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I know there has been a lot of discussion about this but wanted to share my experience in case anyone has any ideas.  My GFC500 "hunts" 50-200fpm whether it's in altitude hold mode in level flight in calm air or in a VSI and airspeed climb/descent.  This issue has been going on ever since I had it installed about 3 months ago.  I've read read extensively about the controls sticking and have had it checked by several experienced people without any luck. We have also confirmed that all my settings are correct and have even changed certain ones trying to isolate the issue.  It's been a minor annoyance until several days ago where I was climbing through IMC in VSI mode with some light turbulence and it tried to take me on a 1k fpm rollercoaster where it basically almost put the plane in a dive.  I clicked off the AP before it really started oscillating and hand flew.  My avionics shop has put in more of these autopilots than almost anyone in the country and has been great.  However, I'm getting pretty frustrated at this point (especially since my many AMUs new Garmin system tried to kill me in IMC).  See below for a representative video of the typical porpoise in altitude hold mode in smooth air:

Any ideas @TrekLawler or @KLudwick?

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I believe @donkaye had something similar with his install that got fixed.  Check that cable tensions are actually in spec and lubricate the yokes in the blocks is a good starting place, even if they don’t feel like they are hanging or sticking. Hunting and overshooting was tracked to the slop and resistance to movement in the system IIRC.  The other system to check is your static system. The connectors on the back of the garmin units are finicky.  

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1 hour ago, bradp said:

I believe @donkaye had something similar with his install that got fixed.  Check that cable tensions are actually in spec and lubricate the yokes in the blocks is a good starting place, even if they don’t feel like they are hanging or sticking. Hunting and overshooting was tracked to the slop and resistance to movement in the system IIRC.  The other system to check is your static system. The connectors on the back of the garmin units are finicky.  

Thanks, tensions and yokes have been checked already.  They also checked the static connections and all seemed ok.

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There is a documented problem (with a service bulletin) in certain Pipers that have a static port on the same vane with the pitot. That was ultimately a software fix. 

But there is also a (maybe unofficial) recall on a batch of servos. We had all of the servos replaced under warranty by Garmin at a total cost of zero, and the porpoising problem has not returned. 

(Note this is in a PA-28.)

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1 hour ago, Davidv said:

Thanks, tensions and yokes have been checked already.  The static system is probably the next stop.

 

10 minutes ago, Yetti said:

Is it instrumentation error  or is the plane actually going up and down.

 

The plane is actually moving up and down which makes for fun long trips!

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20 minutes ago, Davidv said:

 

The plane is actually moving up and down which makes for fun long 

 

Try using alternate static and see if that stops the porpoise.  Pro tip- do this before engaging the AP so you don’t induce a wild ride at the onset of the pressure change when you pull the knob. 
 

What’s your G5 or other backup instrument saying with altitude- do they agree during the porpoises?  

 

 

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43 minutes ago, Davidv said:

 

The plane is actually moving up and down which makes for fun long trips!

free roller coaster ride

What 81X said makes lots of sense.

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It would be interesting to measure the static pressure with an inline monitor.   Since there are two ports on either side of the plane  wonder if there is a differential pressure on either side of the plane.  A damper in the line may be needed.   Which of course is what the alt static will get you since it is single source.

Is the A/P creating the issue or is it following it's sensors.   Hmmm

Edited by Yetti
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Often, around here... on other AP models...

1) altitude variations with increasing sinusoids... are the control system fighting something that isn’t right... like...

2) Bad static input... some people have a rubber hose attached to the static system, connected to the AP... the rubber hose goes bad... essentially leaves the AP getting bad static information from inside the cockpit...

Consider checking what line is connected to the back of the AP, and is it leak free... a good static system check may be an easy route to check this too...

3) Bad servos... the attitude servo gets used a lot.  Some use a clutch that has a tendency to wear... the wear in the system is out of tune with the controller... the older response time of the BK APs starts showing with increasing porpoise... 

An example of really odd responses... find a clutch that got CorrosionX on it...

 

So...

4) What happens when... the plane begins to porpoise, the pilot manually stops the porpoise... reengages altitude hold, or VS climb... how long before the uncontrolled behavior begins again?

5) Was there any re-used servos part of the install?

6) Since the attitude Servo and trim Servo can be capable of masking each other... check both...

7) Ever want to mount a camera in the tail to watch over the two servos back there?

8) Expect that it could be anything including a dirty wire connector... a good visual inspection in the tail is easy...

9) Visual inspection behind the panel... not so easy...

Good luck with the next step...

PP thoughts only, not a mechanic...

Best regards,

-a-

In the end... the modern GFC should be able to hold a steady climb, or altitude without wandering... it’s computer speeds are incredible quick compared to anything that came before them...

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3 hours ago, Davidv said:

Thanks, tensions and yokes have been checked already. 

Don’t just check the yoke shafts.  Actually clean them.  @donkaye is probably the most experienced Mooney pilot on this site.  His yokes (also M20M, BTW) felt perfectly normal to him but his GFC500 was exhibiting the exact same problem you’re describing.

Not sure his method for cleaning his, but a good start would be silicone spray lubricant applied to the shaft and wiped clean.  Apply more and move the yokes in and out, and wipe clean again.  Repeat a few times and go for a test flight.

Good luck, please keep us posted.

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On 2/7/2021 at 6:08 PM, Andy95W said:

Don’t just check the yoke shafts.  Actually clean them.  @donkaye is probably the most experienced Mooney pilot on this site.  His yokes (also M20M, BTW) felt perfectly normal to him but his GFC500 was exhibiting the exact same problem you’re describing.

Not sure his method for cleaning his, but a good start would be silicone spray lubricant applied to the shaft and wiped clean.  Apply more and move the yokes in and out, and wipe clean again.  Repeat a few times and go for a test flight.

This.

As far as cleaning, I ended up getting the softest steel wool available, dousing it in a WD40-like substance (so the steel wool is almost dripping wet) and giving both yoke shafts a few gentle rubs from one end to the other of the section that goes through the eyelet.

I still have the replacement eyelets I ordered on the shelf.

Edited to give credit where it is due: FYI, the recommendation for steel wool and WD40 suggestion came from Dan at LASAR; he also recommended wiping it all down with silicone sprayed into a rag.

Edited by tmo
Edited to give credit where it is due (Dan R @ LASAR)
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I had my 252 at the installer last week.  Avionics is his business.  He had a Beech that did what you are describing.  They found the controls were binding slightly.  

My GFC500 would hunt when I made a rapid change in IAS on climb.  However, seems like that went away with the last software update-or maybe I am less aggressive with the change?

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I'm sure you did this too, but update the software in the G5.  I had a small oscillation in the IAS climb and on the glideslope.  Updating the software fixed it for me.  I think there were updates very recently that improved some hunting behaviors.

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6 hours ago, tmo said:

This.

As far as cleaning, I ended up getting the softest steel wool available, dousing it in a WD40-like substance (so the steel wool is almost dripping wet) and giving both yoke shafts a few gentle rubs from one end to the other of the section that goes through the eyelet.

I still have the replacement eyelets I ordered on the shelf.

Edited to give credit where it is due: FYI, the recommendation for steel wool and WD40 suggestion came from Dan at LASAR; he also recommended wiping it all down with silicone sprayed into a rag.

Can't remember, but Brass or bronze wool for chrome.   Which are probably not obtainable anymore.     I would probably use green or red scotch brite pads as now you have specs of metal all over your carpet.

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Garmin Field Service engineering (the techs who support avionics installers) have been recommending a) ensuring rigging is in spec, b) check for control binding (great info above) and c) update all software. The G5s have had at least three different software updates to deal specifically with this. Latest is version 6.82.

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On 2/7/2021 at 9:54 AM, Yetti said:

it makes you marvel at the couple of resistors on a board of the Brittian system.  

This is an interesting lesson in expectations.  If the altitude hold on our Brittain maintained the selected altitude within 20' using climb rates of 50-200 fpm as shown in the OP's video, I'd consider it to be doing a fantastic job - much better than expected.  For us, it's a pretty good day when it holds altitude within 100' or so, which is all it promises.

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2 minutes ago, Vance Harral said:

This is an interesting lesson in expectations.  If the altitude hold on our Brittain maintained the selected altitude within 20' using climb rates of 50-200 fpm as shown in the OP's video, I'd consider it to be doing a fantastic job - much better than expected.  For us, it's a pretty good day when it holds altitude within 100' or so, which is all it promises.

Com'on man it is a hardware only solution.  In a very dynamic environment I might add. 

Edited by Yetti
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Any slop or friction in a control system causes some degree of controllability issues. Before installing an autopilot, I would expect the shop to verify that the control system is rigged and functioning properly. However, if your airplane doesn't have noticeable control issues like a sticky yoke or lots of free play due to worn rod ends, the autopilot should be able to fly it as well as you do.

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  • 1 month later...

The early Garmin GFC-500 autopilots Had endemic servo issues. Although I never had a porpoise issue, I had everything else.

Before the saga ended, three out the four servos failed. Although the labor was on my ticket, Garmin without hesitation provided new ones.

Now all is well, and the autopilot is functioning as intended. Just a suggestion, you might want to  check those servos.

 

Best

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7 minutes ago, DAVIDWH said:

The early Garmin GFC-500 autopilots Had endemic servo issues. Although I never had a porpoise issue, I had everything else.

Before the saga ended, three out the four servos failed. Although the labor was on my ticket, Garmin without hesitation provided new ones.

Now all is well, and the autopilot is functioning as intended. Just a suggestion, you might want to  check those servos.

 

Best

What did Garmin's defective servos cost you in labor?

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