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GFC 500 pitch oscillations by Mooney model


PT20J
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GFC 500 pitch oscillations by Mooney model  

12 members have voted

  1. 1. I have experienced autopilot pitch oscillations in ALT or PIT vertical mode, and my model is:

    • M20J
      7
    • M20K
      3
    • M20M
      1
    • M20R
      0
    • M20S
      1


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Mooney changed the elevator control system design from trim assist bungees (M20J and earlier) to a variable downspring and bob weight (M20K and subsequent). Some people have reported that the autopilot oscillates in pitch in ALT and PIT vertical modes (let's leave other more complicated modes out for now - maybe a later poll). I'm curious if these occurrences are more prevalent with different models having different trim systems.

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Just had the 500 installed. I don't have the the large oscillations that I've seen in videos'. However, I do have very minor ones that seem rather abrupt. We believe it is either inadequate lubrication of the control shaft or the tension of the control cables is to tight/loose. 

My guess is the data will show this might be an issues across all models in your poll. I'd be careful of sample bias too. 

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I sometimes have very minor, almost imperceptible pitch oscillations in cruise in my M20J.  I have all four servos, including YD.  You can only tell by looking at the tape on the G5.  Anecdotally I think they're linked to lubrication--they only seem to appear and get worse the longer it is after the controls received a good lubing.  Only after it's been a while can you feel them, otherwise most people probably wouldn't know.

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10 hours ago, ZuluZulu said:

I sometimes have very minor, almost imperceptible pitch oscillations in cruise in my M20J.  I have all four servos, including YD.  You can only tell by looking at the tape on the G5.  Anecdotally I think they're linked to lubrication--they only seem to appear and get worse the longer it is after the controls received a good lubing.  Only after it's been a while can you feel them, otherwise most people probably wouldn't know.

My Century 41 when it works does that, and the S-Tec I had in a C-210 did so also. I think it might be normal, seems that you have to be off altitude by just a little then the AP corrects, then you get off by just a little and another correction. Maybe if they didn’t have this little dead band they would be constantly correcting?

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30 minutes ago, Greg Ellis said:

Are these pitch oscillations isolated to the models in this poll?  I have a 1963 C model with the GFC500 and I have no pitch oscillations at all but wonder if I am lucky or is that the norm with the older models.

Well, I was just curious how many instances there actually are. So far, none. ;) I suspect it’s quite rare which means it is unlikely a design issue. That narrows it down to an airframe issue, an installation issue, or a component issue.

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35 minutes ago, Greg Ellis said:

Are these pitch oscillations isolated to the models in this poll?  I have a 1963 C model with the GFC500 and I have no pitch oscillations at all but wonder if I am lucky or is that the norm with the older models.

I wonder if it's worse in heavier airplanes.  I have read that those servos are pretty wimpy.

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3 minutes ago, PT20J said:

Well, I was just curious how many instances there actually are. So far, none. ;) I suspect it’s quite rare which means it is unlikely a design issue. That narrows it down to an airframe issue, an installation issue, or a component issue.

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Seems like I have read quite a few posts complaining of this.  Didn't you have it early on?  If so, how did you get rid of it?  Or how do you keep it at bay?  Do you lubricate your yoke shafts frequently?

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18 minutes ago, Fly Boomer said:

Seems like I have read quite a few posts complaining of this.  Didn't you have it early on?  If so, how did you get rid of it?  Or how do you keep it at bay?  Do you lubricate your yoke shafts frequently?

No, I never had an issue in ALT or PIT. IAS can hunt a bit if the airspeed is increasing or decreasing at engagement, or if the thumbwheel is rapidly moved to a setting greater than +/- 5 KIAS after engagement, but it will generally settle out after 2 cycles which is the natural long-term longitudinal response of the airplane.

I replaced both yoke eyeballs when I re-did the panel. I was amazed at how much greater friction the old ones had. I do keep all the heim bearings and trim components lubricated and occasionally apply some silicone spray to the control shafts which are chrome plated and smooth. I have followed Mooney’s current recommendation and have not lubricated the new nylatron eyeballs.

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I think the small long period pitch oscillations are a Phugoid, which most GA airplanes have. Hand flying you never notice it most likely as you dampen it out, but if you ever get into real smooth air and turn loose of everything you’ll notice she will begin a slow climb or descent and you will be adding small trim changes, you just can’t quite get perfect trim. That’s the Phugoid.

https://en.wikipedia.org/wiki/Phugoid

‘I suspect however you’re asking about the recent FAA email? Isn’t that only on engagement, not any other time?

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I occasionally experience very minor pitch oscillation. I noticed it some on a cross-country trip last week while eastbound over the Mojave desert in the morning. I don't know if it was the autopilot of the environment, though it seemed to lessen when I turned northeast. I'm just out of annual and lubricated everything except the yoke control shaft. I have roll and pitch servos, no auto trim (luckily, this week...) or yaw damping. I did disconnect the autopilot and seemed to have less oscillation, so I don't think I was out of trim. (Incidentally, the up/down arrow manual trim indicators are tight enough that if I follow them, when I disconnect, I'm pretty well trimmed.) I guess the upshot is that I get enough oscillaiton to notice, but not enough to think about it too much. 

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Comparing older systems to newer systems….

The computing power of the newer system, and its speed for calculations….

The GFC when working correctly is amazing….

20years ago… we would be lucky to get data points to work with every second….

Then the slow computers took time to calculate…

Yet… the old stuff worked pretty well…

 

It shouldn’t be that hard to figure out what isn’t working to expectations….

Big G has a pretty large technical group with incredible knowledge, and skill…

Big G also has competitors that are hoping to find out when and where Big G drops the ball….

 

I hope this doesn’t end up as a B-school case of basic capitalism at work…   :)

 

If going to the next larger servo drive solves the issue… is it only the one servo? Or do all of them need to be replaced?

 

PP thinking out loud only, I don’t have a dog in this fight…

Best regards,

-a-

 

 

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according to G, here are the (non exhaustive) possibilities :

  1. lubrication
  2. tension/instattion of the AP "bridles"
  3. static source anomalies (which make it seem like the plane is climbing/descending, and the AP overcorrects)
  4. vibration in the panel (see the GFC500 thread for a video copied from Beechtalk)

 

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This performance is not normal and totally unacceptable to me.

As I reported in another MS post, four guys from Garmin flew over and spent almost 3 hours reviewing my airplane and the avionics installation and found nothing relevant. (Btw, it is not vibration; my panel is not shock-mounted.)

When changing from ALT to Pitch, the oscillations stop immediately and the ride is as smooth as glass. 

Regarding the Poll: ten reports in the Poll could be significant since only a fraction of Mooney GFC500 owners are MSers and post. I too would like to know how many installations do NOT suffer from oscillations, maybe even by model. Also, I don’t see any reference in the Poll to the short bodies. Bottom line: the data are in Olathe. 

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@PeytonM This is what Trek wrote me the other day: "As for the issues on Mooneyspace, we're working on a number of these and even went and looked at Peyton's plane, we know for sure in his case the problem is static data related, and the installer is looking at doing some different things with the static lines and possibly another static port on a different location of the plane." [I added a missing comma for clarity]

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At the risk of a minor threadjack, I will point out that Garmin determined early in the GFC500’s certified life that they had produced a bunch of bad servos and they had a quiet recall of the affected units. We had a GFC500 installed in the Piper right after the STC became available, and the AP started the pitch oscillations within a few months. Garmin replaced all of the servos at their cost, and the problem has not returned.

I have to wonder whether some of the affected aircraft have the “bad” servos installed?

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1 hour ago, toto said:

I have to wonder whether some of the affected aircraft have the “bad” servos installed?

my plane was in the shop when the servos were recalled, and I got a new set of them before they were ever installed. but I do have pitch oscillation

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I hope all you people with this pitch oscillation problem can keep the heat on Garmin long enough to get it fixed.  From this distance, I can't tell whether they don't know how to fix it, or it would be more expense than they can stomach.  Either way, it's a pitiful situation with the king of the hill in avionics (no pun).

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As someone who has been in the product business, I'd like to give them the benefit of the doubt. They need to keep the affected customers satisfied while they figure out if there is a bigger issue, and show optimism to the marketplace. 

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