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Old 07-21-2017, 08:15 AM
blderubba blderubba is offline
Join Date: Jul 2017
Location: Niles, OH
Posts: 2
Default Compass Magnetic Anomaly

Hello RVers,
This is my first time on this forum. I built my standard build kit RV7A in 5 years. It has been flying since 2009 with over 480 hrs. O-360 Superior engine with fixed pitch Sensenich. The plane is fantastic. I just recently rebuilt the panel with G3X 10" & 7" screens, GTX45R, GTN650, etc. I have one issue that the Garmin X team cannot resolve. During my climb out I get a message that says magnetic compass anomaly and that I should check the installation. I installed the GMU22 using the Garmin mounting bracket on the aft deck per their instructions. I have calibrated the magnetometer 4 times now in three different compass rose locations. The interference test shows that the rudder movement is most likely the culprit. If I move the rudder left at a quick pace, the result is 92% of limit. If I move the rudder left at a slow pace, the result is 31% of limit. I thought of degaussing the rudder cables,eyelet, and etc. but I am not sure that is the problem since a compass needle really does not deflect much at all near them.

I read forums involving the RV10 from a few years ago but nothing about any problems with the GMU22 and the RV7A series.

Has anyone installed a GMU22 on the aft deck of a 7A and had either similar issues or non at all?

Thank you for reading. Safe & Happy flying!
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Old 07-21-2017, 09:05 AM
Raymo's Avatar
Raymo Raymo is offline
Join Date: Nov 2014
Location: Richmond Hill, GA (KLHW)
Posts: 1,669

Welcome to VAF!

A similar question was posed here and answered by Garmin. The search feature here works okay but other search engines work better. Just enter the information and include VansAirforce in the search string.
RV-7A - Slider - N495KL - First flt 27 Jan 17
O-360-A4M w/ Catto 3 blade NLE, AFP FM-150 FI, 1 PMag, Vetterman Trombone Exh, SkyTech starter,
PlanePower Alt, FlightLines Interior, James cowl & plenum
All lines by TSFlightLines

"The object of the game, gentlemen, is not to cheat death: the object is not to let him play."
Patrick Poteen, Sgt. U.S. Army
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Old 07-21-2017, 09:52 AM
dave4754's Avatar
dave4754 dave4754 is offline
Join Date: Oct 2009
Location: Edson, Alberta, Canada
Posts: 430
Default That is true

Funny you should mention that Raymo as often when i Google some question i have it will show me a VAF thread which did not show up on my VAF search.

Thanks for confirming this happens and that I am not totally computer illiterate.
Dave Cobb
=VAF=2018 Donation - and a bit extra for great work!
Empennage RV 7 - Tipper
Wings complete

Fuse nearing completion, Canopy complete
AME reviewed and repaired O-320-Lyc installed
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Old 07-21-2017, 10:00 AM
fl-mike's Avatar
fl-mike fl-mike is offline
Join Date: Jul 2005
Posts: 932

Do you get the error message regardless of cardinal heading?
Only in a climb?
Magnetic lines of force are not horizontal. (Dip)
Mike W
Venice, FL
RV-6A. Mattituck TMX O-360, FP, GRT Sport EFIS
N184WM reserved (RV-8)....tail kit done, slow-build wing done, slow-build fuselage started.
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Old 07-31-2017, 07:48 AM
blderubba blderubba is offline
Join Date: Jul 2017
Location: Niles, OH
Posts: 2
Default Compass nomaly

Hi Mike,

I apologize for the delayed reply. I have been traveling. The error message usually pops up about 200 to 400 AGL on climb out and on my base to final around 500 to 800 AGL. I am looking for a degaussing tool to purchase and give that a try.

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Old 07-31-2017, 10:58 AM
BillL BillL is online now
Join Date: Sep 2007
Location: Central IL
Posts: 4,301
Default Thanks for the reminder!!

I have an old radio shack VCR/cassette tape degausser that I inherited. It worked great on a 10-Friend's seat belt harness. I have not checked my rudder cables, and getting ready to be airport bound - thanks for the reminder.

Oh - it could also be the elevator horns. I just checked mine and they turn the compass parallel to the long direction of each member.

eBay - -

Look up the instructions, it is not heavy duty rated.

Op Lims in hand 12-7-17
1st Flight 1-27-18
33/40 hrs
Aerobatics DONE!!
2017 Dues Paid
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Old 07-31-2017, 11:12 AM
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Dbro172 Dbro172 is offline
Join Date: Apr 2012
Location: West Fargo, ND
Posts: 1,014

I had the same issue on my RV-9A. I tried many recalibration as well as trying to demagnetize (degauss) the rudder cables and elevator horns, it got better but still got magnetic anomaly during climbout.

Ultimately I ended up moving the magnetometer forward a few feet and has been trouble free since. I made a shelf out of two angles spanned between the lingering.
Derek Hoeschen
EAA Tech Counselor
RV-9A #92103 - N803DK
G3X, Superior XO-320, Dual Pmags, Catto 3B

RV-8 #83565 - N184DK - building
1968 Mooney M20C - Sold
1956 C-182 - Sold
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Old 07-31-2017, 02:42 PM
moosepileit moosepileit is offline
Join Date: Dec 2010
Location: Floyds Knobs, IN
Posts: 239

I had a 3 foot section of my left rudder cable magnetize somehow just after purchasing my RV-6, 200 hours after an EFIS upgrade from 6 pack by the builder.

I fly in mag heading not track mode, so it was obvious as it skewed then went fully Bermuda triangle. This is where a separate autopilot that has its own AHRS/gyros and can display and fly its own gps tracks comes in handy if you are single EFIS/magnetometer/AHRS paneled.

Luckily, the section could be moved outside the tail once dropping the cable from the rudder. Older GRT magnetometer in the tail under the fiberglass fairing.

A $9 bathroom shaded pole fan motor with a wood spindle was just big enough to do the degauss in 2 passes.

Nothing else was magnetized in the tail area.

Worked great. Learned a bunch.
RV-6 N91GG, bought from builder.
O-320, slider, carb, mags, FP

Last edited by moosepileit : 07-31-2017 at 02:46 PM.
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Old 07-31-2017, 05:23 PM
mike newall's Avatar
mike newall mike newall is offline
Join Date: Jan 2005
Location: Yorkshire, England
Posts: 1,602

Semi related and somewhat humorous...

Retrofitted a Garmin 695 to our RV7 a while ago, all was well and it is a super piece of kit.

A short while later, my buddy said, "why is the standby compass 30 out ?"

I looked, we had a fiddle and couldn't fix it. It is a Falc*n gauge, so we put it down to remote hacking ;-)

Then, I was working on our RV8 and installing the similar unit. I placed the GPS aerial close to something metallic and it jumped over - yep, the GPS puck aerial has a magnetic base

So, our little whiskey compass is just looking at the GPS aerial.

Some things are easy to figure, some take a coincidence
"I add a little excitement, a little spice to your lives, and all you do is complain!" - Q

Donated twice in 2017
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Old 07-31-2017, 05:29 PM
Tankerpilot75 Tankerpilot75 is offline
Join Date: Jan 2015
Location: Oklahoma City, OK
Posts: 273
Default Similar problem with GRT & Dynon magnetometers

Several years ago I had a similar problem on my RV7A after I purchased it from the original owner/builder. Both magnetometers were mounted on the deck shelf below the vertical stabilizer. Heading would vary every time I talked on the radio and/or moved my rudder. Chased that dog for several months until a long conversation with the GRT tech rep convinced me to move my magnetometers to the wing tips. Once I did that heading issues disappeared.

Not sure if this is your problem but in your setup configuration there is probably some way to check "counters" to see if there are decreases or increase number count rates to confirm. Just my two cents. Good luck.
Jim Harris, RV7A, 2nd owner, N523RM
IO-360, Hartzel CS prop, Older Aerotronics built panel Dual GRT Horizon WS, Garmin 340, 335, Dual 430s, Stratux hard wired to WS, Dynon D10A (backup EFIS)
Retired - Living the dream - And going broke!
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