GFC500 problem

[Jblanton]

I don't have electric pitch trim.  My IAS climb, and altitude hold is near perfect.  I don't miss it either.  It is just three swipes on the wheel when I level off, and three swipes when I descend.  The GSA 28 servo is weak for Mooney pitch trim so it is geared accordingly.  Maybe the pitch trim is too slow and it is fighting the pitch servo, but what do I know.  I don't know if you can disable electric trim for a flight, or at least put your hand on the wheel.  

[rbp]

3 minutes ago, Jblanton said:

Maybe the pitch trim is too slow and it is fighting the pitch servo, but what do I know. 

I do know because the data is the logs and have analyzed them. That’s not the problem.

[Jblanton]

The Data is an effect, not a cause.  That is something I have had to realize maintaining modern jets.  

[PeytonM]

I have elect trim. When in ALT, my trim wheel does not move at all when established and level.
 

While in ALT, if you wedge a couple fingers in between the yolk shaft and the panel, you can feel the shaft move every so slightly, but enough to cause oscillation. If you go to PITCH mode, the movement and oscillation stops immediately. 

[rbp]

On 6/7/2023 at 6:31 PM, Jblanton said:

The Data is an effect, not a cause.  That is something I have had to realize maintaining modern jets.  

Lol. Where did I say it was the cause?   I am looking at the torque numbers with and without oscillations.  

[RoundTwo]

Coming home from the PPP in Lexington yesterday, I finally experienced pitch oscillations, for awhile. Maybe my experience can shed some light on the root cause analysis.

While heading home and dodging areas of precipitation, my wife and I were front seat with our dog in the back seat. At one point, it was necessary for my wife to get in the back to calm the dog. We learned that opening the vent window will immediately cause him to go into shakey-leg mode. As soon as she climbed into the rear, the oscillations began. It was a slight porpoising with a cycle of a few seconds. The trim was rock solid, so it was definitely the pitch servo. As soon as the CG was returned to its original location, the oscillations ceased. While it was happening, I tried some minor trim adjustments thinking if I moved to a more positive, or negative, servo torque, it might make a difference. I couldn’t say definitively that trim adjustments helped.

Anyway, my oscillations seem to be related to longitudinal stability which correlates well with the gain adjustment observations.  Maybe the real driver is trim angle which was caused by moving the CG aft requiring more nose down trim. 
Maybe this additional data point will be helpful in the ongoing analysis.

[PT20J]

It might be an interesting experiment to try loading the airplane to the aft CG limit and seeing what the effect might be. All airplanes have decreased longitudinal stability at aft CG.

[PeytonM]

I have noted more oscillation when I had 2 passengers in the back seat and only me in the front. I calculated the CG depending on where I put the two passengers: CG moved aft 2 inches, from 46” to 48” if I put both in back, but nowhere close to the aft limit of about 50”.

You’ve got to fuel lightly and put some pretty heavy folk in the back to get her out of limits. 

[Ragsf15e]

28 minutes ago, PT20J said:

It might be an interesting experiment to try loading the airplane to the aft CG limit and seeing what the effect might be. All airplanes have decreased longitudinal stability at aft CG.

I also wonder how sensitive it is to getting the pitch trim set very precisely for itself.  Example (and yes, different system, but hear me out) - on my stec 30a, if I set alt hold in stable cruise and then slow down (or speed up) ~2 knots, the “trim”warning will not activate and the unit will be able to hold altitude, but it has a very similar oscillation to what you guys have shown as the pitch servo is “fighting” to hold altitude.  If i make a very minor trim adjustment, it’s steady.

Is the pitch trim on the gfc500 setting itself very precisely as airspeed or cg change or does it have a less precise adjustment schedule?

[PeytonM]

Email exchange yesterday with Trek at Garmin indicates they are still looking for a J/K to bring in for testing. I reported to him that flying with a passenger in the back seat (CG moving 2” aft, but well within the envelope) resulted in more noticeable oscillations. In fact my passenger asked about it, unsolicited. He was right: it was worse than when I fly by myself. 
 

[PeytonM]

I only have only one issue and I’m far from the only one. Oscillations, and it appears to be CG sensitive. I count at least a dozen posters, and those are just from MS. And those are only the ones that understand that there should be NO OSCILLATIONS! I’m guessing this issue is much more prevalent than we think. BeechTalk has a thread on it, too.  I’ve flown in other models of AC with the GFC500, and it’s smooth as glass.
 

I’m the only one who continues posting about it. I guess everyone else is just accepting the issue as “oh well,” and Garmin is hoping the issue will just die and go away.

They want a plane for about two months and up to 20 hours. After being out of service for 3 months for the installation, I’m not about to give it up for even more (unspecified) time. Last year, Garmin sent a Baron-load of guys down here to look at my plane for several hours and found nothing. Frankly, I’m not sure why any of us should have to give up their plane for any time. Do you think that would be the approach in any other industry?  With all the EIS data Garmin have, I would venture they know how to solve it. So do the installers. 

[Pinecone]

Are they offering any compensation?

I would be willing to volunteer my K, for a free GFC-500.

[ArtVandelay]

Are they offering any compensation?
I would be willing to volunteer my K, for a free GFC-500.

They’re looking for someone with problems. If I had issues that couldn’t be fixed, it would be worth it to get them to fix it.

[PeytonM]

And you saw my reply.

[blaine beaven]

I’m looking at installing a GFC500 in my 1978J this winter. @PeytonM did you ever get your problem solved?

[ArtVandelay]

I’m looking at installing a GFC500 in my 1978J this winter. [mention=8500]PeytonM[/mention] did you ever get your problem solved?

I have a 78J and don’t have any issues with the GFC500 with latest software updates, FWIW. Please make sure you have no sticking of the controls and trim.

[PeytonM]

I believe Garmin is still looking for a J/K with the oscillation for testing. I continue to fly my plane. 

[Ragsf15e]

To be fair to garmin, it’s hard to solve a problem that they can’t duplicate without having an aircraft with the actual problem.  At least they would be willing to address it if someone could give up their airplane, but fully understand that might not be easily done.

[ZamF16]

On 5/16/2023 at 6:35 AM, rbp said:

unfortunately this started happening yesterday when I leveled off

 

rbp, this is exactly the issue I am having with my M20M (G3X and GFC500).  My oscillations increase over time to sometimes as much as +/-300 fpm.  Did you ever get it resolved, or does the saga continue?

Zam

[LANCECASPER]

It would be interesting to see on every one of these Mooneys where there is oscillation, who were the shops that did the install? What if most of the problems were installed by only a few shops? 

Also the installations where there have been no oscillation problem, who were the shops that did the install?

 

If it's not related to specific shops then pressure needs to be put on Garmin to get this right. Twenty Mooneys in a caravan landing near Olathe, KS on a Monday morning, all with Go-Pro footage shared with AOPA, might be a start .. lol

[PeytonM]

Garmin released Optional Gain settings ver5 a few weeks ago to address this issue. It fixed mine and I know of a couple J/K’s down-under that it fixed.   I’m sure planes are scattered all over the world! Unfortunately the Addendum is only J/K, but I bet longbodies are next. They’ve done a similar Gain Addendum for Cirrus. Check out the thread