I suspect it is a design and/or magnetometer-quality issue. They have decided to fit everything in a tight space, and with everything else that is going on in there electrically, it must be an impossible balancing act.

I have a Dynon Skyview with a separate ADHRS unit (which contains the magnetometer). This is located remote from the EFIS, but not very far away and is still behind the panel and I have not had any trouble with stable magnetic compass using this installation. Even so, Dynon offers a separate magnetometer for problematic installations. Dynon’s setup also allows for installing a second ADHRS unit for redundancy in a stack, although I have not done this.

I think that uAvionix wanted to make a splash by doing everything in a very small self-contained unit and they have found out by now that stray EM energy is impossible to block within a small distance of a pile of electronics, screens and power supplies.  They should have anticipated this in the design phase. Sounds like a marketing-department imposed constraint.

You may be able to mitigate the “precession” by identifying any component that changes its EM emission profile over time. The only one of these I can think of is the flat screen, as it may build up static electricity on it’s surface over time. Maybe finding a way to discharge this, like a small bare ground wire wrapped around the perimeter of the screen? Just thinking out loud here.

Cheers,

Jay