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Weird G3X ADAHRS Problem

Amazon-1

Active Member
I have a nearly new G3X touch system in my RV-14A. It mostly works flawlessly except when it doesn't. The engine is a Lycoming YIO-390-EXP33 Thunderbolt with a constant speed propeller from Whirlwind (HRT-74).

When I am doing my run up at the end of the runway prior to takeoff, I advance the throttle to 1700rpms for the mag check. I then pull the prop control out to course pitch. The RPMs rapidly drop (as they should) and the ADAHRS immediately tumbles (typically completely upside down).

Several seconds later it rights itself, but if I pull the prop again it will tumble again.

I saw it do the same thing once in the flair for a touch and go but generally in flight I do not have any problems.

During installation I did the vibration, interference and calibration tests on the ADAHRS and the magnetometer as per Garmin's installation manual and they all passed.

The G5 which gets pitot and static air from the same source does not tumble.

Christof at SteinAir had me redo the calibration but that did not help.

I have a video I made of it tumbling just in case anyone thinks I have been drinking.

Any thoughts, recommendations etc. appreciated.

SteinAir said it could be a bad GSU25 and they could RMA it but I thought I would see if anyone has any other ideas first.

Thanks and happy new year!
 
Where is the GSU-25 and the magnetometer mounted? Are they anywhere near the prop cable? I would try to move the prop cable without the engine running and see if you get the same response.

Jay
 
The prop cable is not really near the GSU-25

The GSU 25 is mounted on the back of the PFD about a foot from the prop cable. The throttle and mixture cables which are nearly identical to the prop control cable do not cause the problem.

I think the rapid change in air flow over the pitot may have something to do with it. Even though the airplane is stationary, the airspeed shows some speed due to the propwash and it rapidly changes when the ADAHRS tumbles.

bruce
 
Resonant vibration

My guess is that there is some resonant vibration that occurs when the prop is being cycled and this may be disturbing the GSU-25. Can you temporarily put some extra bracing around the support bracket to see if it makes any difference? I've previously experienced airframe momentary vibrational "shudders" when cycling the prop in another aircraft.
 
I think the rapid change in air flow over the pitot may have something to do with it. Even though the airplane is stationary, the airspeed shows some speed due to the propwash and it rapidly changes when the ADAHRS tumbles.

bruce

If this is true then your airspeed will be off in cruise, too. Easy to check - Tape over the pitot opening and see if the problem goes away. And if that’s the fix, you probably need to move the pitot more outboard.
 
Relocating the GSU 25 may be needed

It's most likely vibration.
Garmin does not recommend mounting the GSU 25 to the back of the PFD.
Garmin specifically recommends mounting the GSU 25 to a solid structure. Page 19-6 in the G3X installation instructions covers this.
 
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It's most likely vibration.
Garmin does not recommend mounting the GSU 25 to the back of the PFD.
Garmin specifically recommends mounting the GSU 25 to a solid structure. Page 19-6 in the G3X installation instructions covers this.

Yeah the installation instructions were very clear on this.
 
+1 for almost surely vibration related. Relocate the ADAHRS or brace the panel.
FYI, the G5 will tumble too. Seems susceptible to high level white noise, like N2 venting in a temperature chamber. Don't ask me how I know...
 
+1 for almost surely vibration related. Relocate the ADAHRS or brace the panel.
FYI, the G5 will tumble too. Seems susceptible to high level white noise, like N2 venting in a temperature chamber. Don't ask me how I know...

Ditto on posts #2 and #9. My engineer brain says that when you reduce the prop that you are hitting an resonance range of the sheet metal that the ADHARS is mounted to and makes it go wonky. Brace where it's mounted or move the ADHARS, and it will probably disappear.
 
Before you completely rework your setup try a few things. My GSU25 is mounted to the back of my left GDU-460 on my 14A and passes all tests and works perfectly. YMMV but my mileage is good! So yours could be too.

A simple start would be to use longer screws and rubber washers to vibration isolate the -25; of course that might make it vibrate more too depending on the mode. Easy enough to try. Can also make sure all the screws you have in are tight, similar idea.

Find someone else with a -25 and swap out for another unit. Maybe yours is more sensitive than another one? Maybe it is the unit? What if it isn't the mount point after all?

It would be pretty easy to connect a brace from the top of the instrument panel over to the sub panel and stiffen the assembly in a way that doesn't interfere with the canopy. You might not want new holes though.

You could also remove it from the GDU, put velcro on it, and velcro it to the sub panel somewhere; just to see what changes out of curiosity. Or make extender cables and stick it on the side-wall of the fuselage below your left knee. I've thought that one good place for it (or somethings anyway) is on the tunnel cover low and forward. If you fabricated some extension cables you can play with it.

But I wouldn't go to the effort of finding a new permanent home until you know that is really the problem.
 
I would agree that bracing the panel is an easy option to try.
The AHRS must be rigidly mounted to the structure, mounting it on rubber isolators or with Velcro is asking for trouble.
 
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In the G3X manual, and on the configuration page, there is a vibration test. run it and see what the result is.

DAR Gary
 
What Walt said. Steer away from rubber shock mountings. In fact most modern digital ADHRS manufacturers suggest rigid mounting going so far as to recommend removing rubber shock mounts from floating instrument panels. On Cessnas those little rubber shock mounts are replaced with solid bushing and machine screws. That shouldn't be a factor in a 14A.

One down side of mounting the GSU 25 on the back of the display is it makes it three times as hard to remove the GDU for maintenance since the GSU and it's associated pitot/static plumbing has to come for the ride too. However since JDA_BTR has a 14A with component mounting similar to yours without trouble it might come down to the panel's physical structure. He might have a thicker panel, braces or added stiffeners and different display location (few inches in all directions).

Since your airplane is an 14A (and therefore don't have to keep the stick back on run up) you could try another test. Try grabbing the bottom edge of the panel with your left hand while your right hand cycles the prop. Basically you will be temporarily bracing the panel from flexing and also temporarily changing the natural frequency of the panel. If you can't grab the bottom edge you could also try pressing in on the panel next to the GDU display.

You could also pull the breaker on the GSU 25 to force the system to revert to the G5 as a AHRS backup, as designed, and see if you get similar results.
 
All good ideas here. My intention wasn't to permanently have it velcro'd or rubber-washer mounted. It was to change the scenario and see what changes with it, in an experimental sort of way.
 
I think i have found the problem

As most everyone said, the likely cause is excessive vibration of the GSU25. Today I checked the bolts that attach the GSU25 to the back of the GDU460. I found two were very loose (not even hand tight) and two tighter then hand tight but looser than wrench tight. I tightened them all and tested the system by running the engine at 1800 rpms and pulling the prop control about a dozen times. It did not flop once. Too yechy to fly today, but I will check it several more times when the weather improves. I am optimistic that the problem is solved.

Thanks to all that helped me solve this.
 
As most everyone said, the likely cause is excessive vibration of the GSU25. Today I checked the bolts that attach the GSU25 to the back of the GDU460. I found two were very loose (not even hand tight) and two tighter then hand tight but looser than wrench tight. I tightened them all and tested the system by running the engine at 1800 rpms and pulling the prop control about a dozen times. It did not flop once. Too yechy to fly today, but I will check it several more times when the weather improves. I am optimistic that the problem is solved.

Thanks to all that helped me solve this.

Great news...back to basics with the trouble shooting...I bet you got it
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So the screws were loose. I assume they were tight once? Did you add some split ring lock washers or some other prevention so they don’t get loose again?
 
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