Trim stuck on approach, works after landing

[StevieDee]

I had a stuck trim issue today and I can’t figure out what cased it.  I have an M20V with G1000NXi. Everything was working fine with the autopilot.  On the LPV approach I tried to capture both VNV and GP.  It did not want to capture.  When I switched the autopilot off on the G1000, the electronic trim and manual trim were stuck in the full forward position, so I had to pull back on the yoke.  I tried to power cycle the autopilot on the toggle switch.  Trim still stuck.  I did not cycle the flight director and did not pull the autopilot circuit breaker.  Used about 20lbs of force in the flare and needless to say the landing was rough.  After the landing, the trim was back to working fine.  Clearly the servo got stuck.  What could have caused it in the first place?  And what caused it to become unstuck? What is the fix?

[PT20J]

It sounds like a trim runaway. In case it repeats, you might want to review the emergency procedures in the manual. Generally, the drill is to press and hold the autopilot/trim disconnect button on the yoke (which will remove power from the trim servo. Even if the servo is somehow mechanically jammed, you will be able to override the clutch manually), then trim manually, and finally pull the autopilot circuit breaker before releasing the button on the yoke. 

After the incident, does the GFC 700 pass preflight test?

Skip

[StevieDee]

46 minutes ago, PT20J said:

It sounds like a trim runaway. In case it repeats, you might want to review the emergency procedures in the manual. Generally, the drill is to press and hold the autopilot/trim disconnect button on the yoke (which will remove power from the trim servo. Even if the servo is somehow mechanically jammed, you will be able to override the clutch manually), then trim manually, and finally pull the autopilot circuit breaker before releasing the button on the yoke. 

After the incident, does the GFC 700 pass preflight test?

Skip

Thank Skip.  I looked at those emergency procedures in the manual and I’ll be more prepared next time.  I’ll do a preflight of the GFC 700 next time I fly.  There were no messages before I shut down and the landing could have shaken the jam loose.

Edited August 5, 2023 by Stephen De Klerk

[toto]

For whatever it's worth, my one experience with stuck trim was a mechanical issue - the trim jackscrew was frozen in a cruise setting.  My first indication of a problem was the AP disconnecting (presumably because the trim servo exceeded the force limit).  Neither electric trim nor manual trim worked, and my landing required a lot of back pressure on the yoke.  In my case, the issue did not resolve after landing, but the away-from-home shop was familiar with Mooneys and had it resolved in about an hour of shop time by regreasing the jackscrew.

[StevieDee]

6 minutes ago, toto said:

For whatever it's worth, my one experience with stuck trim was a mechanical issue - the trim jackscrew was frozen in a cruise setting.  My first indication of a problem was the AP disconnecting (presumably because the trim servo exceeded the force limit).  Neither electric trim nor manual trim worked, and my landing required a lot of back pressure on the yoke.  In my case, the issue did not resolve after landing, but the away-from-home shop was familiar with Mooneys and had it resolved in about an hour of shop time by regreasing the jackscrew.

I am going to ask my mechanic about this next week.  I didn’t grease the landing but the landing may have degreased the jackscrew!

Edited August 5, 2023 by Stephen De Klerk

[N201MKTurbo]

When the trim is taken to its limit, it can get stuck by jamming the trim indicator screw against its stop. My electric trim will jam it if I run it to the limit. I can always get it loose, but it takes a considerable force on the trim wheel.

It might be worth it to run the electric trim to the stops and see if the servo can get it loose and if not, how much force it takes to get it loose.

[PT20J]

42 minutes ago, toto said:

For whatever it's worth, my one experience with stuck trim was a mechanical issue - the trim jackscrew was frozen in a cruise setting.  My first indication of a problem was the AP disconnecting (presumably because the trim servo exceeded the force limit).  Neither electric trim nor manual trim worked, and my landing required a lot of back pressure on the yoke.  In my case, the issue did not resolve after landing, but the away-from-home shop was familiar with Mooneys and had it resolved in about an hour of shop time by regreasing the jackscrew.

The jackscrew is supposed to be lubricated and inspected at annual, but it often isn't. Some of the autopilots (the Bendix-Kings for sure) have trim servos with a lot of torque. If the trim gets stiff, and you always use the autopilot or manual electric trim, you won't notice a problem until it gets so bad that the servo clutch slips. I always reset the trim to the takeoff position by hand after landing using the trim wheel so that I can feel if the trim system is getting stiff.

[PT20J]

15 minutes ago, N201MKTurbo said:

When the trim is taken to its limit, it can get stuck by jamming the trim indicator screw against its stop. My electric trim will jam it if I run it to the limit. I can always get it loose, but it takes a considerable force on the trim wheel.

It might be worth it to run the electric trim to the stops and see if the servo can get it loose and if not, how much force it takes to get it loose.

Certainly true for older airplanes that didn't have the stepped stop nuts, but a M20V should be nonjamming.

[N201MKTurbo]

1 minute ago, PT20J said:

Certainly true for older airplanes that didn't have the stepped stop nuts, but a M20V should be nonjamming.

You would think so, but the evidence says otherwise.

[kortopates]

I would agree this has to be a mechanical jam in the trim. Not electrical or AP servo issue. I too have never seen a jam since installing the newer SB jam nuts which should be standard on the Ultra. But like Rich said, maybe not.


Sent from my iPhone using Tapatalk

[PT20J]

13 hours ago, kortopates said:

I would agree this has to be a mechanical jam in the trim. Not electrical or AP servo issue. I too have never seen a jam since installing the newer SB jam nuts which should be standard on the Ultra. But like Rich said, maybe not.


Sent from my iPhone using Tapatalk

If it was mechanically jammed, I don't think it was due to the stops. When I had the forward trim gearbox out of my airplane, I spent some time observing how the stepped stops work and I just can't see anyway they can jam.

But, in the OP's case, his trim ended up full nose down. There is no way that it would get there if the autopilot didn't drive it there, and if it was a trim runaway, the servo might have been holding it against the stop. I'm not sure why it would clear after landing, though. 

[kortopates]

22 minutes ago, PT20J said:

If it was mechanically jammed, I don't think it was due to the stops. When I had the forward trim gearbox out of my airplane, I spend some time observing how the stepped stops work and I just can't see anyway they can jam.

But, in the OP's case, his trim ended up full nose down. There is no way that it would get there if the autopilot didn't drive it there, and if it was a trim runaway, the servo might have been holding it against the stop. I'm not sure why it would clear after landing, though. 

You're right too much we don't know. But I was betting electric trim commanded down. But any example I come up with the AP commanding it down, with the pilot pulling back should cause AP to disconnect. Really too little info to go on.

[N201MKTurbo]

If it was a trim runaway, the clutch on the trim servo should have allowed the trim to be adjusted even with the servo running full nose down, of course the trim would run back as soon as you let go of the wheel. This assumes the trim disconnect switch wasn’t working. It would be very rare to have failure where the trim disconnect didn’t work.

[Fly Boomer]

12 hours ago, Stephen De Klerk said:

I am going to ask my mechanic about this next week.  I didn’t grease the landing but the landing may have degreased the jackscrew!

Great answers above, but I’ll just add that, according to DMAX, if you have TKS, it washes lubricant from everything in the tail, and things need to be lubricated more often than once per year.

[Schllc]

Are you sure it was stuck?  Did you try to manually trim?  You said you turned the autopilot off and then back on.  In the g1000 and NXI, models, the electric elevator trim only works with the autopilot on, and when you cycle the power it easily takes a minute or two, to start working again. 
this doesn’t explain why it didn’t capture the glide slope, but is almost certainly why the trim switch stopped working during your approach. 
 

[StevieDee]

2 hours ago, Fly Boomer said:

Great answers above, but I’ll just add that, according to DMAX, if you have TKS, it washes lubricant from everything in the tail, and things need to be lubricated more often than once per year.

No TKS on this plane.

[StevieDee]

2 hours ago, Schllc said:

Are you sure it was stuck?  Did you try to manually trim?  You said you turned the autopilot off and then back on.  In the g1000 and NXI, models, the electric elevator trim only works with the autopilot on, and when you cycle the power it easily takes a minute or two, to start working again. 
this doesn’t explain why it didn’t capture the glide slope, but is almost certainly why the trim switch stopped working during your approach. 
 

The manual trim was stuck too.

[Schllc]

24 minutes ago, Stephen De Klerk said:

The manual trim was stuck too.

Not to nitpick, but when you say manual do you mean manually using the trim switch or actually reaching down and adjusting the trim wheel on the floor?

I didn’t even know a servo could freeze like that, my understanding is the clutch is supposed to release with “x”#’s of pressure. 
I’d be very curious to understand how this can happen. 

[StevieDee]

1 hour ago, Schllc said:

Not to nitpick, but when you say manual do you mean manually using the trim switch or actually reaching down and adjusting the trim wheel on the floor?

I didn’t even know a servo could freeze like that, my understanding is the clutch is supposed to release with “x”#’s of pressure. 
I’d be very curious to understand how this can happen. 

The trim wheel on the floor refused to move in either direction.

[PT20J]

1 hour ago, Stephen De Klerk said:

The trim wheel on the floor refused to move in either direction.

So, there are two mysteries here:

1. How did the trim get full down? Normally during slowing and configuring for an approach the autopilot should be trimming nose up.

2. What was preventing the trim wheel from moving?

It is well known that some autopilot trim servos have enough torque to cause the trim system to jam at the stops if the system has the old style jam nuts rather than the newer step stops. So, one scenario that might explain this is that there was a trim runaway that caused the trim to run down to the stop and it jammed. On a long body, the elevator system has a bob weight and down spring and elevator goes full down when there is no air load. Perhaps this relieved some tension in the trim system after landing and allowed it to free up. It's impossible to tell without investigation, but if this were my airplane, I would check that the trim stops are stepped and verify that they cannot jam, and I would inspect the entire trim system for any wear and make certain it is properly lubricated. And, I would be wary of the autopilot and watch for any unexpected trim wheel movement.

Skip

[StevieDee]

3 hours ago, PT20J said:

So, there are two mysteries here:

1. How did the trim get full down? Normally during slowing and configuring for an approach the autopilot should be trimming nose up.

2. What was preventing the trim wheel from moving?

It is well known that some autopilot trim servos have enough torque to cause the trim system to jam at the stops if the system has the old style jam nuts rather than the newer step stops. So, one scenario that might explain this is that there was a trim runaway that caused the trim to run down to the stop and it jammed. On a long body, the elevator system has a bob weight and down spring and elevator goes full down when there is no air load. Perhaps this relieved some tension in the trim system after landing and allowed it to free up. It's impossible to tell without investigation, but if this were my airplane, I would check that the trim stops are stepped and verify that they cannot jam, and I would inspect the entire trim system for any wear and make certain it is properly lubricated. And, I would be wary of the autopilot and watch for any unexpected trim wheel movement.

Skip

In addition to the control/runaway possibilities and the mechanical/jam possibilities that have been mentioned, I also don’t want to discount the possibility of an electrical issue with the wires or the servo motor.  The plane had been standing for seven months in Arizona and there were a few electrical issues that worked themselves out when it started to fly again.  Thank you everyone for your thoughtful responses.

Edited August 6, 2023 by Stephen De Klerk