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The Earth's magnetic poles are constantly shifting, resulting in a significant distance between the true and magnetic poles. This means that while using them as references for heading at lower latitudes makes sense, using them as such while flying in polar regions causes often unacceptable inaccuracy.

In a simulator, I noticed that many modern airliners have a toggle to switch between referencing true and magnetic north as appropriate; but at what distance from a pole would this toggle be activated?

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    $\begingroup$ Whilst the north magnetic pole does shift it is a slow process, and not the reason for using true north at the poles. The reason is that close to the poles the compass will try to point at the ground instead of moving freely. $\endgroup$
    – Ben
    Dec 1, 2017 at 2:42
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    $\begingroup$ I hadn't considered that. Good point! $\endgroup$
    – Jules
    Dec 1, 2017 at 2:44

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In Boeing aircraft the default is to select MAG. When the aircraft enters the polar region it will auto switch to TRU. The pilot can also select TRU when outside the polar region.

The polar region is defined as north of 82 deg N latitude or south of 82 deg S latitude. It also includes the region north of 70 deg N between 80 deg W longitude and 130 deg W longitude and the region south of 60 deg S between 120 deg E and 160 deg E longitude.

Polar Regions, B777 FOM

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  • $\begingroup$ What is the reason for the additional areas below the 82°? $\endgroup$ Dec 4, 2017 at 23:06
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    $\begingroup$ @Czechnology Those are the areas around the magnetic poles. Near them magnetic compasses (and heading) are pretty much useless. $\endgroup$
    – Gerry
    Dec 5, 2017 at 0:17
  • $\begingroup$ @Gerry: So why is most of the true-heading area referenced to the geographic poles rather than the magnetic poles? $\endgroup$
    – Vikki
    Jan 3 at 16:50
  • $\begingroup$ @Vikki Because True Heading is defined to be referenced to the geographic poles. The Mag poles drift and the magnetic variation in the area doesn't follow a consistent pattern so it would be difficult maintain up-to-date magnetic variation charts. Most navigators do their underlying computations in true and then apply mag var along the computed route. True heading is directly tied to the Lat/Lon coordinates so it makes sense to use it. Converting it to Mag heading makes it easy to fly with the use of a mag compass. Near the poles, the extreme mag var makes the compass is pretty much useless. $\endgroup$
    – Gerry
    Jan 3 at 17:44
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    $\begingroup$ @Vikki The mag pole has (was) been shifting at a pretty steady rate so they have planned updates to the model every few years based on an acceptable error. Recent shift have been greater than expected so they did an update before the next planned update, hence an unplanned update. $\endgroup$
    – Gerry
    Jan 4 at 4:09

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