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In the southern hemisphere you would undershoot when turning towards a southerly heading and overshoot towards a northerly heading, and the opposite in the northern hemisphere. Why is this?

Pondlife
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Aget Aviano
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1 Answers1

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There is a good video that goes over the topic here ~2:45 into the video. It has to do with the way a magnetic compass responds to turns. When turning north the compass lags when compared to the tun of the physical aircraft so you need to overshoot the heading based on what the compass is showing. When turning south the compass leads the turn and you must roll out early to make your desired heading.

This site offers a nice explanation of why this happens in the compas

Northerly turning error is due to the mounting of the compass. Since the card is balanced in fluid, when the aircraft turns, the card is also banked as a result of centrifugal force. While the card is banked, the vertical component of the Earth's magnetic field causes the north-seeking ends of the compass to dip to the low side of the turn. When making a turn from a northerly heading, the compass briefly gives an indication of a turn in the opposite direction. When making a turn from the south, it gives an indication of a turn in the correct direction but at a faster rate.

Similar explanations show up in this thread. Simply put its not the turn that causes the error but the banking of the aircraft. By rolling the compass housing to one side we allow the compass to tilt differently than it would in level flight. Since the pole is generally beneath the aircraft the roll allows the compass to pitch in a slightly different way causing the error we see.

Dave
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  • I didn't see the video actually explain why, except briefly mentioning the “magnetic dip”, but without even saying what it is. – Jan Hudec Aug 26 '18 at 17:11
  • Yeah, ive only known that it is cause by the dip. But how does this dip exactly work/function and why does it happen. Thus how does it lag the compass. – Aget Aviano Aug 30 '18 at 03:46
  • @AgetAviano I found a decent explanation and added it above, if its still unclear I will try and expand more. – Dave Aug 30 '18 at 05:03
  • @AgetAviano if you want a really detailed answer you may want to ask on Physics.SE – Notts90 Aug 30 '18 at 06:44
  • @Dave thanks alot for the additional information. I understand the reason for the lag/lead better now. – Aget Aviano Aug 31 '18 at 02:30
  • Related -- http://aviation.stackexchange.com/a/97267/34686 -- with a caution -- it's not the direction you are turning that matters. It's whether the aircraft's instantaneous heading contains a northerly or southerly component. This pertains to the second and third sentences of this answer. Likewise, for deciding whether to roll out "early" ("undershoot) or "late" ("overshoot"), the key factor is whether your intended final heading has a northerly or southerly component, not whether you are turning to the north or to the south. – quiet flyer Jan 31 '23 at 14:51
  • Similarly, re the quoted content in this answer, the direction you are turning from only matters when describing the behavior of the compass as you are initiating the turn. When deciding whether to "overshoot" or "undershoot" on the roll-out, all that matters is whether the final target heading contains a northerly or southerly component. For example, when trying to roll out on a SW heading, you'd plan to "overshoot" regardless of whether you were turning left ("from the north") or right ("from the south"). – quiet flyer Jan 31 '23 at 14:56