Measured In Degrees True?

Thu Jan 31, 2013 2:20 pm

Simple way to remember it is "If you read it it's true"

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