winds, magnetic vs true

[Keith Smith]

I apologize in advance if this has been covered before, just let me know if it's been debated and ruled on.

METARS and winds aloft forecasts contain winds, measured relative to TRUE north. My understanding is that ATIS, and any wind reports from TWR positions are supposed to be MAGNETIC (so that they'll be immediately useful to pilots)

Magnetic variation in the bay area is roughly 15 degrees east, so TRUE winds of 290/10 (a 'right crosswind' at SFO) should actually be reported to pilots on final as 275/10 (a 'left crosswind').

I realize this seems like a picky point, but since VATSIM aspires to be as realistic as possible, I though I might bring this up.

I hope I'm not ruffling any feathers, this being my first day of school an' all, but it's something I've wanted to ask ARTCC staff about for a while now.

For what it's worth, I have confirmed in the AIM that METARS are 'true'. I've also confirmed in x-plane that the metar is interpreted as 'true' because I see the conversion to magnetic when I check the weather within the sim (or use the fancy display on the 757). I always twitch a little when the tower then says "winds xxx/10, cleared for takeoff". I'm always tempted to say, "no they're not!!" :)

Maybe ZOA can be the first ARTCC to have a policy of reporting the winds in magnetic degrees. Pilots will flock from all over the world around to experience the wonder that IS an accurate wind report on final.

Keith

[Stephen Zedalis]

Hi,

I understand that reporting the magnetic wind direction may be more useful to pilots.

And taking it from the Aeronautical Information Manual Chapter 4 Section 4-2-10 it says "The three digits of bearing, course, heading, or wind direction should always be magnetic. The word "true" must be added when it applies" Which is less than clear. It seems to imply at the beginning of the statement to always use magnetic, but then contradicts that by telling you what to do when reporting true values.

FAA Order 7110.65N states the following in Chapter 3 Section 3-9-1 b.2 " Surface wind from direct readout dial, LLWAS, or automated weather observing system information display. (May be omitted if pilot states "have the numbers.") " So it says to report right off the AWOS or other direct readout. (With no menetion of conversion to magnetic) This is for takeoff clearance. The equivalent landing clearance section which comes later only mentions surface wind direction and velocity.

To muddy the waters even further. per the VATUSA Voice Lesson, when reporting wind direction, report to the nearest 10 degrees. So a report of 275/10 would be inappropriate.

So, I believe there is room for interpretation. For the real ATC's out there what is reported in the real world?

[feathers]

I'm not a for real ATC, but I'm in the process (in the pipeline, if I was a movie?). It is correct that winds reported from the tower should be magnetic, while winds reported by the ASOS/AWOS and winds aloft systems are in true. This makes sense to me, because when using METAR data and WA data, you'd probably be charting your course on your chart and/or bustin' the wind correction side of your E6-B, and who wants to convert when your chart is in true north?

Yet, one wants magnetic winds for takeoff and landing, because the runways are laid out on magnetic headings, so you don't have to do that conversion in your head... the compass says 270 and there is runway 27 right in front of you. Ditto for the winds. It is my understanding that in the tower cab, there is an indicator of wind speed and heading for the airport, and I would have to make the leap that this reads out in magnetic, rather than true. That way you (the tower controller) can just read it off to your traffic.

Which brings me around to probably the ultimate point of this post: VATSIM concessions. VATSIM is teriffic. It's probably the best thing I ever accidentally tripped over and fell into. It strives to hit the realism mark and nails it in probably 98% of all categories. But it's not perfect. The biggest concession for tower controllers is not having an 'out of the cab' view. I'm sure it's possible, but it probably isn't going to arrive for free. Until then, we make believe and use ASRC's radar to move planes around and clear them for takeoff and landing.

Personally, I always thought of giving out the true winds right off the METAR as being another concession, because we don't have a readout from a sensor on the airfield itself. In the same area, our winds only change every hour... a real sensor on the airfield would give you instant results and would probably swing and dance over the course of 5 minutes. As it is, we do the best we can with what we've got.

My feathers aren't ruffled or anything, I'm just explaining my point of view. I hope those last two paragraphs down sound like I'm preaching or upset. I'm not. Actually, it would be interesting to report the winds in magnetic. However Stephen has a good point: Winds are reported in 10-degree divisions. Since the METAR winds are already rounded, subtracting 15 would always land you on a xx5 heading and at least myself personally I'd always wonder which way to round it. ^_^

But here's a question in return: If a virtual pilot is using a true-weather feeder for their sim (bear with me, I've never simmed on-line and I just have a general sense of how it all works together) and getting the same weather in the sim that we're getting on the METAR reports... aren't their airfield winds (as reported by the airfield ASOS/AWOS) in the sim true heading as well? If you start feeding them magnetic winds are your reports going to be 15 degrees off from what they have in the sim? Unless they somehow correct the METAR reports for...

... ahhhh! Too complicated!! Can we go back to discussing the runways at KSFO? ^_^

James

[Keith Smith]

But here's a question in return: If a virtual pilot is using a true-weather feeder for their sim (bear with me, I've never simmed on-line and I just have a general sense of how it all works together) and getting the same weather in the sim that we're getting on the METAR reports... aren't their airfield winds (as reported by the airfield ASOS/AWOS) in the sim true heading as well? If you start feeding them magnetic winds are your reports going to be 15 degrees off from what they have in the sim? Unless they somehow correct the METAR reports for...

Your last sentence hit the nail on the head (at least for X-plane). I can personally confirm that XPlane takes the wind from the metar and applies local magnetic variation. I'm absolutely sure about that.

When I give wind reports to pilots, I've taken to just subtracting 10. It's not perfect, but it's closer to reality than giving winds in true north.

Truth be told, it's unlikely to have a big impact on any pilot's life on VATSIM, but I thought it was a worthwhile discussion. I also agree that procedures on VATSIM are going to different from real life from time to time. It's just good to acknowledge where those differences are, and try to minimize them where possible. For me, subtracting 10 from the METAR is something I can do on the fly without interrupting my groove, so I do it.