GFC500 problem

[201Mooniac]

16 hours ago, PT20J said:

I believe that it is only the trim torque that is set to 100% but I can check the next time I am out at the hangar, or perhaps someone else knows.

The pitch torque is set to 85% for the M20J

[PeytonM]

I’ve had a conversation with a MSC and an avionics shop in the Twin Cities. They have a couple long body Mooneys with the same symptoms: small but continual pitch changes that they could feel. They strongly believe a change in AP parameters will fix this. My installer is at his whits end.  I get it: he has an unhappy whom he cannot help.

Other than an email from Trek a couple weeks ago, I can tell there are no solutions forthcoming. Garmin wants to “100% beyond a shadow of a doubt make sure that it’s not an airframe related issue.”  

I am still planning on flying it to the Summit (over 6 hours of this), so if any if you are planning to be there, look me up and we can commiserate! 

Br, Peyton 

 

[PT20J]

Trek should be back in the office tomorrow. I would call him and have a conversation about it. It’s interesting that some Mooneys seem to do this while most do not. There must be something about the autopilot/airframe interface that is not completely understood.

Skip

[Marc Taylor]

I have a G5, G500txi, GTNxi (4) servos installed in my 252.  It started oscillating some time ago.  Several trips to the shop, sent the information off the G5 card to Garmin and still no cure.

I finally realized the ball was just a tick or two from centering up.  Tweaked the trailing edge of the tail, ball centered up and yesterday, no oscillation could be felt.  A couple of hunts when making abrupt altitude changes, but then centered up.

Occasionally I could see a movement in the G5 & G500txi altitude window (maybe 5 feet), but could not feel it in the yoke or the ride.  Probably turbulence.  I only flew a little over an hour, so the real test will be a long cross country, but I flew in both heading and Nav (GPS).  Also VNAV and RNAV approach.  If I had not seen this with my own eyes, I would not believe it would make that much difference.

Hopefully, this is the cure, but I will report back when I get a chance to complete a longer flight.

[Ragsf15e]

3 hours ago, Marc Taylor said:

I have a G5, G500txi, GTNxi (4) servos installed in my 252.  It started oscillating some time ago.  Several trips to the shop, sent the information off the G5 card to Garmin and still no cure.

I finally realized the ball was just a tick or two from centering up.  Tweaked the trailing edge of the tail, ball centered up and yesterday, no oscillation could be felt.  A couple of hunts when making abrupt altitude changes, but then centered up.

Occasionally I could see a movement in the G5 & G500txi altitude window (maybe 5 feet), but could not feel it in the yoke or the ride.  Probably turbulence.  I only flew a little over an hour, so the real test will be a long cross country, but I flew in both heading and Nav (GPS).  Also VNAV and RNAV approach.  If I had not seen this with my own eyes, I would not believe it would make that much difference.

Hopefully, this is the cure, but I will report back when I get a chance to complete a longer flight.

I’m curious about this as you have the yaw damper… was it not holding the ball centered in cruise?  Or maybe it was just working harder than it needed?

[PeytonM]

I was not able to look at the yolk and see movement; only when I jammed my index finger in between the yolk shaft and the instrument panel, could I feel it & see it.

I imagine this is a fleet-wide issue, but no one I know as polled all Mooney GFC500 installations. If data from those installs exist, it would be helpful. Many may have the issue, don’t realize it, or write it off as “just the way it is,” and accept it. All I can tell you is: 1) if I turn the AP off, the plan flies straight and true and if I turn the AP back on, I get the oscillations; 2) my old Century 2000 flew Alt Hold solid as a rock. 

It’s the autopilot system. 

[carusoam]

Marc has made an interesting observation above…

The comment is related to rudder trim…

LBs have powered rudder trim…

Do LBs have the same AP challenge?

Does it change with rudder trim applied?

Something to experiment with if the opportunity arises…

PP thoughts only…

Best regards,

-a-

 

[PeytonM]

Yes, LB do have the same challenge. Please see my post earlier today. 

[carusoam]

6 minutes ago, PeytonM said:

Yes, LB do have the same challenge. Please see my post earlier today. 

If able…?

See if adjusting the rudder trim changes  anything regarding the alt hold oscillation…

Kind of an odd request….
 

But, Marc T posted a solution to his challenge by altering the fixed rudder trim on his plane…

PP guessing at best… 

Best regards,

-a-

[rbp]

I have stopped using IAS climbs because of the oscillations. Now using onlly VS in climb.

over the weekend, we used VNAV mode and saw the following oscillations. Switched to VS descent and no oscillations. 

Tagging @TrekLawler

[EricJ]

2 hours ago, rbp said:

I have stopped using IAS climbs because of the oscillations. Now using onlly VS in climb.

over the weekend, we used VNAV mode and saw the following oscillations. Switched to VS descent and no oscillations. 

Tagging @TrekLawler

That kinda suggests that it's rate-based if it tracks the rate more stable than anything else.

Edit:  maybe.   Regardless, it appears to not be very well tuned for Mooney use.

 

[donkaye]

2 hours ago, rbp said:

I have stopped using IAS climbs because of the oscillations. Now using onlly VS in climb.

over the weekend, we used VNAV mode and saw the following oscillations. Switched to VS descent and no oscillations. 

Tagging @TrekLawler

Although I haven't plotted mine, I do not have the same issue.  Mine varies maybe +/- 1 knot in smooth air.  So, it's either an installation issue, servo issue, or "sticking" of the elevator due to lack of generously lubricating the yoke and elevator every few hours of flight time.

[PT20J]

11 hours ago, donkaye said:

Although I haven't plotted mine, I do not have the same issue.  Mine varies maybe +/- 1 knot in smooth air.  So, it's either an installation issue, servo issue, or "sticking" of the elevator due to lack of generously lubricating the yoke and elevator every few hours of flight time.

Don, I'm intrigued by the fact that many (including mine) seem to work fine but others have this pitch oscillation issue.

I seem to recall that you initially had an issue that was resolved. If my memory is correct, what was the issue and solution?

Thanks,

Skip

[donkaye]

2 hours ago, PT20J said:

Don, I'm intrigued by the fact that many (including mine) seem to work fine but others have this pitch oscillation issue.

I seem to recall that you initially had an issue that was resolved. If my memory is correct, what was the issue and solution?

Thanks,

Skip

The main issue with mine was that the installer didn't put in the bracing brackets for the trim servo.  That solved most of the issue.  I still had some oscillation on approaches.  That was resolved with the help of Paul Maxwell, who found the same issue resolved by simply lubing the yoke and elevator.  I now lube mine every few weeks.  I was told that Garmin needed to totally redo the flight control system on the Bravo they used to do the certification.  There  can be no resistance or sticking on the elevator.  Mine yoke "glides" now and the elevator moves effortlessly.

[rbp]

1 hour ago, donkaye said:

That was resolved with the help of Paul Maxwell, who found the same issue resolved by simply lubing the yoke and elevator.

I’m going to try this

[ArtVandelay]

The main issue with mine was that the installer didn't put in the bracing brackets for the trim servo.  That solved most of the issue.  I still had some oscillation on approaches.  That was resolved with the help of Paul Maxwell, who found the same issue resolved by simply lubing the yoke and elevator.  I now lube mine every few weeks.  I was told that Garmin needed to totally redo the flight control system on the Bravo they used to do the certification.  There  can be no resistance or sticking on the elevator.  Mine yoke "glides" now and the elevator moves effortlessly.

Do Bravos have the springs on the elevators like the J?

[PT20J]

5 hours ago, ArtVandelay said:

Do Bravos have the springs on the elevators like the J?

Not exactly. The long bodies have a bob weight and a down spring that adjusts tension with trim. That’s why they sit on the ramp with the elevator deflected full down.

Skip

[Marc Taylor]

Don

What lubricants are you using?  I have heard several different recommendations for the yoke and control hinges.

thank you

 

[PT20J]

Per the SMM, hinge ball bearings should be lubricated with light oil, rod ends with Tri-Flow, guide blocks with MIL-PRF-23827C type II (Aeroshell 7). Later manuals state that the black plastic (Nylatron) ball joints at the instrument panel  should be kept dry. Sometimes it helps to lube the control shaft with silicone spray. Often most of the friction is in the ball joints and the best solution if they are sticky is to replace them.

Skip

[Ragsf15e]

4 minutes ago, PT20J said:

Per the SMM, hinge ball bearings should be lubricated with light oil, rod ends with Tri-Flow, guide blocks with MIL-PRF-23827C type II (Aeroshell 7). Later manuals state that the black plastic (Nylatron) ball joints at the instrument panel  should be kept dry. Sometimes it helps to lube the control shaft with silicone spray. Often most of the friction is in the ball joints and the best solution if they are sticky is to replace them.

Skip

But just know that replacing that nylatron ball joint is major surgery… Skip got to replace his when the whole panel was apart anyway.

[PT20J]

7 minutes ago, Ragsf15e said:

But just know that replacing that nylatron ball joint is major surgery… Skip got to replace his when the whole panel was apart anyway.

It would be a royal pain with everything assembled, although there is a thread here somewhere describing how to do it. Before I had them replaced, I removed the retaining cup from the rear for better access and cleaned them as best I could with contact cleaner and then sprayed them with silicone spray which improved them. Still, when they were removed and compared to new, I was surprised at how much greater the friction was in the old ones. Even the new ones are not frictionless as they appear to be machined from solid material and the machining leaves very fine groves on the ball and socket surfaces. I suspect that these groves collect lubricant and dust which becomes embedded in the plastic over time increasing the friction. This may be why Mooney changed the recommendation to just leave them dry.

Skip

[PeytonM]

“I was told that Garmin needed to totally redo the flight control system on the Bravo they used to do the certification.”

Above from post on Wednesday. If true, we are all hosed. Why would you do this for the certification and then expect everything would go well with all Mooney installations??  Let’s see: long body, newer, and let’s rework the flight control system! What about short & medium body airplanes with different elevator balancing systems, not to mention 20, 30 years older?

[PT20J]

8 hours ago, PeytonM said:

“I was told that Garmin needed to totally redo the flight control system on the Bravo they used to do the certification.”

Above from post on Wednesday. If true, we are all hosed. Why would you do this for the certification and then expect everything would go well with all Mooney installations??  Let’s see: long body, newer, and let’s rework the flight control system! What about short & medium body airplanes with different elevator balancing systems, not to mention 20, 30 years older?

According to a Garmin post on BeechTalk, when they get a "project" airplane in for STC, the first thing they do is go through the airframe and correct any maintenance issues, especially friction and excessive play in the controls.

Since the vast majority of Mooney GFC 500 installations work just fine, an issue with a particular airframe must be a bad component, an installation issue, or an airframe issue. According to some BeechTalk threads, the most common installation issue is bridal cable tension. Some airframe issues were due to static system restrictions but the most common seemed to be excessive friction in the elevator controls. 

Mooney's are known to have elevator stiction at the eyeball bearing where the yoke goes through the panel. Mooney has had several designs of this bearing over different models, has changed lubrication recommendations on the latest design, and started chrome plating the shaft presumably to make it smoother. I think this indicates that there is a continuing issue that Mooney keeps trying to resolve.

If I had a porpoising issue, I would first check the bridal cable tension and then blow out the static lines and do a static leak check. If that didn't fix it, I would try disconnecting the pitch servo connection to the elevator push-pull tube and crawl in the tail and move the push-pull tube by hand so I could feel what the servo feels. If I felt an stiction, I would then investigate the cause.

Skip

[Ragsf15e]

35 minutes ago, PT20J said:

According to a Garmin post on BeechTalk, when they get a "project" airplane in for STC, the first thing they do is go through the airframe and correct any maintenance issues, especially friction and excessive play in the controls.

Since the vast majority of Mooney GFC 500 installations work just fine, an issue with a particular airframe must be a bad component, an installation issue, or an airframe issue. According to some BeechTalk threads, the most common installation issue is bridal cable tension. Some airframe issues were due to static system restrictions but the most common seemed to be excessive friction in the elevator controls. 

Mooney's are known to have elevator stiction at the eyeball bearing where the yoke goes through the panel. Mooney has had several designs of this bearing over different models, has changed lubrication recommendations on the latest design, and started chrome plating the shaft presumably to make it smoother. I think this indicates that there is a continuing issue that Mooney keeps trying to resolve.

If I had a porpoising issue, I would first check the bridal cable tension and then blow out the static lines and so a static leak check. If that didn't fix it, I would try disconnecting the pitch servo connection to the elevator push-pull tube and crawl in the tail and move the push-pull tube by hand so I could feel what the servo feels. If I felt an stiction, I would then investigate the cause.

Skip

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

[EricJ]

12 minutes ago, Ragsf15e said:

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

It's a contraction of "static friction".