Remote magnetometer drift

[mem235]

Installed magnetometer in rear of plane. Getting variegated heading indications between 4-13 degrees off, measured on ground at rwy and rose. The readings change with motor rpm and also when landing or position lights turned on/off. The battery and master switch solenoid is also located in rear of plane, at least 24” from magnetometer but on same shelf. Installer replaced shielded wiring for landing and position lights, moving grounding location closer to fuselage. Led landing light. Unstable inaccurate heading persists.
Looking for some advice to resolve. Also, any experience measuring interference with digital guass meter or Hall effect?

 

[Carl_Froehlich]

Just a really bad mount location - not much you can do other than move it to a better spot.

Perhaps go up and mount a bracket on the near bulkhead. 12” could make a big improvement.

Carl

 

[mem235]

Any experience/lessons learned in mounting magnetometer further back in fuselage or in wing. Thinking a bracket would be needed but what would be best way to mount either to bulkhead or wing spar and still allow required alignment

 

[GKC Aviation]

I have mounted the magnetometer in the tail before, mounting it on a small shelf that spans the tail, attached to the mid stringer each side. Folded the edges for stiffness. I think I have a photo or two somewhere.
On Cessna's, I have mounted it to a rib outboard of the fuel tank. Built a bracket for it rivetted to a rib.

 

[tniel7331]

Installed magnetometer in rear of plane. Getting variegated heading indications between 4-13 degrees off, measured on ground at rwy and rose. The readings change with motor rpm and also when landing or position lights turned on/off. The battery and master switch solenoid is also located in rear of plane, at least 24” from magnetometer but on same shelf. Installer replaced shielded wiring for landing and position lights, moving grounding location closer to fuselage. Led landing light. Unstable inaccurate heading persists.
Looking for some advice to resolve. Also, any experience measuring interference with digital guass meter or Hall effect?

What Antenna is on the top of the tail ? You might trying running it away from that especially if it's a com Antenna. Not the ELT but the 1 on top that runs left side nothing should be more then 2 inches from an Antenna cable and if cim the further the better

 

[Rhino]

If the antenna cable routing was an issue, you'd most likely see the problem occur or change when transmitting. This appears to be electrical noise from the aircraft systems. As a general rule though, it probably isn't the best idea to route the 236 wires alongside an antenna cable. But if this problem only occurs when other aircraft systems are powered on, I would suspect a ground loop. Your comment about moving the ground location makes it sound as if you are not using a common ground point, making a ground loop more likely. If you do have a common ground location in the aircraft, temporarily routing a ground wire there might be a fairly simple troubleshooting step.

While I wouldn't suspect a magnetic issue considering the symptoms you describe, it is not impossible. You have the 236 mounted next to the ELT, which undoubtedly has ferrous metal in it. The flight control cables are probably ferrous as well, but moving the controls would likely cause the problem to change if they were causing interference.

 

[mem235]

I really appreciate the feedback. The antenna cable in back left is elt, the one immediately above is com and appears bundled with magnetometer cable. There is nav antenna in top tail.
The heading variation changed 20 deg even with additional 200 rpm from idle so i wonder if the battery charging cable is properly isolated from magnetometer cable. Dynon emphasized separating transponder cable but they were not aware that i also had battery back there. unfortunately plane is 90 mi away at dreaded avionics shop. Their solution was to get led landing and position lights.
Any experience with alternators? Friend suggested i rule out busted diode, that the resultant ac current would largely go unnoticed but would screw up computer electronics

 

[Rhino]

I'd rule out the diode first because that's relatively simple. But excessive noise from an alternator usually shows up in your audio system. Can you hear the engine RPMs over your headset? That's often a grounding issue as well.

 

[Rhino]

If this were me, and feel free to ignore it if you want, I would create a new (preferably non-metallic) shelf for the magnetometer between those two upper longerons a bit further aft (using upper gets it away from the flight control cables). Then I'd better isolate all the lines going to the magnetometer, and use a common ground point (not the fuselage).

 

[tylerpattison]

1. Based on your writing I suspect the skin of the aircraft is probably carrying some stray return currents that are interfering with your compass.
2. Consider installing a compass app on your smartphone to help evaluate local interference when evaluating mounting locations.
3. Suggest mounting the magnetometer (as others have suggested) away from aircraft skin and any high-current wires. (hint: sometimes the skin is a high-current wire, per say)
4. Be careful with flight control turnbuckles. They can sometime become magnetized. Again, use a compass of some kind to verify. If magnetized use this magnet on a drill trick to degauss the turnbuckle without removing it to help eliminate issues. Worked like a treat for me on a Bonanza once.

 

[Rhino]

Don't let the magnet touch the turnbuckle when doing that. Touching it can actually magnetize the turnbuckle.

 

[mem235]

I am not getting engine rpm noise in headset any more, the p lead and capacitor/filters were just replaced. I am going to build shelf, and re do magnetometer wiring, sheathing the entire lenght of dynon magnetometer cable w/ground at each end. Sheath will include area behind panel. Very concernow about energized skin. I hope shelf removes that variable. I am confused about suggestion of compass app on cell phone as i presumed those were based on cell tower geolocation vs magnetic north. In any case my cell app compass doesn’t work when in plane. I shutter to think that i have energized skin. Amazon has inexpensive digital guass meter or just use boyscout compass to prove quality of shelf location
The idea is now with everyone input that i am dealing with two problems, maybe three(skin effeft). Magnetic interference (elt lithium battery and steel in casing; steel control cables) and cable routing (influence from charging current to battery and and all other cables)
Cant wait to get back in air, new motor and i am going to fly the crap out of it. Been really difficult dealing with local avionics shop that i chose. There were many other problems
Thank for all input. Past and future

 

[Rhino]

If you're using a common ground point for your avionics systems (and you should), most folks, Dynon included, recommend you only ground the shielding at the end of the cable that's connected to your common ground point. Less chance of a ground loop that way.

 

[maartenversteeg]

Just a boyscout compass (you must still have one somewhere in a drawer or the attic) should do, you are not performing a high resolution measurment, you are just trying to prove that there are no magnetic fields near the installed parts.

 

[mem235]

Ok good. Regular compass and I will look into common ground with mechanic. Under panel many devices share a ground i know that position lights were grounded to first rib. I failed to mention problem #4: adahrs/magnetometer alignment- got deer in headlight look on that one. Shop suggested that the airplane rear shelf alignment was implicit

 

[Rhino]

The alignment probably is good. Your issue appears to be more related to proximity with other stuff rather than alignment.

If this problem occurs on the ground, I'd also perform another troubleshooting step. Remove the ELT entirely and see if that changes anything. If it doesn't, then you've eliminated the possibility that the proximity with the ELT is part of the problem. That won't rule out potential issues with the proximity of the wiring, but it will tell you if ferrous metals in the ELT are part of the problem.

 

[mem235]

Update: plane was moved to different shop. Here is what we found
1- the original avionics shop replaced position and landing light cables. When that didnt solve problem they sent the -236 magnetometer back to dynon for warranty. The -236 unit was tested and found to be acceptable.
2- the original mount location was found to have significant magnetic interference from steel control cables, not from the elt. I have to change location to one that is magnetically benign.
3- my plane has battery mounted in rear, behind baggage compartment. The magnetometer cable was routed in close proximity to the battery charging cable, in some places less than 2 inches. We measured magnetic interference from this charging cable to 8 inches. I will have to move the magnetometer cable away from battery cable.
4- the magnetometer was not aligned with axis of airplane, measured over 13 degrees out on longitudinal axis. The adahrs unit was also not level but within 3 degrees.

All the requirements for magnetometer location, cable location, and alignment are succinctly spelled out in installation manual.
I will update again once corrections are finished, with photos.

 

[Rhino]

13 degrees. Wow. Wondering if they even read the manual.