GMU 11 Magnetometer Failing Self Test

[AerostarDriver]

I have been trying to figure out how to get my GMU 11 to past a self test. I had intended to mount the GMU 11 to the avionics rack in the tail of my M20E but that is looking like an increasingly unlikely place to mount it. With the GMU 11 mounted directly on the Avionics Rack my Whelen LED strobes where causing a 1666% out of limit interference every time they flashed. My solution was to place the GMU 11 on a standoff tray that increased the height off the unit to 3.5 inches from the tray, this allowed the self test to pass but now I am at 106% limit when I raise or lower the flaps. I feel that using the standoff is a band-aid to the larger problem. I don't believe their should be this much EMI on the rear most of the trays in the back. I can find numerous examples of it being located there with no issue. I have it located about 3 feet from the closest current carrying wire bundle.

Does anyone have any ideas on how to address the magnetic interference like moving a ground, potentially a bad ground or ground loop? Anyone have an examples of GMU11 mounting in the tail that is not on the avionics tray?

[bradp]

Shielded wires for your strobes. Eliminate ground paths. Bring everything to the battery or a ground block then to the battery. That should help but no guarantee. I put mine in the wing to avoid this issue. 

[Ragsf15e]

Jeez, glad have hydraulic flaps and strobes are on the wings!  Mine is there on a tray.

good luck!

[EricJ]

1 hour ago, AerostarDriver said:

Does anyone have any ideas on how to address the magnetic interference like moving a ground, potentially a bad ground or ground loop? Anyone have an examples of GMU11 mounting in the tail that is not on the avionics tray?

Mine is on a fabricated shelf a bit above and forward of the static port.   Shop put it there.   Works fine.

Yes, my tail is filthy.   It lived outside in the Nevada desert for twenty years, so that's dust storm samples. 

[dzeleski]

I dont have mine installed yet but I know my shop also installs them in the wing for this exact reason.

[kpaul]

A shop installed mine in the tail, Garmin moved it to the wing when doing the GFC500 testing.

[jetdriven]

That shelf back there is very noisy, even things like Pitot heat make it get crazy because mostly they avionics shelf handles all the current back to a post and then to the negative battery terminal. So there’s so much skin current back there it just kills the magnetometer. I put the one I installed in the right wing. 

[BDPetersen]

Mine went on the shelf in the tail of my C, but it was not until I ran the ground from the strobe directly to the battery that I eliminated the problem of which you speak.

[AerostarDriver]

2 hours ago, BDPetersen said:

Mine went on the shelf in the tail of my C, but it was not until I ran the ground from the strobe directly to the battery that I eliminated the problem of which you speak.

My tail strobe ground is run all the way back to the battery but I still see the spike in EMI when the tail strobe flashes. The other strobes I can only assume are run just to the airframe somewhere.

10 hours ago, dzeleski said:

I dont have mine installed yet but I know my shop also installs them in the wing for this exact reason.

My GMU11 was failing the self test in the wing when I made up the test rig but was passing in the tail, now that I have everything wired and reinstalled, it is now failing the self test.

10 hours ago, EricJ said:

Mine is on a fabricated shelf a bit above and forward of the static port.   Shop put it there.   Works fine.

Yes, my tail is filthy.   It lived outside in the Nevada desert for twenty years, so that's dust storm samples. 

I am going to see what I can whip up to copy your setup, I was trying to avoid drilling the skin or a longeron. I may try the other side as my tail strobe wire runs close to where your mount is. Thanks!

[sdflysurf]

11 hours ago, EricJ said:

Mine is on a fabricated shelf a bit above and forward of the static port.   Shop put it there.   Works fine.

Yes, my tail is filthy.   It lived outside in the Nevada desert for twenty years, so that's dust storm samples. 

 

You ain't seen nothin - I had birds nesting in my tail when we bought it.  yuck!

We are installing ours in the co-pilot side wing today.  I'll get you a picture.  Our strobes power supply was causing a lot of interference in the tail.

[OR75]

Before wasting time building and installing brackets .... ask your shop to place an old whiskey compass in potential location and start turning on and off all the electrical around while looking at what the compass does 

Easy to see when there are interferences 

[AerostarDriver]

2 minutes ago, OR75 said:

Before wasting time building and installing brackets .... ask your shop to place an old whiskey compass in potential location and start turning on and off all the electrical around while looking at what the compass does 

Easy to see when there are interferences 

I did use a whiskey compass, I get no movement in the needle in the original location when following Garmin's interference test plan. 

[drifter001]

Finally got my Mooney squared a way last week. Did ground test on the ground and HsI was tracking perfect. Did a flight to Ogden the following day with bow issues. Day after that I had an instrument flight lesson and noticed I had a 15 degree offset between HsI and ground track on gps unit while on the ground. Plane partner flew today and confirmed 15th free offset on hsi ti whiskey compass and ground track. Anyone have any ideas what may have caused it?

[N201MKTurbo]

45 minutes ago, drifter001 said:

Finally got my Mooney squared a way last week. Did ground test on the ground and HsI was tracking perfect. Did a flight to Ogden the following day with bow issues. Day after that I had an instrument flight lesson and noticed I had a 15 degree offset between HsI and ground track on gps unit while on the ground. Plane partner flew today and confirmed 15th free offset on hsi ti whiskey compass and ground track. Anyone have any ideas what may have caused it?

The calibration procedure should fix that.

[drifter001]

30 minutes ago, N201MKTurbo said:

The calibration procedure should fix that.

It was calibrated prior to our departure to Ogden. Do you think it somehow lost its calibration?

[N201MKTurbo]

9 hours ago, drifter001 said:

It was calibrated prior to our departure to Ogden. Do you think it somehow lost its calibration?

Where is it mounted? Is it possible something is interfering with it? 
 

The calibration procedure is simple and easy to do. If I recall, you start with airplane pointing north with the engine and all avionics on and then taxi in a circle.

[drifter001]

1 hour ago, N201MKTurbo said:

Where is it mounted? Is it possible something is interfering with it? 
 

The calibration procedure is simple and easy to do. If I recall, you start with airplane pointing north with the engine and all avionics on and then taxi in a circle.

It’s mounted in the tail. I got really good at making right turns during the process actually lol. I’ll give that a shot.