Trim servo on GFC-500

[John Mininger]

A few months ago, I had a major avionics upgrade done on my J. Overall I am more than pleased with the install. One chronic issue that I’m having though is the pitch trim servo on the GFC-500 is not moving the trim up much past the take-off indication point before it just stops. The old autopilot that was removed was an S-Tec 55 and that pitch trim servo had no problem moving the trim to wherever I wanted it.

So, I’m not sure if there’s a problem with the GFC-500 servo itself, or there just too much friction in my trim system that the Garmin servo can’t handle it. It does seem tight with the hand wheel when it goes much past the take-off indication.

The tech who installed GFC-500 said that it does feel tight, but not as tight as some of the Saratogas that he’s worked on. He said the GFC-500 servo works OK on them. He suspects a bad servo.

Anyone else run into this problem?

John

[LANCECASPER]

8 minutes ago, John Mininger said:

A few months ago, I had a major avionics upgrade done on my J. Overall I am more than pleased with the install. One chronic issue that I’m having though is the pitch trim servo on the GFC-500 is not moving the trim up much past the take-off indication point before it just stops. The old autopilot that was removed was an S-Tec 55 and that pitch trim servo had no problem moving the trim to wherever I wanted it.

So, I’m not sure if there’s a problem with the GFC-500 servo itself, or there just too much friction in my trim system that the Garmin servo can’t handle it. It does seem tight with the hand wheel when it goes much past the take-off indication.

The tech who installed GFC-500 said that it does feel tight, but not as tight as some of the Saratogas that he’s worked on. He said the GFC-500 servo works OK on them. He suspects a bad servo.

Anyone else run into this problem?

John

You have an early 201 (1977) and I've heard that there are some variances throughout the J model that require modifications. Your installer should call Garmin and verify this.

[John Mininger]

1 minute ago, LANCECASPER said:

You have an early 201 (1977) and I've heard that they are some variances throughout the J model that require modifications. Your installer should cal;l Garmin and verify this.

Thanks. He was going to call Garmin. But I haven't heard back from him yet.

John

[PT20J]

The Mooney trim system is geared pretty low (takes a lot of turns of the trim wheel from full up to full down) and also has more friction than conventional trim systems since it has to move the entire tail with a jackscrew.

The GFC 500 has a spec of 24 seconds max. for the M20J for the trim run time stop to stop. In order to meet this spec, Garmin uses a very large sprocket on the servo and a much smaller sprocket on the trim tube to “gear up” the servo and increase its speed. But, this reduces torque available. Even with the torque setting at 100% as specified in the STC, there isn’t much reserve. You can tell this by placing your hand on the trim wheel when operating the electric trim on the ground and noticing how little pressure you have to apply to stop the trim motion.

So, after checking that the installation is correct (servo chain tension and alignment, servo setup parameters) the thing to do is to go through the trim system and find and remove sources of excess friction. 

[John Mininger]

12 minutes ago, PT20J said:

The Mooney trim system is geared pretty low (takes a lot of turns of the trim wheel from full up to full down) and also has more friction than conventional trim systems since it has to move the entire tail with a jackscrew.

The GFC 500 has a spec of 24 seconds max. for the M20J for the trim run time stop to stop. In order to meet this spec, Garmin uses a very large sprocket on the servo and a much smaller sprocket on the trim tube to “gear up” the servo and increase its speed. But, this reduces torque available. Even with the torque setting at 100% as specified in the STC, there isn’t much reserve. You can tell this by placing your hand on the trim wheel when operating the electric trim on the ground and noticing how little pressure you have to apply to stop the trim motion.

So, after checking that the installation is correct (servo chain tension and alignment, servo setup parameters) the thing to do is to go through the trim system and find and remove sources of excess friction. 

Thanks Skip.

Do you know if you release the trim switch and then trim-up again, does the 24 second timer re-set for another 24 seconds? Because when I do that, the servo still has trouble moving the trim up on the second try.

John

[cbarry]

The way you describe the situation makes me think the trim may be out of rig.  Do you have full trim range or do you have more trim down than up, for example?  After I had the GFC500 installed in my Eagle, the trim was out of rig simply due the installer not getting the trim rigged properly. It is my understanding that the trim jack screw should be ran fully to a stop (either up or down) prior to removal to avoid guesswork when installing the trim servo and trim control rod.  I ended up flying to and getting this solved by the fine work done by Don and Paul Maxwell—now it’s correct. 

[PT20J]

1 hour ago, John Mininger said:

Thanks Skip.

Do you know if you release the trim switch and then trim-up again, does the 24 second timer re-set for another 24 seconds? Because when I do that, the servo still has trouble moving the trim up on the second try.

John

No, the time range (actually 16-24 seconds) is an installation validation measurement to be sure that the manual electric trim is operating properly. There isn't a timer -- you just manually run the trim to a stop and then actuate the electric trim and time how long it takes to run to the opposite stop and then repeat in the other direction.

On the ground, it takes more force to trim nose up because you are lifting the tail with the jackscrew.

[PT20J]

1 hour ago, cbarry said:

The way you describe the situation makes me think the trim may out of rig.  Do you have full trim range or do you have more trim down than up, for example?  After I had the GFC500 installed in my Eagle, the trim was out of rig simply due the installer not getting the trim rigged properly. It is my understanding that the trim jack screw should be run fully to a stop (either up or down) prior to removal to avoid guesswork when installing the trim servo and trim control rod.  I ended up getting this solved by the fine work done by Don and Paul Maxwell—now it’s correct. 

This is a common problem with installers not familiar with Mooneys. It is easy to check on a J: just put the trim in the take off position on the trim indicator and the trim assist bungees should cause the elevator to align with the stabilizer. This check doesn't work on later models that have a down spring and bob weight because the elevator always hangs down when the airplane is parked.

[Ragsf15e]

1 hour ago, PT20J said:

This is a common problem with installers not familiar with Mooneys. It is easy to check on a J: just put the trim in the take off position on the trim indicator and the trim assist bungees should cause the elevator to align with the stabilizer. This check doesn't work on later models that have a down spring and bob weight because the elevator always hangs down when the airplane is parked.

I don’t mean to take a big detour from the op, but I’ve been thinking of a question about the gfc500 and trim… with an older model that didn’t come with electric trim, is it a requirement to add the electric trim(switch, wiring, etc) to have the trim function or can you have the trim servo installed but only manual trim for the pilot?  

[PT20J]

12 minutes ago, Ragsf15e said:

I don’t mean to take a big detour from the op, but I’ve been thinking of a question about the gfc500 and trim… with an older model that didn’t come with electric trim, is it a requirement to add the electric trim(switch, wiring, etc) to have the trim function or can you have the trim servo installed but only manual trim for the pilot?  

Trim is an option, but if you install it, I don't believe that the manual electric trim switch is optional. And you have to put the AP disconnect/trim interrupt switch on the yoke anyway. Garmin's solution is to mount a bracket on the yoke to hold the switches. They list a small toggle switch option for the trim switch for this mounting method. I know a local flight school that did that on two of their C-172s which also have small yokes with no room for switches.

[Ragsf15e]

3 minutes ago, PT20J said:

Trim is an option, but if you install it, I don't believe that the manual electric trim switch is optional. And you have to put the AP disconnect/trim interrupt switch on the yoke anyway. Garmin's solution is to mount a bracket on the yoke to hold the switches. They list a small toggle switch option for the trim switch for this mounting method. I know a local flight school that did that on two of their C-172s which also have small yokes with no room for switches.

Thanks. The bracket makes sense too as that’s how the two stec switches are mounted on my setup now.

[dzeleski]

14 minutes ago, Ragsf15e said:

I don’t mean to take a big detour from the op, but I’ve been thinking of a question about the gfc500 and trim… with an older model that didn’t come with electric trim, is it a requirement to add the electric trim(switch, wiring, etc) to have the trim function or can you have the trim servo installed but only manual trim for the pilot?  

The gfc manual says it’s optional and not required, but I’m not sure why you wouldn’t install it. The trim switches do a few extra things for safety though. Hitting the trim switch with the AP on kicks the AP off as well as stopping a commanded trim runaway. You always have the CB as well but having the switch there is nice.

[PT20J]

7 minutes ago, dzeleski said:

The gfc manual says it’s optional and not required, but I’m not sure why you wouldn’t install it. The trim switches do a few extra things for safety though. Hitting the trim switch with the AP on kicks the AP off as well as stopping a commanded trim runaway. You always have the CB as well but having the switch there is nice.

Thanks. I couldn't find that in the  GFC STC documentation. This appears to be from the G5 Pilot's Guide.

[dzeleski]

5 hours ago, John Mininger said:

A few months ago, I had a major avionics upgrade done on my J. Overall I am more than pleased with the install. One chronic issue that I’m having though is the pitch trim servo on the GFC-500 is not moving the trim up much past the take-off indication point before it just stops. The old autopilot that was removed was an S-Tec 55 and that pitch trim servo had no problem moving the trim to wherever I wanted it.

So, I’m not sure if there’s a problem with the GFC-500 servo itself, or there just too much friction in my trim system that the Garmin servo can’t handle it. It does seem tight with the hand wheel when it goes much past the take-off indication.

The tech who installed GFC-500 said that it does feel tight, but not as tight as some of the Saratogas that he’s worked on. He said the GFC-500 servo works OK on them. He suspects a bad servo.

Anyone else run into this problem?

John

Yep similar issue, the chances of the airplane passing the ground test in the up direction are slim even on a well rigged and maintained trim system. That being said you still have a problem here, it should make it stop to stop without issues and stopping. It might just be on the slower side.

We had to completely pull apart my trim system cleaned, re lubed, and re rig everything in the tail to get it as close as we could. Garmin was called and they advised on my airframe slightly slower on the ground is ok. With wind loading in the air it’s completely to spec.

After all this work my manual trim has never been easier and the ground test just barely fails in the up direction. If I recall correctly it took <30 seconds up rather then the 24 that is written.

My serial is 24-0095, 77 J.

Edited March 16 by dzeleski

[John Mininger]

39 minutes ago, dzeleski said:

Yep similar issue, the chances of the airplane passing the ground test in the up direction are slim even on a well rigged and maintained trim system. That being said you still have a problem here, it should make it stop to stop without issues and stopping. It might just be on the slower side.

We had to completely pull apart my trim system cleaned, re lubed, and re rig everything in the tail to get it as close as we could. Garmin was called and they advised on my airframe slightly slower on the ground is ok. With wind loading in the air it’s completely to spec.

After all this work my manual trim has never been easier and the ground test just barely fails in the up direction. If I recall correctly it took <30 seconds up rather then the 24 that is written.

My serial is 24-0095, 77 J.

Interesting. I have to admit, I've never tried going stop to stop on the ground. I noticed the problem when triming up on short final. And it doesn't just slow down, it stops. My serial is 24-0278, 77 J.

I'm going to PM you.

Thanks

John

[dzeleski]

9 minutes ago, John Mininger said:

Interesting. I have to admit, I've never tried going stop to stop on the ground. I noticed the problem when triming up on short final. And it doesn't just slow down, it stops. My serial is 24-0278, 77 J.

I'm going to PM you.

Thanks

John

That’s very much not good. You need this looked at ASAP. It could also be a servo issue there was a recall but if you just got it installed you probably aren’t affected. Still possible that you got a bad or weak servo.

If your shop hasn’t done a lot of mooneys I would ask them to reach out to a shop that has to get some advice. Also tell them to call garmin.

What you can do is inspect the grease on the jack screw. Mine was absolutely thrashed and was 50% of the problem.

Edited March 16 by dzeleski

[John Mininger]

8 minutes ago, dzeleski said:

That’s very much not good. You need this looked at ASAP. It could also be a servo issue there was a recall but if you just got it installed you probably aren’t affected. Still possible that you got a bad or weak servo.

If your shop hasn’t done a lot of mooneys I would ask them to reach out to a shop that has to get some advice. Also tell them to call garmin.

What you can do is inspect the grease on the jack screw. Mine was absolutely thrashed and was 50% of the problem.

What was the other 50%? We did spray some avgas on the threads of the jackscrew as best we could to clean it. We then put some grease back on it. That didn't seem to help much.

John

[PT20J]

Do the easy things first:

1. Clean and lube the jackscrew in the tail

2. Clean and lube all the u-joints in the trim shaft

3. Clean and lube the screw at the forward gear box. (This is a good time to check that SBM20-88A has been complied with to install non-jamming stop nuts)

4. Check that the trim wheel chain is not too tight and lube the shaft for the trim wheel.

This will free things up in most cases. Worst case, you might need to remove the front gear box and the aft jackscrew assembly and clean and lube.

Some models had a trim friction adjustment on the trim wheel. I do not believe that any M20Js had this, but it might be worth checking.

[dzeleski]

30 minutes ago, PT20J said:

Do the easy things first:

1. Clean and lube the jackscrew in the tail

2. Clean and lube all the u-joints in the trim shaft

3. Clean and lube the screw at the forward gear box. (This is a good time to check that SBM20-88A has been complied with to install non-jamming stop nuts)

4. Check that the trim wheel chain is not too tight and lube the shaft for the trim wheel.

This will free things up in most cases. Worst case, you might need to remove the front gear box and the aft jackscrew assembly and clean and lube.

Some models had a trim friction adjustment on the trim wheel. I do not believe that any M20Js had this, but it might be worth checking.

Exactly this. Only thing I’ll add here is use the correct lube in the correct spot as per the service manual.

I had nothing broken or “wrong” just a pure lack of maintenance on the trim system.

[Ragsf15e]

2 minutes ago, dzeleski said:

Exactly this. Only thing I’ll add here is use the correct lube in the correct spot as per the service manual.

I had nothing broken or “wrong” just a pure lack of maintenance on the trim system.

That’s pretty common.  The manual trim works pretty well for a good long time after it’s been cleaned/lubed and the trim system doesn’t really show up as an issue until it’s really hard to trim.  Some places might skip the yearly maintenance that could keep it running easier.

[Pinecone]

On 3/16/2024 at 3:03 PM, PT20J said:

Trim is an option, but if you install it, I don't believe that the manual electric trim switch is optional. And you have to put the AP disconnect/trim interrupt switch on the yoke anyway. Garmin's solution is to mount a bracket on the yoke to hold the switches. They list a small toggle switch option for the trim switch for this mounting method. I know a local flight school that did that on two of their C-172s which also have small yokes with no room for switches.

My FBO has a 172 with Garmin setup (G3X, GTN-650Xi, G-5, GTX-345, GFC-500) with that setup.

Glad it is not my plane.  Seems cobbled together.  But it works.  Trim switch is a reach with my thumb.