Welcome Guest. Sign in or Signup

5 Answers

Formula for UNOS Magnetic Compass Error

Asked by: 28456 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?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +1 Votes Thumb up 2 Votes Thumb down 1 Votes



  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.

    0 Votes Thumb up 1 Votes Thumb down 1 Votes

  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.

    +19 Votes Thumb up 19 Votes Thumb down 0 Votes



  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.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  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.

    +6 Votes Thumb up 6 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.