Compass variations

marc17522

New Member
Joined
Jun 17, 2011
Messages
7
When I turn on my fuel pump the compass goes from 126 to 40 deg turn the fuel pump off and it goes back to the 126 deg heading. I have not calibrated it yet but did put a vertical card compass on top of the ADAHRS and it had no variation with the fuel pump on.
 

perryc

New Member
Joined
Nov 26, 2009
Messages
15
I put the ADAHRS behind the panel too being my SIR compass worked well there and am have no success in getting it calibrated.  I fear the module is just more sensitive to magnetic field interruptions than a regular compass.  Not looking forward to replumbing to get the pitot, static and AOA lines rerouted (big PIA 'cause I have backup steam AS/Alt but I fear this is our fate unless Dynon will sell us just a remote compass module D10/D100 style.

Perry
 

dynonsupport

Dynon Technical Support
Staff member
Joined
Mar 23, 2005
Messages
13,226
Marc - If the fuel pump affects the ADAHRS, then its magnetic field is interfering. The card compass may not be as sensitive (or as accurate) as the ADAHRS's magnetic sensors for a variety of reasons. As a rule of thumb, you can use a card/whiskey compass to rule out a bad location (if that other compass also deviates), but even if it passes basic test you may not have a sound location for the ADAHRS. Without going in to too much technical detail - the ADAHRS can see the full 3D magnetic vector. This lets it be correct in all attitudes, but that also means that its sensitive to variations that a card/whiskey compass may not always show in a static test. Though the card compass may look good on the ground, once aircraft dynamics (pitch, bank, yaw) come into play, you'd likely get poor performance from a card compass in that same location as well.
 

marc17522

New Member
Joined
Jun 17, 2011
Messages
7
I also have a hydraulic pump at the other end of the fuselage that also swings the compass when running.

Marc
 

meljordan

Active Member
Joined
Mar 23, 2005
Messages
1,367
Location
Tucson, AZ
Marc,

It is not just the pump itself that can be the problem, but also the wires to the pump. If the wires run close to the ADHARS it will also impact the compass when the current flows through them. Check to see if any wires that carry periodic currents are near the ADHARS. The more current in the wire, the bigger the impact. So when you turn on something big like a pump, or lights or landing gear, if those wires are anywhere near the ADHARS you are going to get a compass deviation.


Best,
Mel Jordan
 
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