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  1. #1
    ismertr Guest

    Background on why we have both true and magnetic winds?

    Just wondering what the rational is behind winds aloft being true and other forcast winds being magnetic?

  2. #2
    Motivated Guest
    If you are getting winds aloft from aviationweather.gov they are true. Where are you getting your forecasts? I'm not sure if you are using flight planners, but the columns normally progress in true course, winds & finally... magnetic correction.

  3. #3
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    My recollection is that all forecast winds are in true, so that includes winds aloft and TAFs. (Um, what other wind forecasts are there?) METARs give winds in true, also.

    Where you'll hear magnetic winds is over the radio, e.g. AWOS, ATIS, and tower wind checks.

    The motivation for magnetic winds seems fairly easy to speculate on, it matches up with runways, which are magnetic, and when we're in the cockpit all our heading indications are in magnetic.

    As for why we don't just use magnetic for all the forecasts, too, I'm not sure I have a particularly good answer for that.

    The advisory circular "Aviation Weather Services" is a good reference for stuff like this:
    http://rgl.faa.gov/Regulatory_and_Gu...her%20services
    -harry

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    I've always gone with the idea that because variation can change greatly over a relatively short distance, that true is used for winds aloft. I don't have any source for that except speculation.

    I had a friend who was a FSS briefer and he said that if something was transmitted digitally, it was true, and if something was transmitted verbally, it was magnetic. That seems to hold true (or maybe magnetic) for the cases I have seen.

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    To follow up on harry's note, the reason I've been given for true winds aloft is along the same lines as using UTC - airplanes go fast and variation would be changing constantly. Makes sense to me.

  6. #6
    MikeELP Guest
    I like the adage "If it's in print..it must be true"!

    As noted previously, as regards winds aloft forcasts, since they usually cover a very large area that may involve several changes in magnetic variation it seems logical to give the the direction in true.

    As for why they don't put METARS and TAF's (which cover a very small area) in magnetic is a big ????


    Mike

  7. #7
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    ... if something was transmitted verbally, it was magnetic...
    I've heard this as "over the radio is magnetic".

    Your saying raises the question: "if I call a briefer and ask for winds aloft, do I get it in true or magnetic?" The original report is true, does the briefer really convert that to magnetic in order to read it to me? I don't think so.

    My saying raises the question: "if I call the phone number associated with an airport's AWOS, do I hear the same thing I hear over the radio (i.e. magnetic), or do I hear it converted to true?"

    I have a feeling that neither "saying" is always true ... err, I mean, correct.
    -harry

  8. #8
    Warever Guest
    Quote Originally Posted by hm View Post
    METARs give winds in true, also.
    Except when they're not.

    I had this discussion with my first flight instructor and then we looked at the METAR for that particular airport and it directly coincided with what ATIS was saying. She had no explanation for WHY (and she was a weather NUT... just LOVED everything about it), but for some reason, they were evidently giving the METAR in magnetic (I don't remember how we determined that they were both magnetic as opposed to both true). We looked at some other local airports and there was a difference.

    Quote Originally Posted by hm View Post
    As for why we don't just use magnetic for all the forecasts, too, I'm not sure I have a particularly good answer for that.
    I did a Google search and found an "answer" on another forum. Don't shoot the messenger; I'm just relaying what I read elsewhere which may or may not be true.

    The answer was that most forecast products are delivered not only for pilots but for scientists, researchers and others. Since magnetic deviation changes so much from place to place and because it actually changes slowly in any given place, they're all given in true figures so ANYONE ANYWHERE could use those figures directly against a map. The "verbal" products used mainly by pilots are in magnetic simply because they're easier for us to use.

    Like I said, not MY answer, but it does sound like a reasonable explanation.

  9. #9
    EMC Guest
    Well it has to be for flight planning. We plot a true course over the ground, so to determine our wind correction we need true winds. Once we have a corrected true course, yes, we have to correct with variation, but we are still trying to fly a true course plotted on a true map.

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    Quote Originally Posted by Warever View Post
    Except when they're not.

    I had this discussion with my first flight instructor and then we looked at the METAR for that particular airport and it directly coincided with what ATIS was saying. She had no explanation for WHY (and she was a weather NUT... just LOVED everything about it), but for some reason, they were evidently giving the METAR in magnetic (I don't remember how we determined that they were both magnetic as opposed to both true). We looked at some other local airports and there was a difference.
    That made me think of something a naval aviator said his flight instructor stressed to him. Everyone is trying to kill you. The controller, the mechanic, the other pilots, the aeronautical engineer who designed the aircraft, and now we can add the weather man to the list. (What am I thinking; the weather man was already near the top of the list.)

    His point was that once you believe everyone is trying to kill you and act appropriatly, your odds of living to a ripe old age are greatly increased.

  11. #11
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    a little saying...

    Jason miller had a little podcast on this. This is easy to goof up on the written or the oral.
    "If it is written it is true. If it is spoken it is magnetic." Just remember that. Because
    You might have a question on flight planning and u need to remember which is which.

    true is for planning. When in the air u don't want to need to remember to magnetic variation on final.

  12. #12
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    http://www.cockpitgps.com/VOR_magnet...ps_and_obs.htm

    I found the site listed above while looking for something else. I thought that while it doesn't speak directly to the subject of this thread (winds), that it does give some interesting information that may be useful to you.


 

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