The G3X "Miscompare"

[PandaCub]

So I've been flying my RV-8 for over two years now with nary a complaint about the G3X. Last week, however, Betty interrupted my cross-country reverie with "Miscompare!" followed by the dreaded tritone beep of the autopilot going AWOL. A quick search of this site shows it's not an unknown issue.

I've made no changes that might explain why this would crop up now, and I'm not sure what any of the yellow messages really mean (see screenshot).

Bad module maybe? Software needs an update? Any suggestions appreciated.

Obviously, to have this happen during an autopilot-coupled instrument approach would not be the favorite part of my day.

 

[AZSteve053]

Do you have two ADAHRS installed ? If so, something is out of comparison limits between the two. You could assign ADAHRS 1 to PFD & ADAHRS 2 to MFD for a manual comparison.
Could be airspeed or altitude or heading or gyro...
Suggest doing a search on "miscompare" in latest Pilots guide. unlikely software.
Does the "miscompare" remain or was it momentary.
You could also download data & compare that way too...
Hope this helps.

 

[Richard Connell]

Out of the blue does seem strange.
My bet is hardware fault or vibration.
Miscompare can also be airdata too so a loose or disconnected hose is worth checking.

There was a bad batch of GSU25s floating around a few years back plus a SB for noise (acoustic vibration)
I went through 6m of this tedium. A very forgettable experience.
In short they are extremely sensitive to vibration. Random combinations of airspeed, power and RPM and mounting stiffness make a difference.
How many GSUs do you have? 1or 2? Where are they mounted?
How have you configured miscompare monitoring in the configuration?

If I was presented with your situation now I’d:
1. Double check your GSU(s) are bolted down tight and haven’t loosened.
2. Check pitot/static/aoa into each GSU and G5.
3. Make sure the back shells are tight to the GSUs and G5.
4. check the SBs and the hardware mod levels and serial numbers of your GSUs.
5. Download the data and send it to Garmin. It logs AHRS performance at 1Hz too.

 

[tareed]

So I've been flying my RV-8 for over two years now with nary a complaint about the G3X. Last week, however, Betty interrupted my cross-country reverie with "Miscompare!" followed by the dreaded tritone beep of the autopilot going AWOL. A quick search of this site shows it's not an unknown issue.

I've made no changes that might explain why this would crop up now, and I'm not sure what any of the yellow messages really mean (see screenshot).

Bad module maybe? Software needs an update? Any suggestions appreciated.

Obviously, to have this happen during an autopilot-coupled instrument approach would not be the favorite part of my day.

View attachment 62617

View attachment 62618

Check that your GMC 507 was calibrated. It has an ADHRS also. Had this issue with G5s and GFC500. Mine was never calibrated causing the issue on rare occasions.

 

[Walt]

Assuming you have a data card in the PFD, I would suggest sending that flight data to the G3X folks, lots of times they can pick out the offending culprit.
"[email protected]"

 

[Richard Connell]

Check that your GMC 507 was calibrated. It has an ADHRS also. Had this issue with G5s and GFC500. Mine was never calibrated causing the issue on rare occasions.

This wont be the cause of this particular issue.
The GMC507 AHRS will disconnect the AP if there is an issue but it won’t throw a miscompare CAS.
The miscompare CAS is a result of GSU vs GSU or G5