Discrepancy between DI and RXP GTN750 heading

[rog943 14]

After superb help from several members I now have the GTN750 up and running.  And it's awsome.  Thanks guys.

Strange to find though there is a difference between the Direction Indicator and the Heading shown on the 750 screen.  I'm being picky at 2-3 degrees but then I'm being picky.  I wonder where the two get their data from and maybe I have to configure yet something else?  I even wondered if it was a parallax error but no - I changed my position and no difference.

Can I anyone help get rid of it?

[RXP 2,338]

Hi,

I'm glad you're up and running now, and I thank you for your kind words!

Can you please be more specific and tell us whether this is happening all over the world or only in a particular region and which?

What is the mag var in this region?

[rog943 14]

Hi Jean-Luc,

Me again!  I'll get it perfect yet.

It happened to be Leeds/Bradford.  I just taxied round a bit and the Di reads more than the 750 figure on all headings by 2 degrees.  Can't really see that the Mag Variation would affect the difference.  I've looked in the set up screen (Shft+Rt Clk) and there seems to be a link here between the HSI and the 750.

[rog943 14]

I just nipped over to Toronto and it is the same.

[Bert Pieke 7,585]

Does pushing the "D" key make a difference?

[RXP 2,338]

3 hours ago, rog943 said:

Can't really see that the Mag Variation would affect the difference

If both the GPS magvar and the SIM magvar difference is more than 0.5 deg, depending on how the gauge is rounding the value you can see up to 2 deg difference (say GPS is 1.51 and is rounding down and gauge is 2.01 and is rounding up (bug), you might see GPS 1deg and gauge 3deg. More realistically a difference of more than 1 deg can show up as 2 deg diff.

Edited May 15, 2021 by RXP

[Bert Pieke 7,585]

One thing I am not clear on... Are you comparing the Track on the GTN to the Heading on the DI?  Those are not necessarily the same.. 😉

[glennav8r 19]

Have you set the DI off the magnetic compass? Are they the same.

[rog943 14]

Morning, guys (it is here).

Somewhere glennav8r, I'm sure I read something about a sensor that adjusts the DI so there is no gyro drift.  That right?  When I was flying it was a constant check.  On the sim I've become used to the sensor theory so I seldom check.  It could be, but right after switching on?

Bert, while musing on this and dropping off to sleep I did think about this.  Yes I am comparing the Trk and the DI.  That was a natural thing to do surely, but what is the subtle difference?  Is it what Jean-Luc referred to?

In the end is it something I just have to live with?  Or can I sync them?

[rog943 14]

To add to this I notice that in Settings/Config settings that I can adjust the HSI deviation - lateral and/or vertical.  Is this something to explore?

[RXP 2,338]

6 hours ago, rog943 said:

Yes I am comparing the Trk and the DI.

The DI heading is geo-magnetic and can deviate (calibration needed)

The TRACK is True +- the mag var from the GPS database.

In practice, both the sim and the gps are using a mag var model and there could be small discrepancies between the two which can lead to up 2 deg truncated values difference as explained above and is normal (at least in the sim). IRL, the magnetic sensor and the GTN mag var model could have a small difference as well theoretically.

Actually, can someone with a real GTN confirm they are not always matching IRL?

Edited May 15, 2021 by RXP

[Bert Pieke 7,585]

In addition, in flight, the TRK and HDG differ because of wind correction.

TRK is the actual track flown over ground!

HDG is the direction that the nose is pointing.. 😉

[RXP 2,338]

1 hour ago, Bert Pieke said:

In addition, in flight, the TRK and HDG differ because of wind correction.

+1 for the correct answer!

which in fact makes my IRL question above nearly impossible to answer in practice LOL

[fppilot 4,368]

17 minutes ago, RXP said:

+1 for the correct answer!

It's just Bert being Bert!  LOL!

[rog943 14]

Thanks for the contributions.  In the end is my flying good enough to make this small difference matter?  Of course it is🙂!  Hmm.

Seriously though it's good to see how it all comes together.  I'm happy.

However It does still leave the question of what the HSI deviation setting actually is and what it might do - either lateral or vertical.  Shall I leave both at 1.0?