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Well, isnt that one of the reason's one would TIME a turn? because of the compass error??
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Normally for standard compass error, one would either overshoot the turn if turning to a northerly heading, or undershoot the turn if turning to a southerly heading, ie- OSUN. That means for turns to, say, 290 or 070 (northerly), one would overshoot the turn by about 10 degrees, and the compass would roll back to the correct heading. If turning to a 350 or 010 heading, overshoot by about 30 degrees and you'll be safe. Same for southerly turns, but undershoot them. BL is: the closer you're rolling out to east or west, the lesser you have to over/undershoot.
Timed turns are for when your DG or compass, or both, are inop. By setting a 3 degree/second turn rate (or standard rate turn), you can hack the clock, roll into a turn, set standard rate and by counting the seconds elapsed, know how much heading change has gone by.
MD