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Erroneous Compass Readings With Heated Pitot/AOA (Read 1063 times)
Mar 17th, 2013 at 7:18pm

Tom Thibault   Offline
Full Member

Posts: 172
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I have always had the compass swing left about 12 degrees whenever the Pitot heat cycled on.  Finally, I trouble shot this to root cause this week and eliminated the problem completely.

The problem occurred because my airframe is all composite and I had installed all aluminum lines between the Pitot head and the ADAHRS unit.  In fact, the power return wire from the Pitot, the Pitot tube and aluminum lines are all electrically common.  Since the ADAHRS fittings and the case are also common to the Skyview network ground, this provided a path for some of the Pitot heat current to return through the ADAHRS to A/C ground even though there is a dedicated power return wire to the main A/C ground bus.  This flow causes a magnetic field to be generated that swings the compass.

The fix was to insert some non-metallic line in the Pitot and the AOA tubes.  In fact, I did this at the Pitot tube end and at the ADAHRS end.

Mike Huff has confirmed this with the engineers and the installation manual should be getting new guidance on the topic.

My supposition is that an all metal A/C would not see this problem even with all metal lines because the grounded airframe would carry any current back to the battery that did not go through the Pitot heat power return.

I would like to hear from any composite aircraft owners that have all metal lines from Pitot to ADAHRS about there experiences.
 
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Reply #1 - Nov 6th, 2018 at 3:18am

Snowbirdxx   Offline
New Member


Posts: 6
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I have erratic compass readings on a Husky using the Dynon heated pitot tube and controller and garmin G5s. I have no metal tubings from the probe to the AHRS. But I have the GMU within 3 ft from the Probe. I need to check if I wound the wired up in a circle, which would be stupid. i LL POST RESULTS AS SOON AS THE PROBLEM IS SOLVED:
 
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Reply #2 - Nov 6th, 2018 at 7:21am

airguy   Offline
Senior Member
SkyView Flight Tester
Gods Country - west Texas

Gender: male
Posts: 371
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Winding the power feed wire in a circle is not required to create a magnetic field that could throw off the compass reading, the heater itself in the probe will do that, and even a perfectly straight piece of wire carrying a DC current will do that.

The only way to completely eliminate a field gradient caused by power wires in proximity to flux gate would be to wrap them in mu-metal for some distance either side of the flux gate.
 
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Reply #3 - Nov 6th, 2018 at 10:09am

RayInGA   Offline
Expert
I love aviation!
Richmond Hill, GA

Gender: male
Posts: 531
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I don't know if it helped but I used a drill to twist all the wires to the Dynon heated pitot to help cancel the magnetic field. Then again, my ADAHRS are in the tail, far away from any electrical noise.
 

Ray Eaker
RV-7A flying since 27 Jan 2017
Dual Skyview 1000T with all available Dynon VFR goodies
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Reply #4 - Nov 8th, 2018 at 7:55pm

RV711AC   Offline
Junior Member

Posts: 64
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Is the pitot heat grounded locally? If so try moving the ground closer to the fuselage or directly to the battery ground. I have seen gnu 11s respond to current in wing skins. The G5 has a survey mode to assist in finding issues. You can run test grounds externally to test without too much effort although vthe final solution will need to be validated.
 
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