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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?

Jules
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    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. – Ben Dec 01 '17 at 02:42
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    I hadn't considered that. Good point! – Jules Dec 01 '17 at 02:44

1 Answers1

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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

Gerry
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  • What is the reason for the additional areas below the 82°? – Czechnology Dec 04 '17 at 23:06
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    @Czechnology Those are the areas around the magnetic poles. Near them magnetic compasses (and heading) are pretty much useless. – Gerry Dec 05 '17 at 00:17
  • @Gerry: So why is most of the true-heading area referenced to the geographic poles rather than the magnetic poles? – Vikki Jan 03 '22 at 16:50
  • @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. – Gerry Jan 03 '22 at 17:44
  • @Gerry: It's difficult to maintain up-to-date magnetic-variation charts, but people do so anyways, since these charts are essential for navigating with a magnetic compass. And, since the rationale for the true-heading zones existing is the extreme magnetic variation and high magnetic inclination at high magnetic latitudes, it would make more sense for these zones to be centered on (the current locations of) the magnetic poles, rather than being mostly centered on the geographic poles with relatively-small cutouts for the magnetic poles. – Vikki Jan 03 '22 at 19:37
  • @Vikki It's just not about having the mag var charts. Aside from the issues with the compass due to the vertical component near the poles, the fact that the mag var changes dramatically over relatively short distances makes the use of a compass impossible. At typically cruising speeds, the mag var would drive constantly changing headings. Take a look at the map here from NOAA. Zoom in on the polar map. Lay a course line and try to compute a heading. – Gerry Jan 03 '22 at 22:24
  • @Gerry: Mmm, makes sense when you explain it that way. Also, from that link: "Due to unplanned variations in the Arctic region, scientists have released a new model to more accurately represent the change of the magnetic field" - are there planned variations in the earth's magnetic field? – Vikki Jan 03 '22 at 23:55
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    @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. – Gerry Jan 04 '22 at 04:09