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5 Answers

Formula for UNOS Magnetic Compass Error

Asked by: 1488 views General Aviation

30 deg undershoot/overshoot is used in the US because it's roughly in the 30 deg latitude region. What is the rule of thumb formula for calculating the turn error for magnetic compass use in other latitudes?

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5 Answers

  1. Mark Kolber on Nov 01, 2016

    The same. Even in the US at different latitudes. The amount of N-S copass error is roughly equal to degrees latitude.

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  2. MasterFlight on Nov 01, 2016

    Use the latitude. I live in the US near 45N latitude, and recommend 45 degrees lead/lag when turning through north/south up here. At the equator, turning errors would be negligible.

    But let’s also consider the forest through the trees. Compass turns are inherently sloppy. If you roll out within 10-15 degrees of your target, that is really well done. You would then want to fine tune your heading with timed turns.

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  3. Best Answer

    Kris Kortokrax on Nov 01, 2016

    There is a rule of thumb. It can be found on page 5-13 of the Instrument Flying Handbook.

    It is 15 degrees + half of the latitude. If you are in the 30 degree region of the U.S. (which would be around the Gulf, or south Texas) the rule works perfect. 15 + 15 = 30. If you live in my area (latitude 41 degrees), it would be 15 + roughly 21 = 36 degrees.

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  4. Drew on Nov 01, 2016

    Kris, that was what I was looking for. I think the formula that I came across before was more complicated, but I like this one better. Like others have said, it probably doesn’t make too much of a difference in the big scheme of things. Thank you.

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  5. Brian on Nov 11, 2016

    To add to Kris’ response:

    That formula applies to turning to a north or south heading. We rarely turn to those exact headings so what about the rest of them? Using 30 for north and south as a starting point here are the rest of them:

    360/180 = 30 degrees (15 + 1/2 lat)
    330/030/150/210 = 20 degrees ((15 + 1/2 lat) * .66)
    300/060/120/240 = 10 degrees ((15 + 1/2 lat) * .33)
    090/270 = 0 always

    If you’d like, draw a compass on paper and throw those numbers in to make it easier to see.

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