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Garmin G3X Autopilot Suddenly Dives

gmcjetpilot

gmcjetpilot

Well Known Member
Garmin G3X in a new RV-12 (Experimental) . Doing Phase 1 tested Autopilot. Heading works fine. Did V/S to climb 500 ft and it captured. Then 30 seconds later it started to dive. Disconnect autopilot the trim was nose down, significantly. Any ideas. This was first and only use, Knocked off using it until I can figure out what is going on. LESSON never use autopilot close to ground.
 
gmcjetpilot said:
Garmin G3X in a new RV-12 (Experimental) . Doing Phase 1 tested Autopilot. Heading works fine. Did V/S to climb 500 ft and it captured. Then 30 seconds later it started to dive. Disconnect autopilot the trim was nose down, significantly. Any ideas. This was first and only use, Knocked off using it until I can figure out what is going on. LESSON never use autopilot close to ground.
Click to expand...
Hi,
I had the same problem with my autopilot when it was new. Not installed by me. As I recall it I changed the setting for pitch trim to reverse. After that it worked fine. Really a surprise first time the fault appeared. In my aircraft I saw the pitch trim indicator move down until autopilot disconnected.
 
FlyingSquirrel said:
the autotrim engaged and is running the wrong direction, which causes a runaway condition. There is a configuration that let's you reverse the auto trim direction which will correct this.
Click to expand...
Brantel said:
Yep, autotrim is backwards most likely!
Click to expand...
Thank you all... I will pass that on to the builder.... It was kind of scary to disconnect autopilot and have it fight back pressure (building as airspeed is higher). I will report back if that fixes it. Proceeding with caution at altitude. Again autopilot at low attitude has limits in airlines.... 400 feet roll and 1000 feet pitch would be my self imposed min altitude in GA. Practice and exercise autopilot and trim disconnects.
 
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gmcjetpilot said:
Thank you all... I will pass that on to the builder.... It was kind of scary to disconnect autopilot and have it fight back pressure (building as airspeed is higher). I will report back if that fixes it. Proceeding with caution at altitude. Again autopilot at low attitude has limits in airlines.... 400 feet roll and 1000 feet pitch would be my self imposed min altitude in GA. Practice and exercise autopilot and trim disconnects.
Click to expand...
I can pretty much guarantee that it will fix it.
There are checks in the avionics set up instructions for this, and one of the steps must have obviously been missed.
 
gmcjetpilot said:
That is when we found the issue, using the manual. We are less than 3 hours into Phase 1.

GOOD NEWS.... Yes the configuration change did it, Autopilot is functioning well now, captures and holds altitude. Thanks again. Now to calibrate AOA.
Click to expand...

Well if you were already 3 hours into phase 1 of inflight testing then you skipped the section in the installation manual for "On the Ground" checks that would have caught this misconfiguration. I suggest you go back and make sure you did not skip any other checks that are intended to find issues before flight.
 
gmcjetpilot said:
Garmin G3X in a new RV-12 (Experimental) . Doing Phase 1 tested Autopilot. Heading works fine. Did V/S to climb 500 ft and it captured. Then 30 seconds later it started to dive. Disconnect autopilot the trim was nose down, significantly. Any ideas. This was first and only use, Knocked off using it until I can figure out what is going on. LESSON never use autopilot close to ground.
Click to expand...
The other part that was missed was Step 2 of autopilot tuning from Appendix A: disconnect auto-trim until AP is properly tuned:

1715095184263.png
 
Brantel said:
Well if you were already 3 hours into phase 1 of inflight testing then you skipped the section in the installation manual for "On the Ground" checks that would have caught this misconfiguration. I suggest you go back and make sure you did not skip any other checks that are intended to find issues before flight.
Click to expand...
This is an RV-12is question, so like some other RV topics there is some variance in how builders learn and what they expect. To be fair, I think this would be easy to miss for someone that is closely following the PAP as the guide for ground checks. As far as I can tell, this isn't covered there. That said, this scenario also seems to indicate the PAP wasn't being followed either since (I assume) the config file from Van's has the auto trim disabled per the Garmin instructions.

Either way it seemed worth mentioning in this thread for future searches that the G3X Installation Manual has important and relevant info that could be missed by RV-12 builders only referencing the KAI and PAP.
 
N8DAV8R said:
This is an RV-12is question, so like some other RV topics there is some variance in how builders learn and what they expect. To be fair, I think this would be easy to miss for someone that is closely following the PAP as the guide for ground checks. As far as I can tell, this isn't covered there. That said, this scenario also seems to indicate the PAP wasn't being followed either since (I assume) the config file from Van's has the auto trim disabled per the Garmin instructions.

Either way it seemed worth mentioning in this thread for future searches that the G3X Installation Manual has important and relevant info that could be missed by RV-12 builders only referencing the KAI and PAP.
Click to expand...
You’re right, it is not covered in the PAP because there are a number of different avionics configurations.
But there is an avionics set up guide document that is intended to be used for the specific avionics kit that is installed.
www.vansaircraft.com

Downloads - Van's Aircraft Total Performance RV Kit Planes

Van’s provides a variety of downloadable items in support of kits and flying aircraft. Choose from the type of downloads you need below. RV-12/12iS avionics software downloads Dynon Skyview Garmin G3X Dynon D180 RV-12 and RV-12iS Electrical System Diagrams RV-12 misc. documents Canopy Warning...
www.vansaircraft.com www.vansaircraft.com
I am pretty sure that it covers this detail.
 
rvbuilder2002 said:
You’re right, it is not covered in the PAP because there are a number of different avionics configurations.
But there is an avionics set up guide document that is intended to be used for the specific avionics kit that is installed.
www.vansaircraft.com

Downloads - Van's Aircraft Total Performance RV Kit Planes

Van’s provides a variety of downloadable items in support of kits and flying aircraft. Choose from the type of downloads you need below. RV-12/12iS avionics software downloads Dynon Skyview Garmin G3X Dynon D180 RV-12 and RV-12iS Electrical System Diagrams RV-12 misc. documents Canopy Warning...
www.vansaircraft.com www.vansaircraft.com
I am pretty sure that it covers this detail
Click to expand...

Except, it mostly doesn't cover this detail. I had that document in mind as a supplement to the PAP, I went through all of these steps a few times to be careful. With respect to trim adjustments it's pretty vague and aside from calibrating the trim indicator on the screen it mostly tells the builder to just trust Van's default. Pasted below is the entirety of trim reference in the avionics setup guide.


Screenshot 2024-05-07 at 10.06.00 AM.png
 
Another detail to mention here is that the OP may be stating that this is an E-AB RV-12. Can't be sure though, he just says (Experimental) which is true with E-LSA and E-AB. That being said, if it is E-AB there is no telling where the builder deviated from the original RV-12IS KAI and design intent. Depending on how far the builder deviated, the default config file, PAP and the supplements may not appropriately apply.

Also the readme for the G3X installations has this guidance included:

Capture.PNG
 
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