Crosswind components & Variation
 

Long time since school and haven't operated in too many areas where Magnetic variation is significant. Can someone remind me as to whether we correct ATC broadcast W/V's to Mag for crosswind component calcs or is it TAF's & Actuals?
Thanks.

TAFs and actuals need correcting:

"Anything written is True"

ATC/Tower are in deg magnetic.

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.....unless the person transmitting it is reading it from something written.

Food for thought indeed! I was just watching a Fairbanks Alaska pilot brief which mentioned the subject due to 26' of variation. So anything written down needs correcting to MAG.
To carry on with the thread, the wind that ATC gives these days ( At LON STN ) anyway, is an average reading for the last 7 mins, or was it 9 mins. Is this wind also MAG? I always ask for an instantaneous wind if it's looking grey or lumpy, so I assume that is MAG.

In the Great White North the ATIS reports winds averaged over 2 minutes (Mean wind). Winds given by Tower controllers are averaged over the previous 5 seconds (Now winds). Both are MAG.

FSS when giving TAFs/METARS will state that fact and under those circumstances I assume(correctly or otherwise http://www.pprune.org/ubb/NonCGI/eek.gif ) that they are reading them and that winds are given as True.

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edited for appalling typing

[This message has been edited by Code Blue (edited 12 March 2001).]

This is how it works:

Aircraft used to only carry a magnetic compass.

Because the aircraft used a magnetic compass, the runway directions are marked for their magnetic heading (that's why they change from time to time.)

Because pilots are interested in the wind relative to the runway, and the runway sirection is magnetic, the wind on the ATIS and on Tower reports is always given in magnetic.

When pilots fly from A to B, however, they are usually looking at a map, and as the magnetic variation changes with geographic location, maps are always orientated to True North, so forcast winds are always written with reference to True North.

Some aircraft navigation systems display the wind in True North only, some in Magnetic, and some in either, depending on the configuration!