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SB-00028: GSU25C G3X Noise Induced Erroneous Attitude

rcarsey

Well Known Member
Van's has published SB-00028, which addresses erroneous attitide indications from the GSU25C (the AHARS sensor under the baggage floor access panel). It applies to certain SLSA s/n's and ELSA with Garmin avionics shipping dates between March 2019 and June 2021.

https://www.vansaircraft.com/wp-content/uploads/2021/06/SB-00028-RV-12-G3X-Noise-Induced-Erroneous-Attitude.pdf

Details related to the flight restrictions prior to compliance can be found in the service bulletin document.

An example of the erroneous behavior on my aircraft can be found here.
 
Interesting. I'm sure Van's confirmed this fix before publishing, but it sure doesn't look like a very effective isolation scheme to me. Isn't the head of the screw touching the airframe bracket and the other end with the nut touching the ADAHRS? That's a direct vibration path, at least in the rebound direction. To be fully isolated you would need another bushing between the bolt head and the bracket, or a sleeved bushing. I'm curious what the frequency of concern was.

I'm hoping to take delivery of my avionics from Stein in early to mid July, so I guess I will get an affected unit. Would be nice to ensure a proper fix at initial installation and not wait for flight testing.

I will contact Van's and report back anything of note.
 
I've had 60+ hours of trouble free operation of my GSU 25C/G3X since first flight of my RV12iS in October 2020. The iS engine runs very smoothly with a dynamic prop balance achieving vibration levels of 0.02IPS.

On my last 3 flights I experienced progressively worsening erratic attitude indications with marked pitch deviations observed on the PFD even while in level flight. Multiple AHRS ALIGN messages were noted. Both Garmin and Van's technical support suggested I install the vibration isolators per SB-00028. Neither had any thoughts why the problem might start after 60+ hours of normal operation.

I'll comply with the SB this weekend and report results here (as well as to Van's and Garmin Technical Support).
 
Interesting that Garmin calls out the cause as "acoustic noise energy." :rolleyes:

If you look at the CubCrafter's SB, they use a proper isolator.

It's possible that the CubCrafter's isolators are too tall to use in the RV-12, since the aileron torque tubes are just beneath the GSU25C.

But in any case, I'm confident that the factory has tested the fix.. especially since the factory had the benefit of testing out Cub's solution (published in Feb 2021).

I'm happy that Garmin finally fessed up to there being an issue. Though I'm unsure if ALL GSU25C's are affected, or just a certain revision of them. Either way, I'm surprised the issue made it out of Garmin's product testing regime. And I wonder what is different inside the GSU25D which is FAA-PMA'd and more expensive, but otherwise appears to be the same device.
 
Thanks for the link. Interesting that Garmin calls out the cause as "acoustic noise energy." :rolleyes:

If you look at the CubCrafter's SB, they use a proper isolator.

And if a proper ADAHRS installation was done there would be no isolator... the ideal installation is with a very solid mechanical connection between the box and some very stiff structure on the airframe.

Garmin engineering was initially somewhat resistant to using any type of isolation in the mounting of the ADAHRS.

Extensive testing proved that this mounting method works well for the installation that is used on the RV-12iS. Making it further isolated than depicted could actually make the problems worse depending on the the isolators that were used.
 
It's possible that the CubCrafter's isolators are too tall to use in the RV-12, since the aileron torque tubes are just beneath the GSU25C.

I asked if that was the case in my email to Van's.

But in any case, I'm confident that the factory has tested the fix..

I would assume so, but I'm still surprised it works and am curious from an engineering standpoint. Personally I'd still prefer to find a proper isolating scheme, but hey, if it works, it works.

Though I'm unsure if ALL GSU25C's are affected, or just a certain revision of them. Either way, I'm surprised the issue made it out of Garmin's product testing regime. And I wonder what is different inside the GSU25D which is FAA-PMA'd and more expensive, but otherwise appears to be the same device.

Ah, yes, I forgot to ask them what the permanent fix is since the SB implies it is no longer an issue for kits shipped from Van's after June... or is it that they are just including the isolators now?
 
And if a proper ADAHRS installation was done there would be no isolator... the ideal installation is with a very solid mechanical connection between the box and some very stiff structure on the airframe.

Garmin engineering was initially somewhat resistant to using any type of isolation in the mounting of the ADAHRS.

Extensive testing proved that this mounting method works well for the installation that is used on the RV-12iS. Making it further isolated than depicted could actually make the problems worse depending on the the isolators that were used.

Good to know, thanks for chiming in Scott. And I agree, improper isolation can be worse than no isolation, depending on the failure mode.

P.S. Are isolators the permanent fix, or is there an update to the ADAHRS?
 
I've had 60+ hours of trouble free operation of my GSU 25C/G3X since first flight of my RV12iS in October 2020. The iS engine runs very smoothly with a dynamic prop balance achieving vibration levels of 0.02IPS.

On my last 3 flights I experienced progressively worsening erratic attitude indications with marked pitch deviations observed on the PFD even while in level flight. Multiple AHRS ALIGN messages were noted. Both Garmin and Van's technical support suggested I install the vibration isolators per SB-00028. Neither had any thoughts why the problem might start after 60+ hours of normal operation.

I'll comply with the SB this weekend and report results here (as well as to Van's and Garmin Technical Support).

I installed the vibration isolators per SB-00028 and flew 2+ hours this weekend. The attitude indications were very stable without any of the erratic behavior seen throughout most of the previous 3 flights. I'll report back if the problems recur, but for now I consider the issue addressed. Feedback has also been provided to Van's and Garmin's Technical Support.
 
I installed the vibration isolators per SB-00028 and flew 2+ hours this weekend. The attitude indications were very stable without any of the erratic behavior seen throughout most of the previous 3 flights. I'll report back if the problems recur, but for now I consider the issue addressed. Feedback has also been provided to Van's and Garmin's Technical Support.

I'll echo this too. I did the isolator fix, logged 5 flights (4.2 hours), and its been pretty solid so far.
 
Reviving this old thread from the dead..

150 hours total time and the GSU25C with rubber isolators was working pretty good.

I was contacted by the factory today and it sounds like they worked things out with Garmin to do a no-cost-to-us swap with a revised GSU25C that isn't affected by the "acoustical vibration" problem.

Their e-mail stated that they were going to ship it out (with my approval) and I had until May 31 to ship the original back (with the rubber isolators. seems they want to be sure that nobody uses them anymore). Kudos to the factory for this one and minimizing/eliminating downtime for us pilots.

Seemed like they were mailing people individually, so if you were affected, be on the lookout for an email in the next few days or week. The SB will have a revision.. (probably posted on a Friday as usual).
 
Reviving this old thread from the dead..

150 hours total time and the GSU25C with rubber isolators was working pretty good.

I was contacted by the factory today and it sounds like they worked things out with Garmin to do a no-cost-to-us swap with a revised GSU25C that isn't affected by the "acoustical vibration" problem.

Their e-mail stated that they were going to ship it out (with my approval) and I had until May 31 to ship the original back (with the rubber isolators. seems they want to be sure that nobody uses them anymore). Kudos to the factory for this one and minimizing/eliminating downtime for us pilots.

Seemed like they were mailing people individually, so if you were affected, be on the lookout for an email in the next few days or week. The SB will have a revision.. (probably posted on a Friday as usual).

I received the same email. I wonder why the change? Did they find some sort of manufacturing error with the original GSU25?
 
I received the same email. I wonder why the change? Did they find some sort of manufacturing error with the original GSU25?

I think Garmin had changed some internal parts (MEMS sensors?) to a different manufacturer or model and it bit them.. as the certified version of the GSU25 was unaffected.

GSU 25C ADAHRS (011-02929-50) units with serial numbers 5Q2000000 through 5Q2099999 were affected.

https://support.garmin.com/en-US/aviation/faq/a28Hiu9gNJ9kaTAxHHJW97?keywordSearch=gsu25c
 
I think Garmin had changed some internal parts (MEMS sensors?) to a different manufacturer or model and it bit them.. as the certified version of the GSU25 was unaffected.

GSU 25C ADAHRS (011-02929-50) units with serial numbers 5Q2000000 through 5Q2099999 were affected.

https://support.garmin.com/en-US/aviation/faq/a28Hiu9gNJ9kaTAxHHJW97?keywordSearch=gsu25c

I’m a mentor for a local high school build. We received our avionics kit about 10 days ago and received a GSU25C in the affected serial number range. Have sent an email to the mother ship. Perhaps the parts we got include the SB fix. Dunno yet.
 
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