Magnetometer alignment?

[Eugr]

I was looking at the video that I took during my first taxi with my new Dynon installation and noticed that GPS track doesn’t match the heading on the ground, although I was taxiing straight at that moment. The heading also doesn’t align with the taxiway, seems to be off by ~3-4 degrees.

I performed a compass calibration on the compass rose earlier, and I noticed a couple of degrees difference as well, but I though I wasn’t aligned well with the compass rose itself - it’s in a very confined space at that airport, so I didn’t have much space to maneuver.

So, the question is - could it be that the magnetometer itself is not well aligned with longitudinal axis? Should we try to realign it (it’s a remote magnetometer mounted in the rear)? Or will in-flight calibration help?

 

[jakej]

All bets are off until after you do the inflight calibration - the precision after that is amazing. Just throw it around in left/right hand orbits until you are well over the 100 on the counter, you will be rewarded.

 

[Marc_J._Zeitlin]

All bets are off until after you do the inflight calibration - the precision after that is amazing. Just throw it around in left/right hand orbits until you are well over the 100 on the counter, you will be rewarded.

I wish that were true. I've never had good experience with either Dynon or Garmin's magnetometers from an accuracy standpoint, for either me or my customers. They either drift over time, or have errors in one direction and opposite errors in the other direction. This, after the ground cal and the in-flight cal. And mounting the magnetometers in places FAR away (in composite aircraft) from motors, magnets, high current wiring, etc.

I'm glad that it works for you, but I've got multiple examples of them being relatively inaccurate even when doing everything exactly per the calibration instructions.

Now, pretty much the ONLY think affected by incorrect magnetometer readings will be incorrect wind indications, as well as slightly annoyed ATC folks when they give you a heading and your're 5 - 10 degrees off what they think you ought to be flying, but still - you'd think that it would be more accurate...

 

[Eugr]

All bets are off until after you do the inflight calibration - the precision after that is amazing. Just throw it around in left/right hand orbits until you are well over the 100 on the counter, you will be rewarded.

I thought in-flight calibration was to compensate for magnetic dip in all attitudes. Will it also try to compensate for slightly misaligned magnetometer and/or some static magnetic interference from surrounding metal (if any)? I won’t be in the shop until Tuesday afternoon, so wondering if I should ask my avionics tech to check the alignment or flight test it first.

The magnetometer is mounted in the tail section away from any potential interference sources. See picture below (have no idea why it flipped to the side, but left is top actually). Seemed pretty straight when we mounted it, and will not be easy to adjust there...

 

[jakej]

Marc - with respect, I think there is something going on in your case that is causing the issues you’ve had. My comments are based on experience with ~ 50 RVs installs, never had 1 compass calibration fail & my own aircraft as well as others I’ve done are composite too.

 

[kellym]

I wish that were true. I've never had good experience with either Dynon or Garmin's magnetometers from an accuracy standpoint, for either me or my customers. They either drift over time, or have errors in one direction and opposite errors in the other direction. This, after the ground cal and the in-flight cal. And mounting the magnetometers in places FAR away (in composite aircraft) from motors, magnets, high current wiring, etc.

I'm glad that it works for you, but I've got multiple examples of them being relatively inaccurate even when doing everything exactly per the calibration instructions.

Now, pretty much the ONLY think affected by incorrect magnetometer readings will be incorrect wind indications, as well as slightly annoyed ATC folks when they give you a heading and your're 5 - 10 degrees off what they think you ought to be flying, but still - you'd think that it would be more accurate...

I agree that something is wrong, but I've not seen any drift in my Dynon, and after hardware changes had to calibrate it several times. Second, as a former approach radar controller many years ago, they are not going to notice a 5 degree difference unless the vector is for 40 miles or more. They use 5 degrees to fine tune their issued headings. IIRC most of my headings are within 2 degrees.

 

[Eugr]

So you guys think in-flight calibration may actually eliminate this different? Could this also be a botched ground calibration (e.g. what happens if I don’t align with a compass rose well enough)?

 

[jakej]

So you guys think in-flight calibration may actually eliminate this different? Could this also be a botched ground calibration (e.g. what happens if I don’t align with a compass rose well enough)?

From my experience if it's not too far off then the inflight cal will fix that.

Is the that the ADAHRS network connector on the forward side of the unit ? if so then it's reversed to what it should be - the picture I'm seeing is not clear enough. I don't see the Static, Pitot or AOA lines either however that probably is a photo issue too.

 

[Eugr]

From my experience if it's not too far off then the inflight cal will fix that.

Is the that the ADAHRS network connector on the forward side of the unit ? if so then it's reversed to what it should be - the picture I'm seeing is not clear enough. I don't see the Static, Pitot or AOA lines either however that probably is a photo issue too.

How much is too far off? I estimate mine to be between 4-6 degrees off (will check again when I get to the plane on Tuesday).

As for the unit, it’s not ADAHRS, it’s a remote magnetometer (SV-MAG-236). ADAHRS is installed on the avionics shelf behind the panel.
It is installed correctly, but possibly not aligned precisely...

 

[jakej]

How much is too far off? I estimate mine to be between 4-6 degrees off (will check again when I get to the plane on Tuesday).

As for the unit, it’s not ADAHRS, it’s a remote magnetometer (SV-MAG-236). ADAHRS is installed on the avionics shelf behind the panel.
It is installed correctly, but possibly not aligned precisely...

Thanks, got it. If you don’t fly it you’ll maybe never know - come on, you know you want to fly

 

[kellym]

If your ground calibration is within 4-6 degrees the airborne calibration will reduce the error by at least 50%, which will have you better than most whiskey compasses. In the meantime it certainly is more than good enough for VFR flight, but you crosswind calculations for winds aloft will be somewhat erroneous.

 

[Eugr]

I looked at some other pictures I took during that day, one of those at the compass rose, and there the GPS track on my 430W matched the heading on Dynon (facing South). Could it be some magnetic interference on the taxiway itself?

Anyway, before messing with he magnetometer alignment, I will do ground calibration again and then the flying portion.

 

[Marc_J._Zeitlin]

Marc - with respect, I think there is something going on in your case that is causing the issues you’ve had.

One assumes that there is "something" going on. What is unknown is what that "something" is . If you (or anyone else) has any ideas, I'm all ears. As I said, installed per plans and instructions; both Dynon and Garmin; installed by me and others.

My comments are based on experience with ~ 50 RVs installs, never had 1 compass calibration fail & my own aircraft as well as others I’ve done are composite too.

The cals don't fail - the magnetometers just give somewhat erroneous readings - 5 to 15 degrees off - after all ground and air cals. I'm glad that you're having better results - maybe (for unknown reasons) the magnetometers work better or just more consistently in AL planes than in composite ones, although I have no idea why that would be.

I'm open to suggestions.

 

[sunfish]

Page 20 - 4 calls for the magnetometer and ADAHRS to be accurately mounted at the same pitch angle. I assume your installation complies.

 

[Marc_J._Zeitlin]

Page 20 - 4 calls for the magnetometer and ADAHRS to be accurately mounted at the same pitch angle. I assume your installation complies.

Yep.

 

[jakej]

As I said, installed per plans and instructions; both Dynon and Garmin; installed by me and others.
I'm open to suggestions.

I'm stumped too, having issues with both brands is not nice, IF there was a product problem then there would be a lot more 'talk' on forums IMO.
Is it possible that magnetic tools eg screwdrivers were being used to install the magnetometers/ADAHRS units in your cases ?
BTW - I've done installations in composite aircraft too

 

[Raymo]

I did a very crude ground calibration using my iPhone, then did the inflight calibration. No issues in the past 3.7 years. AD-AHRS mounted in the tail of my RV-7A.

 

[Eugr]

An update - I re-calibrated my magnetometer today and it seems accurate enough now in all directions. I have done only ground portion so far, the plane wasn’t ready for flying yet.

Weird thing though - looks like the georeferenced diagram (both FAA and SA) is a bit off. I’ve had the same issue last time, and it is behaving like that with both Dynon’s GPS and when Dynon gets position information from my panel-mounted 430W. Didn’t have my tablet with me, but forgot that I have Aera 660 with georeferenced charts in the panel and my phone with Foreflight - would be interesting to see if there is the same issue.

I don’t think the GPS is that much off?