Magnetometer and AHARS Calibration

Tonight I worked to calibrate the magnetometer and the AHARS. It was rather windy when i got to the airport, and you need good GPS data to calibrate both, so i had to wait until the storms passed. First up was the AHARS calibration. This was pretty easy. Basically level the aircraft in flight attitude and let the G3x figure out pitch and roll offset. I also needed to do a vibration test on the AHARS once the engine was running. Next I did a magnetic interference test. Basically you start and turn off all the various electrical gizmos that could cause interference and manipulate the controls etc. You do this in 10 second increments. The magnetometer (which is in the left wing tip) passed the interference test.

I taxied to an open part of the airport and used my compass to ensure I was facing due north. I guess you can do the magnetometer calibration with the engine running and just taxi it. But I chose to shut down and just push the tail around. Basically all you have to do it rotate the plane 360 degrees and 30 degree segments. The display tells you what to do and when. Dirt simple. And I got a successful calibration.

Now with the aircraft warmed up, I taxied back to the hangar, tied her to my truck and did a full power run-up for the AHARS vibration test. This also passed. So now I have a fully tested and calibrated AHARS and magnetometer.  I also setup all the airspeed marking limitations tonight in the EFIS.


Instrument Panel Finalization

There is a ton to report in this post. I won’t be able to cover it all. I’ve been working hard to finalize all the electrical wiring and avionics installations. I’ve finally gotten to the area behind the panel and the instrument panel itself. Things that were accomplished in this session: final install of pitot static boa lines, final wrapping and organizing of all wires and cables, removal of instrument panel, carbon fiber wrapping instrument panel, reinstalling all switches and avionics into the panel, labeling all switches, elt nut plate installation, and installation of a few wires remaining for functionality such as fuel pump switch to bus power and power for panel led lights.


So a few days ago, I turned on the avionics to find that nothing was communicating. How could this be? It was all working perfectly, and then something happened? At first i thought perhaps I hadn’t trickly charged the batteries enough and a low voltage was causing some LRUs to drop offline. Nope. Speaking with Garmin they told me to disconnect LRUs and reconnect individually to try to identify a bad unit. That didn’t work either. The only other possible culprit – the CAN Bus. So i pulled the connector on the GDU 460 and buzzed it out. I got the appropriate 60 ohms across CAN HI and CAN LO. Which told me that the CAN Bus was contiguous and terminated properly. Perhaps a short to ground? Bingo. CAN LO was buzzing as shorted to ground. Great. Finding that isn’t going to be easy. The only real way to do so, is to divide the CAN BUS in two, and figure which is the bad half, and keep doing that until the offending segment / connector is isolated. So i did that, and found that the bad area was the GMC 507 to GEA24 to GTR20 segment. The GEA 24 is really hard to reach, and is where i had just been cinching wires together. I buzzed it out, and thought i determined taht was the bad connector. Reinstallation of the connector didn’t solve the problem. But i did it from inside the airplane. Which, given the GEA24 is down by the fuel pump… is difficult. I decided to pull both segments out of the plane so i could diagnose and correct on the bench. Long story short, I isolated the leg that was bad. Then tried to determine which connector was bad. Ultimately, even after cutting both ends off the cable, the cable still buzzed as shorted to ground. I completely tore the cable apart and examined everything with a magnifying glass. I was unable to find the offending section.

I built two new segments and reinstalled them in the plane. With all the troubleshooting this was a major time sink, and quite frustrating. But everything is back to 100% functionality. While human error cannot be ruled out, i was extremely meticulous in testing the cable on the bench. I really think there was a flaw in the cable somewhere that was exposed when i began cinching wires with zip ties.