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(skyvector.com)

When I want to calculate the leg time from GIJ to CRL going along the airway J554, at say FL190, I know that I have to calculate the ground speed first, but in order to do that, should I take 082° magnetic course, 087° magnetic course (reciprocal of 267° from CRL), or should I enter in the CX-2 flight computer (Plan Leg) an average of them?

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What I would normally do is to enter in the CX-2 flight computer the true course (calculated with a plotter) in order to get the ground speed; but I want to know if it is correct to get the true course by converting the known (shown on the chart) magnetic course by adding or subtracting to it the magnetic variation, whatever be the case.

In this case the magnetic variation around GIJ is 1W and the magnetic variation around CRL is 3W, so the true course of the GIJ - BENJO - CRL leg would be 81° and 84°, am I correct? Once again I get confused without knowing if I should take true course 81°, 84° or an average of both.

In the explanations given in the FAA book ATP Test Prep, relative to the nav questions in the performance chapter, the course to be taken, is sometimes the magnetic course that corresponds to the beginning of the leg, other times it is taken from the end of the leg and other times it is an average of the leg's initial and ending magnetic courses.

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  • $\begingroup$ What do you have the winds aloft in? Usually in true headings, no? $\endgroup$
    – Jan Hudec
    Dec 27, 2021 at 22:53
  • $\begingroup$ Practically it should not matter, because you don't have the winds aloft accurate to a degree either. $\endgroup$
    – Jan Hudec
    Dec 27, 2021 at 22:55

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I would calculate that leg in two segments. From GIJ to BENJO flying a course of 082 magnetic (50 NM), thence from BENJO to CRL on a magnetic course 087 (79 NM).

In addition, the magnetic variation is only applicable to hand plotting an arbitrary course using the map's orientation as a reference; IFR charts list true magnetic courses on their airways for ease of reference.

After that, you just need the winds aloft and you're off to the races!

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  • $\begingroup$ What is a “true magnetic course”? Given that “true course” is used to indicate course according to reference ellipsoid as opposed to magnetic course, saying “true magnetic” is confusing. And as far as I can tell, the numbers on the map (where adjacent to VORs) are radials, which used to correspond to magnetic headings but usually don't any more because the VORs are not periodically re-aligned to the changing magnetic declination. $\endgroup$
    – Jan Hudec
    Dec 27, 2021 at 22:52

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