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Problem understanding magnetic heading

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starling View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote starling Quote  Post ReplyReply Direct Link To This Post Topic: Problem understanding magnetic heading
    Posted: December-03-2012 at 7:03pm
I feel I should know the answer to this question and I suspect I'm going to feel pretty stupid when I hear the explanation.

Please see screen capture:

I'm just 10 minutes into my flight  and am flying the stock Cessna with the glass cockpit.
 
In this example I've done a "Direct To" and have a flight plan to EGFF at heading 275 (see A).
The DTK and TRK both show 275 (see B)
I have set the heading bug (just for ref) to 275 (see C)
The GPS magenta pointer is aligned and is pointing to 275 (see C)

However the magnetic heading shows 306 (see D) 
...and the mag heading varies over time even when the other readings remain unchanged.

In the MFD the little plane is shown accurately following the GPS course (see E).

Continuing with flight I arrived at EGFF as per the flight plan.

This always happens. Frustratingly I don't understand why the magnetic heading varies so much from the TRK heading and why the difference between the mag heading and the TRK heading varies over time (minutes). 

I would really appreciate an explanation.

Bruce


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starling View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote starling Quote  Post ReplyReply Direct Link To This Post Posted: December-03-2012 at 8:25pm
...since posting I've repeated the above flight but this time setting the course (CRS-BARO knob) using the single green NAV pointer before pressing the CDI soft-button to switch to GPS. This time the magnetic heading did not vary and stayed pretty much the same as the TRK bearing. The magenta GPS line also pointed directly upwards in line with the plane.

So ok, setting the course using the CRS-BARO knob first works (I tried it both ways a couple of times to prove it to myself) and I suppose ithere is a logic to it - but...

I just don't understand why it was necessary to set the course using the single green NAV needle before changing to the magenta GPS needle?

Bruce
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Bert Pieke View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bert Pieke Quote  Post ReplyReply Direct Link To This Post Posted: December-03-2012 at 10:17pm
Could it simply be the effect of wind?

Your track and heading are definitely different once any wind is blowing.
Bert
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mjrhealth View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mjrhealth Quote  Post ReplyReply Direct Link To This Post Posted: December-04-2012 at 3:22am
The north pole is not a constant, it moves. Do a search on google you can see the track it is taking. True North is a fixed point and doesnt change, it is not set ny magnetic North, The GPS uses true North the Other compass uses magnetic and they do not always agree. In QLD Mag N is 7 deg less than true north. Ie 0 deg Mag is 7 deg True. Have to deal with it all t5he time when setting up sat dishes.The other issue as mentioned in previous post is if the wind is blowing from you side it will blow you side ways, so you have to track slightly more into the wind to maintain course. This is why you have a course heading Pointer, they dont always line up.
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starling View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote starling Quote  Post ReplyReply Direct Link To This Post Posted: December-04-2012 at 4:27am
Bert - Thanks for your help, wind effect could well a factor.

mjrhealth - I did as you suggested and Googled magnetic north - It was quite an eye opener, whilst I knew there was a difference I'd assumed that the difference (and movement) was far more subtle than it is. Thanks for your help.

Bruce
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GrandpaKim View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GrandpaKim Quote  Post ReplyReply Direct Link To This Post Posted: October-16-2013 at 1:52am
mjrhealth, the default GPS displays magnetic north.  I've flown in all parts of the world and the deviation can be over 20.  The magnetic compass has always agreed with this, taking allowance for wind of course.
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