Mark Castle-Smith

Mark Castle-Smith

Member
A general interest question really.

Does anyone know why the AFS 3500 magnetometer has to be aligned with the EFIS in all three axis rather than be aligned with the aircraft axis. Of course the roll and yaw axis are the same for the aircraft and the EFIS, but the pitch axis of the EFIS in the instrument panel on my RV7 and the aircraft pitch axis are different by several degrees. I would have thought that the EFIS wanted to know the actual pitch axis of the aircraft for the Artificial Horizon display rather than the angle of the panel?

Just displaying my ignorance!
 
Don't know for sure about the AFS but Dynon say to align the magnetometer with the unit installed in the panel in all three axes within a tolerance of 1 degree. You calibrate the pitch to take account of the panel angle.

Jim Sharkey.
 
I'll take a swing at this one. I suspect AFS uses a three-axis magnetometer, and they use the dip (vertical component of the earth's magnetic field - remember that from ground school?) to help stabilize the solid-state gyros in the EFIS itself. That's why the magetometer and EFIS should be aligned.

My $0.02...

Dave
 
Magnetometer Alignment

A lot of information at these links below.

http://www.xbow.com/Support/appnotes.aspx
http://www.xbow.com/Support/Support_pdf_files/FAA_Certification.pdf
http://www.xbow.com/Support/Support_pdf_files/CRM500GA_Installation_Manual.pdf

3.5 CRM500GA-[] Alignment with the aircraft
The CRM500GA-[] must be level on the yaw and roll planes of rotation
when the aircraft is in straight and level flight. The pitch axis must be level
to the aircraft when aircraft is leveled for weight and balance measurements.
The CRM500GA-[] will measure gravity and the earth magnetic field
vector along the axes of its sensors. The CRM500GA-[] sensors are aligned
with the baseplate. The baseplate references are noted in the installation
drawing and are used as reference surfaces for aligning the CRM500GA-[]
sensor axes with the aircraft. The CRM500GA-[] should be aligned as
closely as possible with the axes you define in your system. Errors in
alignment will contribute directly to errors in measured earth magnetic field
vector relative to your system axes.
It is important that the CRM500GA-[] be aligned carefully with the
longitudinal axis of the aircraft and NOT with the outer skin of the fuselage.
 
Think of a three axis magnetometer as a standard whiskey compass, but one you can tilt electronically so that it's always parallel to the hoirizon. It doesn't automatically know what way to "look" to find the magnetic vector, it has to be told. Just like you would do with a handheld compass.

What tells it is the EFIS, since the AHRS knows the pitch/roll of the plane. So the EFIS is constantly using it's internal attitude solution to tell the compass what way the horizon is so it can use that to find magnetic north.

The easiest way to achieve this is to assume the compass is bolted to the AHRS, and is always fully parallel to it. In units such as the Dynon EFIS, where the AHRS is in the panel, that means we want it parallel to the EFIS, not the plane. If the compass and EFIS aren't parallel, the EFIS actually ends up telling the compass to look up or down from true level, which introduces an error when the plane is banked.

That's the reason the Dynon EFIS needs the magnetometer parallel to the EFIS, and I assume AFS has the same basic reason.
 
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