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flybubba
16th Sep 2010, 04:42
Sorry if this has been answered before, but why does the course the FMS calculates/shows differ from the one inputed? I seem to remember something about great circle.

BOAC
16th Sep 2010, 08:05
Flyb - these questions of yours need more flesh! Which 'course', how input, give example, which FMS...........

DFC
16th Sep 2010, 10:00
There are a number of reasons why there can be a small difference between what you see on the chart and what you see in the FMS after putting in a route.

It is not down to Great Circle alone since the only time you are not flying a great circle is when you are being vectored by ATC and even then since magnetic is the reference you will actually be following a wandering track which may not be either a great circle or a rhumb line over a long distance (changes in variation). ATC however will have airspace represented on their screen which of course will (when a straight line) be a great circle.

Remember that the FMS uses the local value of magnetic variation and applies this to the local True track required (no wind). The figure represented on the chart will depend on what navaid is being used. If you remember your ATPL theory - for a VOR (tracking a radial), the variation used is that at the station while for an NDB, the variation used is that where the aircraft will be.

Here is an example that is quite simple but hard to explain!!

VOR A (variation 10E) to ONEPT (variation 0) True track 360

ONEPT (variation 0) to VOR B (variation 10W) True track 360

As I hope you can see that when over ONEPT you are directly North (T) of VOR A and directly south (T) of VOR B.

I hope that you can also see that overhead ONEPT you are on the 350 Radial from VOR A and at the same time are on the 190 Radial from VOR B.

Imagine how that would look on a RMI at ONEPT with zero wind - magnetic Heading 360, tail of 1 pointer at 350 and head of the other pointer at 010.........and all correct and all telling you that you are tracking exactly from VOR A to VOR B!!!!!!

So in the FMS speaking very generally, to keep things simple, the FMS works in true for everything right up until the time that it displays something to you and then it applies local variation to give you a magnetic figure. This is why there can be some difference between what you see on the chart and what you get from the FMS.

So now ask yourself if you input a route from ONEPT to VOR B in the above example what will the FMS say that the magnetic track is?

In general these differences are going to be a maximum of 2 degrees in places like Europe. When over the ocean pre-prepared tables are used to crosscheck the tracks between points on the route (LAT/LONG) and/or any other point we have to route to on an amended clearance.

Perhaps if you draw a diagram then it will be a bit more clear. Remember that this is a general example and the manual for your parrticular FMS should have more information.

Most SOPs that I have used over the years require that the crosscheck of the flight plan gives you tracks that are +/- 2 degrees and leg distances that are +/- 2nm when in domestic airspace i.e. Europe or US/Canada etc and I have not come across many cases where there was a significant difference.

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Hope I got the Variation the right way round!! :}

flybubba
16th Sep 2010, 12:49
I understand the variation explanation. But for example, cleared to join localizer. Do a NAV intercept, input intercept course (published loc course), and you will often see different course calculated on FMS. I would be surprised if variation changes so much over short distance.