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Garmin fix for M20J/K GFC 500 pitch oscillations


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I just learned that Garmin has released rev 5 of the GFC 500 Gain Addendum for the M20J/M20K that includes alternate gain settings for installations where the “pitch or pitch trim is overactive.” Dealers will have access to the document.

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Gosh after all these years, adjusting the gain fixes the problem.   I mentioned on BT that someone adjusted the gains in the AFCS of the G5s and got blasted for how illegal that was.  Even though that’s the purpose of gain in a circuit.  

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On 4/2/2024 at 11:43 PM, PT20J said:

I just learned that Garmin has released rev 5 of the GFC 500 Gain Addendum for the M20J/M20K that includes alternate gain settings for installations where the “pitch or pitch trim is overactive.” Dealers will have access to the document.

Excellent! Can’t wait to see what they came up with. Too bad I have an M model, the alternate gains won’t work for me :ph34r: :lol:. To quote Monty Python's Flying Circus, “ROIGHT’.

Cheers,
Rick

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On 4/3/2024 at 2:43 PM, PT20J said:

I just learned that Garmin has released rev 5 of the GFC 500 Gain Addendum for the M20J/M20K that includes alternate gain settings for installations where the “pitch or pitch trim is overactive.” Dealers will have access to the document.

I’ve got to say it is a vast improvement, as it should’ve been when first released!

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What I find interesting is that the vast majority of airframes seem to perform quite acceptably with the original settings that Garmin determined during certification. Some that have problems are probably unresolved issues with the airplane or installation. But, as I understand it, a few have gone through extensive investigation and everything with the airplane and installation seems correct, yet they still have problems.

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12 hours ago, PT20J said:

What I find interesting is that the vast majority of airframes seem to perform quite acceptably with the original settings that Garmin determined during certification. Some that have problems are probably unresolved issues with the airplane or installation. But, as I understand it, a few have gone through extensive investigation and everything with the airplane and installation seems correct, yet they still have problems.

 Being hand built and possibly right at the limit of the specified gain with a (low-ish) powered servo, there’s probably enough variation in the trim and/or pitch control that can cause the issues for a few.  Im glad Garmin hasn’t given up on us because I love the autopilot (as long as it works)!

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I don’t know how many airframes have similar issues; only Garmin knows, and they are not talking! But you are right: I’m glad they didn’t give up on us!
 

If you have a conversation with major installers, they will tell you it’s unpredictable. Wouldn’t surprise me if they issue addenda for other airframes and models: Beech, Cessna, Piper. 

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I don’t know how many airframes have similar issues; only Garmin knows, and they are not talking! But you are right: I’m glad they didn’t give up on us!
 
If you have a conversation with major installers, they will tell you it’s unpredictable. Wouldn’t surprise me if they issue addenda for other airframes and models: Beech, Cessna, Piper. 

But some Mooneys (like the J) have the springs which resist the elevator movement, do others do this?
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1 hour ago, PeytonM said:

“Low-ish powered servos.” Where is this coming from????

You can read about it on B, C, and P forums.  The GFC-600 servos are apparently somewhat larger than the GFC-500 servos, and I have read that the GFC-600 servos are about 1/3 the size and weight of the King servos.  The GFC-600 might have been a better fit for a Mooney, but we are known for being CBs, and Garmin probably maximized their aftermarket Mooney revenue by offering the cheaper solution.

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The 600s are from the certified division of Garmin, of course they are going to be more robust and expensive.

The 500 system is from the experimental division and relies on the G5/G3X to do some of the computation work , the controller is just switches. Completely different system and you can’t just substitute 600 servos.

Before I installed the 500 I had my AP clean/lube every part of the elevator controls and trim system.

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Trek Lawler’s response on 6 May 2023 re GFC 500 and GFC 600 servo differences -

hello everyone,

I'm sorry for the delay in getting to any questions, as you can imagine the last couple of days have been pretty busy. To answer everyone's questions on the difference between the GFC 500 and 600 servos, the differences are very minimal, if you were to take them apart you'd notice the same motors and a lot of the same components. the GFC 500 servos are very capable of flying twins if needed.

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26 minutes ago, Mooney in Oz said:

Trek Lawler’s response on 6 May 2023 re GFC 500 and GFC 600 servo differences -

 

hello everyone,

I'm sorry for the delay in getting to any questions, as you can imagine the last couple of days have been pretty busy. To answer everyone's questions on the difference between the GFC 500 and 600 servos, the differences are very minimal, if you were to take them apart you'd notice the same motors and a lot of the same components. the GFC 500 servos are very capable of flying twins if needed.

On BT, Trek says the following.  These two or three sentences are surrounded by several paragraphs of assurances, but I think this is the nut:

"First thing to address is the difference in the servos.  No, the GFC 500 and 600 servos are not the same and just in different packaging.  I won't go into every detail, but I can assure you they are different.  The GSA 28 (GFC 500) motor is a 30 watt motor and the GSA 87 (GFC 600) is a 50 watt motor and that is about as close as they get at that point."

https://www.beechtalk.com/forums/viewtopic.php?f=21&t=195816&start=15
 

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The difference between the GSA 28 and GSA 87 as advised by Trek on BT & MS has got me thinking even more. His comments differ somewhat, but I notice his post on BT was 2 years before his post on MS and during those years there were servo replacements.

I just wonder if during those 2 years when there were software updates and at least one hardware servo replacement update that I can remember following my own installations in December 2021, if any significant internal changes were made. Shortly after the install, all 4 new servos had to be replaced resulting from the SB recall. At that point the only issue I had was the pitch, which is now resolved with the latest Gain Addendum. This was way before the latest servo replacement within 5 years (which will be my second time replacements) that was announced last year, around the time of Trek's post on MS.

This is only wild wondering on my behalf, but given Trek's post to MS, could it be possible that Garmin may have added some internal, hardware changes during either or both of the hardware replacements to bring the GSA 28 servos closer to the GSA 87 servos than we know of? Maybe a MSer with a contact or two at Garmin could inquire.

I'm happy to be labelled the King of Wishful Thinking :D

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  • 2 weeks later...

I compared the baseline gains from the J/K addendum with the M/R/S addendum and found, with only one exception, the gains are nearly identical. The exception is the pitch trim servo max torque, which is set to 100% on the J/K and 60% on the M/R/S.

In looking at the optional gains it appears they've been adjusted to reduce pitch response rates and magnitudes, increase vertical and airspeed acceleration sensitivity, and to change vertical speed inertial sensing from being rate biased to attitude biased. A similar change was made to one of the Cirrus addendums a year or so ago.

Where this leads me is to expect the optional gain settings from the new Rev 5 J/K addendum that address the pitch oscillation issues will correct the same issues for the M/R/S. However, the M/R/S addendum still sits at Rev 4 and has not been updated with the optional gains, presumably because they haven't been flight tested in a long body.

Has anyone with a long body experiencing the oscillations been approached by Garmin to allow them use of your airplane for this testing?

Cheers,
Junkman

Edited by Rick Junkin
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13 minutes ago, Mooney in Oz said:

Garmin own a long body. 

N430G. A 2001 M20R, registered to Garmin International Inc at Olathe, Kansas.

Yes, they brought it to the Mooney Summit in 2022. It was well appointed including a GFC500 that was working well.

I'm thinking they want/need an airplane that's experiencing the pitch oscillation issues so they can empirically validate the fix. I believe this is what they did for the mid-bodies, although I don't know whose airplane they used. I updated my previous post to add that detail.

Cheers,
Junkman

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6 hours ago, Mooney in Oz said:

Garmin own a long body. 

N430G - a 2001 M20R, registered to Garmin International Inc in Olathe, Kansas.

Yet strangely enough they wanted to use my Bravo in 2018 to certify the GFC500. I decided not to and they used another Bravo from a Mooneyspace member to certify it.

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5 hours ago, LANCECASPER said:

Yet strangely enough they wanted to use my Bravo in 2018 to certify the GFC500. I decided not to and they used another Bravo from a Mooneyspace member to certify it.

Likewise. I was flying with the Caravan to KOSH in 2018 so I too declined their offer/request. Plus I was leaning toward Dynon for my panel upgrade at the time.

The irony is if they had used my airplane they would have tailored the gain set to it and I wouldn’t be having this issue.

Cheers,
Junkman

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