1
$\begingroup$

enter image description here
(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?

enter image description here

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.

$\endgroup$
1
$\begingroup$

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!

$\endgroup$
0
$\begingroup$

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.

It is not correct. When it comes to VOR stations, the 82 and 267 figures on the chart are magnetic radials, not courses.

The FAA Chart Guide notes:

RADIAL OUTBOUND FROM A VHF/UHF NAVAID

All Radials are magnetic

whereas for an LF navaid for example (note the distinction, radial vs. bearing):

BEARING INBOUND TO AN LF/MF NAVAID

All Bearings are magnetic.

and

Compass Roses are orientated to Magnetic North of the NAVAID which may not be adjusted to the charted isogonic values.

As the last quote reads, VOR stations may not be realigned after a change in the magnetic variation, this is also explained in the post 'Is my understanding of magnetic variation and declination (for a VOR) correct?'.

I think your plotter method you mentioned is the better estimate.

FAA InFO

If you want a more official proof that discusses that specific problem, look no further than this FAA InFO. The issue became apparent to some users with RNAV procedures that use a VOR merely as a geometric point, and not a radial source dependent on when it was last aligned. The example they give is rather a big one:

enter image description here

That's almost 10° difference for the same exact course.


By using gcmap.com and the coordinates of the three waypoints, the true headings are 81.6° and 82.3° (geometrically), and the magnetic headings are 86.6° and 88.2°. So GIJ's alignment is off by ~4°. (The station was last aligned in 1965 according to airnav.com.)

Note: The website refers to true heading as initial and accounts for the magnetic variation for the magnetic heading as explained on this page. To view the first decimal place click on 'Show fractional heading'.

$\endgroup$
  • $\begingroup$ A very good explanation, but this airway segment is complicated by the fact it is two legs, not one. The airway includes a waypoint at BENJO at which point there is a small course change. $\endgroup$ – Gerry Sep 30 '18 at 12:57
  • $\begingroup$ I already have BENJO in the gcmap calculation: two true courses of 81.6° and 82.3°. How can I improve the answer further? Thanks @Gerry $\endgroup$ – ymb1 Sep 30 '18 at 14:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.