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Significance of variation in airport chart

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Significance of variation in airport chart

Old 25th Jan 2012, 12:51
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Significance of variation in airport chart

Typically in Jeppessen Airport Chart (10-9) the variation is depicted in the form of one arrow with complete arrowhead and one with half arrow head. I know this is depiction of Variation of the place. But what is significance of this? How would it help a pilot in either takeoff or landing or ground ops?
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Old 26th Jan 2012, 06:59
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1. As the TAF and METAR always give wind direction in degrees true, the variation is needed to convert this into a magnetic value to calculate cross-wind components relative to the (magnetic) runway directions.

2. ATIS broadcasts and 'tower' winds given over the Radio are in degrees magnetic, but you wouldn't normally hear these until you were in the cockpit, so the variation is supplied for pre-flight planning purposes.

Other than that, I agree that a knowledge of the variation plays little part in take-off, landing and ground operations; it's more of an aid to planning.

Hope this helps

Eck
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Old 26th Jan 2012, 07:40
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Have wondered why TAFs and METARs are always in T.
They're localised winds so why not put them M anyway?
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Old 26th Jan 2012, 08:29
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Probably because they are forecast by the Met office using meta data, so it saves them adjusting the variation for all of the different airports out there. A bit of a historical inertia as well, as it's easier to measure true off a chart, so true winds in the "old days" were easier to work with for planning.
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Old 26th Jan 2012, 09:28
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Fair enough and thanks Checks, but one would think with all
the modern software available to forecasters these days that
the wind could be auto adjusted for magnetic variation by the
program as soon as its inserted into the database.
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Old 27th Jan 2012, 06:35
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Jeppessen enroute tracks and tracks on b737 FMCs

Taking this discussion further what are the airway tracks shown on Jepp enroute charts True or Magnetic? What about tracks shown on B737 FMCS, true or magnetic? And tracks shown on computerized flight plans of any airlines?
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Old 27th Jan 2012, 16:54
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Their all Magnetic.
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Old 27th Jan 2012, 17:03
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Their all Magnetic.
Not quite true. In northern arctic areas of Canada and Greenland where variation can be 30-40 degrees or more and change rapidly over sector lengths, then airways tracks and runway headings are often quoted as true (with the letter T after them) and they are very specific when giving bearings and headings.

Then there's the polar grid.......
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Old 27th Jan 2012, 18:02
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Also in a Head Up Display in order for the system to paint the synthetic runway display it needs the localiser track in degrees true. This is normally done automatically by the software but having the variation allows the crew to calculate this value.
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Old 27th Jan 2012, 18:18
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In the GSNX if you didn't have the right variation in it could do some weird and wonderful things if you hoped to setup a persudo votac approach.
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