GFC500 problem

[PT20J]

It is quite possible that the GFC 500 servos are less powerful than a KFC or perhaps Century or S-TEC. Some of these others were designed for airplanes larger than a Mooney and the GFC was designed for the experimental market which airplanes are typically smaller. I saw an old post a while back on BeechTalk where Trek mentioned that the GFC 600 servos are somewhat more powerful than the GFC 500 and I haven't heard the same issue with GFC 600 installations, though I could have missed it. If the GFC 500 servos are less powerful than the legacy servos, it would explain why some airframes worked with older autopilots but ran into pitch oscillations when the GFC 500 was installed. In other words, perhaps the legacy equipment with beefier servos was more tolerant of control friction than the GFC 500.  At any rate, if there is friction in the control system, reducing it cannot help but make the airplane fly better with the autopilot or manually.

[PT20J]

1 hour ago, Ragsf15e said:

Made me look up “stiction”!  It’s real!

Stiction causes a break out force which can be a problem with servo systems. A certain torque is necessary to overcome the break out force, but once the parts start moving this torque is much higher than that necessary to overcome the friction of motion and the assembly accelerates more rapidly than desired and can overshoot the target position before the control system can correct. If the motion oscillates, the stiction becomes an issue every time the direction of motion reverses.

[EricJ]

It does seem evident that in Mooneys the GFC500 is more sensitive to the installation than other systems, and undersized servos would be one possible explanation for that.

[Fly Boomer]

6 hours ago, EricJ said:

It's a contraction of "static friction". 

Is that a portmanteau?

[Pinecone]

I have read in other threads, that a slight tweak in the gain setting solves this issue.  But in certified aircraft, you cannot do this legally, as the STC specifies a certain gain setting.

The experimental guys can tweak this.

[EricJ]

6 hours ago, Fly Boomer said:

Is that a portmanteau?

I think so!

[PT20J]

2 hours ago, Pinecone said:

I have read in other threads, that a slight tweak in the gain setting solves this issue.  But in certified aircraft, you cannot do this legally, as the STC specifies a certain gain setting.

The experimental guys can tweak this.

I have seen threads where posters say this, but I’ve never seen one where someone tried it and it fixed the problem. So, I think tis is just supposition unless you’ve seen a post that I missed (entirely possible). There is a thread on BeechTalk where Garmin says that every one they have seen has been an airframe or installation issue. In one case they mention the shop setting bridle tension with a tensionometer that was miscalibrated.

The GFC 500 may be more sensitive to airframe issues that other autopilots, but the fix would still be to fix the underlying condition.

 

[Pinecone]

I can't point you to it, but I seem to remember on where the shop did try adjusting the gain and it fixed the issue, but Garmin said to stay within the STC, it had to be set back.

It may well be that the G500 is a bit more picky.  

Hopefully, Garmin will put out some airframe specific guidance to the shops about the requirements as they learn them.

[carusoam]

20 hours ago, Fly Boomer said:

Is that a portmanteau?

MS words of the day!

portmanteau and Stiction…

Congrats gents…

Best regards,

-a-

[carusoam]

Check the gain on those tiny servos…

The setting has been rumored to be near maxed out…

If you can check… we can kill the rumor…

 

Best regards,

-a-

[Marc Taylor]

Follow up to previous post.

To get the autopilot to operate completely smooth, it required lubricating the trim wheel bearing.  Removed the cover, used Deep Creep to clean and lube.  (Short term fix until I can get the proper lubrication completed.) You will feel a noticeable difference.  So far, operates very smooth, no oscillation.  Even reduced oscillation when climbing in the IAS mode.  Visited with the installer-he has had several other brands of aircraft that required extensive work on the elevator/trim to get the oscillation stopped.  One airplane trim/elevator required two reworks to get the “stickiness” fixed.  

[rbp]

I still have this problem in IAS and VNAV mode. I can also provoke porpoising in straight and level flight by giving the yoke a good push.

I had the relevant parts lubricated and the bridle retensioned. 

[PT20J]

25 minutes ago, rbp said:

I still have this problem in IAS and VNAV mode. I can also provoke porpoising in straight and level flight by giving the yoke a good push.

I had the relevant parts lubricated and the bridle retensioned. 

What's Garmin telling your dealer?

[rbp]

7 minutes ago, PT20J said:

What's Garmin telling your dealer?

still working through all this

[PeytonM]

Mine, too. 

[PT20J]

On 10/10/2022 at 1:28 PM, rbp said:

I still have this problem in IAS and VNAV mode. I can also provoke porpoising in straight and level flight by giving the yoke a good push.

I had the relevant parts lubricated and the bridle retensioned. 

VNAV and ALT have always been solid on my M20J. IAS is good in smooth air unless I dial in a change of over 5 kts -- then it chases the phugoid. 

The natural phugoid response to a pulse elevator input in the M20J dampens in about 2 cycles with a period of about 30 seconds.

Today, I put my GFC 500 in HDG and ALT and when stabilized applied abrupt elevator force for a 2.5 deg pitch down and released it. The autopilot corrected within 2 oscillations and was steady thereafter.

I repeated with a 2.5 deg pitch up with the same result.

Skip

[jetdriven]

On 9/26/2022 at 8:22 AM, Pinecone said:

I have read in other threads, that a slight tweak in the gain setting solves this issue.  But in certified aircraft, you cannot do this legally, as the STC specifies a certain gain setting.

The experimental guys can tweak this.

That’s the part I don’t understand, Why have gain settings at all,l?? just program the controller to have the gain setting value hardwired in, gain settings are meant to be tweaked for the specific airframe combinations. Seems like a copout on the manufacturers part. 

[PT20J]

Someone on BeechTalk posted a video of the GFC 500 moving the yoke back and forth about a quarter of an inch. This stopped when he pressed on the instrument panel near the GI 275. The ADAHRS, inside the G5 and GI 275, and attached to the back of the G3X, contain accelerometers and might be sensitive to vibration. It would be worth checking, especially if an installation retained the shock mounts on the pilot's panel.

Skip

 

 

[EricJ]

1 hour ago, PT20J said:

Someone on BeechTalk posted a video of the GFC 500 moving the yoke back and forth about a quarter of an inch. This stopped when he pressed on the instrument panel near the GI 275. The ADAHRS, inside the G5 and GI 275, and attached to the back of the G3X, contain accelerometers and might be sensitive to vibration. It would be worth checking, especially if an installation retained the shock mounts on the pilot's panel.

Skip

 

 

That's pretty dramatic.    It might mean that their internal filtering/conditioning of the AHRS signals isnt' very good.   Seen that before a few times.

[carusoam]

Wow!

Dampening provided by a simple finger press on the IP….

Wonder what the plane was doing with all of that yoke motion…?

Also wondering if it would increase AP induced oscillations….

Best regards,

-a-

[rbp]

33 minutes ago, carusoam said:

Also wondering if it would increase AP induced oscillations….

 

from what I have personally experienced, the oscillations only happen in IAS and VNAV modes, not VS

[Ragsf15e]

2 hours ago, carusoam said:

Wow!

Dampening provided by a simple finger press on the IP….

Wonder what the plane was doing with all of that yoke motion…?

Also wondering if it would increase AP induced oscillations….

Best regards,

-a-

You can zoom in on his gi275 ADI and see the airplane pitch oscillate a degree or two.  Doesn’t look like altitude is really changing but id bet it’s annoying and uncomfortable.

[PeytonM]

Hello everyone, As you are aware, I have been experiencing identical symptoms other MSers have reported here: slight pitch oscillations in ALT hold.  I have over 120 hours behind the avionics since installation was completed in late March. My yolk movement was nearly identical as the Beech-poster (I have similar vids), with about a 5-10 foot altitude change up and down, although my frequency seems less. You could see it on the AI and outside the airplane as the nose moves up & down ever so slightly. You could feel it, too.  

Since I last posted on this problem a few months ago, I have exchanged many, many emails with Trek Lawler at Garmin and phone calls with Matt at Mobile Avionics with several suggestions, to no avail.  Both gentlemen get “A-pluses” for their professional and timely responses and commitment to solving this problem. I could not ask for more. The most recent action from Garmin was to send 4 guys (an installer; a pilot and previous Mooney owner who flew certification flights; a customer service person and an installer/engineer) from Olathe to evaluate my airframe and installation. After 3 hours and a spinner to tail inspection: nothing of relevance found.

During their inspection,  I wondered out loud in a sidebar with their certification pilot that maybe a gain change would fix the issue. He kind of bristled at that as a solution, saying there are several inputs involved in the certification, so changing one could have unintended consequences. He also told me that certification flights were done in multiple Mooney models, full GW and at engineering limits of CG (which is beyond what is published in the POH), at altitude. He added that if there were design changes within the same model of Mooney, engineering assessments were done to determine whether more cert flights needed to be flown either side of the SN breaks. Bottom line: lots of certification flying in many (all?) models of Mooney aircraft in various flight configurations, resulting in model-specific config setting. 

After their visit, Garmin asked me to do a flight test: take the plane to a smooth altitude, put the A/P on ALT hold, then put the A/P in PITCH mode (push the ALT button) and see if the oscillations stopped.  They did…immediately!!  After I reported this to Garmin, they suggested installing a “static bulb,“ a small plastic reservoir for the static air. The hypothesis was that there might be slight “surges” in static air to which the A/P reacts, so this bulb might dampen these. After that change, I flew again, but there was no effect on the oscillations. Alas! (Btw, I had also tried alternate static air: no change.)

I’m headed to my avionics shop Thursday to try a few more ideas and fly flight tests.  Fortunately, it’s only a 10 minute ride!  Along the way, I will try pressing the panel near my G5 to see if there is any improvement (a miracle). It sure might be a clue for what may be happening. 
 

And all this was before the GFC500 Service Alert. Now what??

[PT20J]

2 hours ago, PeytonM said:

Hello everyone, As you are aware, I have been experiencing identical symptoms other MSers have reported here: slight pitch oscillations in ALT hold.  I have over 120 hours behind the avionics since installation was completed in late March. My yolk movement was nearly identical as the Beech-poster (I have similar vids), with about a 5-10 foot altitude change up and down, although my frequency seems less. You could see it on the AI and outside the airplane as the nose moves up & down ever so slightly. You could feel it, too.  

Since I last posted on this problem a few months ago, I have exchanged many, many emails with Trek Lawler at Garmin and phone calls with Matt at Mobile Avionics with several suggestions, to no avail.  Both gentlemen get “A-pluses” for their professional and timely responses and commitment to solving this problem. I could not ask for more. The most recent action from Garmin was to send 4 guys (an installer; a pilot and previous Mooney owner who flew certification flights; a customer service person and an installer/engineer) from Olathe to evaluate my airframe and installation. After 3 hours and a spinner to tail inspection: nothing of relevance found.

During their inspection,  I wondered out loud in a sidebar with their certification pilot that maybe a gain change would fix the issue. He kind of bristled at that as a solution, saying there are several inputs involved in the certification, so changing one could have unintended consequences. He also told me that certification flights were done in multiple Mooney models, full GW and at engineering limits of CG (which is beyond what is published in the POH), at altitude. He added that if there were design changes within the same model of Mooney, engineering assessments were done to determine whether more cert flights needed to be flown either side of the SN breaks. Bottom line: lots of certification flying in many (all?) models of Mooney aircraft in various flight configurations, resulting in model-specific config setting. 

After their visit, Garmin asked me to do a flight test: take the plane to a smooth altitude, put the A/P on ALT hold, then put the A/P in PITCH mode (push the ALT button) and see if the oscillations stopped.  They did…immediately!!  After I reported this to Garmin, they suggested installing a “static bulb,“ a small plastic reservoir for the static air. The hypothesis was that there might be slight “surges” in static air to which the A/P reacts, so this bulb might dampen these. After that change, I flew again, but there was no effect on the oscillations. Alas! (Btw, I had also tried alternate static air: no change.)

I’m headed to my avionics shop Thursday to try a few more ideas and fly flight tests.  Fortunately, it’s only a 10 minute ride!  Along the way, I will try pressing the panel near my G5 to see if there is any improvement (a miracle). It sure might be a clue for what may be happening. 
 

And all this was before the GFC500 Service Alert. Now what??

Interesting. Yours does it in ALT but not in PIT. It might be interesting to see what happens in other vertical modes (IAS, VS, VNAV). The op of the BeechTalk thread said his did it in both ALT and PIT.

[carusoam]

Static lines can create interesting oscillations…

Especially when they catch water in them…

or have a leak in their drain valve…

 

A few MSers have reported challenges with alt hold on other APs… where the rubber hose connecting their AP to the static system had dry rotted over the years….

Using the alt static system should add to the oscillations, as the pressure inside the cabin can change often depending on air being let in… and flight coordination…

PP thoughts only…

Best regards,

-a-