RNAV approach TRK not matching DTK

[Karelpatch 29]

Hi,

I was flying the A2A Bonanza with GTN 750 and I loaded a vector approach to RW 17 on KTHV.

As I reached the initial fix, I put my AP in approach mode. The plane aligned to the 166 degrees vector, the CDI was showing a nice full arrow and when I reached the FAF the AP catched the glideslope and started the descent. 

 But from that moment, I watched the TRK moving away from the 166 DTK by a few degrees as we were approaching. At runway level, the track was at 171 degrees, leaving the runway to my left. 

What could have caused the plane to move away from the vector track? It was supposed to track the vector since it even did catch the glide slope. Is this about settings?

Some relevant screen grabs of the track:

https://imgur.com/gallery/b294ijK

First picture, I am on 166 vector.

Second one: I have reached FAF and slowly going off track (171 degrees on the picture.) The leg was activated with the countdown confirming 166 degrees. 

Thanks!

[RXP 2,338]

Hi,

I've seen the other discussion here:

Can you please confirm something:

• I see the GTN shows TRK = 171, and the 'vector line' shows it is pointing out the DTK on the map page.
• However, does the A/P fly you away from DTK or does it stay on the line?
• Furthermore, what about deactivating the Charts overlay and see if this makes a difference?

 

[Karelpatch 29]

25 minutes ago, RXP said:

Hi,

I've seen the other discussion here:

Can you please confirm something:

• I see the GTN shows TRK = 171, and the 'vector line' shows it is pointing out the DTK on the map page.
• However, does the A/P fly you away from DTK or does it stay on the line?
• Furthermore, what about deactivating the Charts overlay and see if this makes a difference?

 

Yes, I posted here because I figured I could get more help.

The A/P did exactly what was shown on the GTN: it flew away from DTK and TRK information seems accurate, as the course needle. 

[RXP 2,338]

In addition to the other questions above, have you by any chance modified the Prepar3D navdata or are you using stock P3D?

[Karelpatch 29]

I have the latest airac data.

[RXP 2,338]

 

1 hour ago, Karelpatch said:

Furthermore, what about deactivating the Charts overlay and see if this makes a difference?

and what about this?

I'm also wondering if there could be a difference in the 'airac data' you're adding in the simulator which could cause this.  This kind of error is generally related to 'magvar' differences between the magnetic models embedded in the device and the simulator, and you may want to cross check what is the magvar reported by P3D and the one reported by the GTN and see if there is roughly 1.5 to 3 deg difference between the two.

[Karelpatch 29]

I’m going to try this when I have the time, ASAP. 

I’m curious: why would deactivating charts overlay change anything?

About the magnetic variation: if this was the case, shouldn’t it cause an issue in many other cases? Also, weirdly enough, the previous 166 degrees leg from IAF to FAF was followed with no issues.

[RXP 2,338]

So that you know we're evaluating this on our end along with you. Charts or magvar could be a factor if there is a bug in the trainer, I'm just trying to check with you in eliminating layer after layer.

 

 

[Karelpatch 29]

You rock, thank you! 

It’s actually mostly curiosity for me at this point. I was wondering if I did something wrong in the way I setup my approach. Or maybe some settings I need to change. 

I haven’t had the time yet to try the GTN 750 in more than a few flights, nor to retry that specific flight. I definitely will. 

If I find I can’t reproduce the issue again or that it’s specific to this airport, this will add another layer of realism: never 100% rely on your avionics. 

[RXP 2,338]

This might indeed be a case of database base error too. I've cross check Garmin's advisories but I've not found anything for this airport for now:

https://www.garmin.com/en-US/aviationalerts/

For example one of the latest:

16 January 2019
Subject:
Neuquen, Argentina
Presidente Peron (SAZN)
VOR Rwy 27
Cycle1901

Incorrect Fix Position

Jeppesen NavData cycle 1901, effective 3 January 2019, contains incorrect data. The position/coordinates for the fix ZN031are incorrect. The fix ZN031 should be NEUQUEN (NEU) VOR DME R-151/D13.0: coordinates S39 08 42.89 W068 0157.48

[Karelpatch 29]

Ok, so I did some tests this morning but I was lacking time:

I reloaded the same approach and I got the exact same behavior (change of track after the FAF to RW leg is activated and I catch the GS.) 

- I searched where the magvar was displayed on the Garmin. I only could find the one on the chart and I compared it with the most recent data and it shows the same magnetic variation. Is there a menu on the Garmin to display the magnetic variation?

- I couldn’t find where to deactivate charts overlay?

- When going through the device informations I discovered that I had corrup/unauthorized databases: Basemap and Safetaxi. Is it normal?

 

Thanks!

[RXP 2,338]

Hi,

Magvar displays in the Unit Setup page (GTN Pilot's Guide 16.7.1 - open the manual and search the PDF for "Magnetic Variation")

Charts overlay is detailed in 9.1.1.6 (search the PDF for this)

As for the database files, can you please detail how you've found out / where was it said, they were corrupted/unauthorized?

[Karelpatch 29]

This is where I found the databases informations:

https://imgur.com/gallery/XCe6F5X

[RXP 2,338]

Ok I get it. This is the "Standby" database information which is not relevant in our case (I guess).

[Karelpatch 29]

7 hours ago, RXP said:

Hi,

Magvar displays in the Unit Setup page (GTN Pilot's Guide 16.7.1 - open the manual and search the PDF for "Magnetic Variation")

Charts overlay is detailed in 9.1.1.6 (search the PDF for this)

As for the database files, can you please detail how you've found out / where was it said, they were corrupted/unauthorized?

So, when I go into the Unit setup, I should see the current magvar on the button for the “Magnetic variation” option?

I will definitely try everything this week. Really curious to see the cause of the issue.