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2 Answers

How to apply magnetic variation over a long distance

Asked by: 3589 views Flight Instructor, Private Pilot

So on a short cross country trip I have been teaching to correct for magnetic variation by using one of the isogonic lines that lies about in the middle of the leg or trip. Is there a rule of thumb on how often to correct for magnetic variation on a much longer trip, say like 800nm? Where we live in Texas they change only half of a degree per isogonic line and so that would be quite burdensome to fill a nav log out with all those small differences.  Thanks

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2 Answers



  1. Skyman100us on Oct 24, 2017

    That is an excellent question. There is not a rule of thumb on how often to apply the variation correction. You would add or subtract the appropriate variation for each leg of the trip closest to the isogonic line as you have been teaching. It is doubtful you would teach your students to plan an 800nm cross country by simply drawing a straight line from point A to point B. Nor would the aircraft you are conducting your private pilot training in have the ability to fly safely for 800nm without a fuel stop.

    It would be a dis-service to your student(s) to teach them to skip steps in aviation because it is burdensome. Cross country planning is an important training tool and developing good habits early on will benefit your student for a lifetime. Your student may not limit their flying future to Texas so should he or she decide they want to fly somewhere other than Texas, say the NW or NE where there is 10-20 degrees of magnetic variation, the necessity of applying variation will be more apparent.

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  2. KDS on Oct 24, 2017

    Your navigation log goes from departure to check point, to check point, to check point, to destination. So what is a long flight is actually a series of smaller legs. Take the midpoint variation for that leg. Besides the change in variation, you will most likely also encounter a change in forecast winds along the way.

    If you really want to blow your mind, read about “great circle routes” versus “rhumblines”. Then there are programs that will give you a series of points that approximate a great circle route by creating a series of rhumblines.

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