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Runaway Trim STEC


Txbyker

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I feel the need to ask about STEC 55X/G-1000 runaway trim.  I have had a few episodes with this over the years and am beginning to think there is an inherit problem with this configuration.  Yesterday, when testing a new GTX345 install with WAAS coupled approach, my trim went full down when I coupled the approach fix.  I disconnected the A/P at the yoke but no longer had pitch trim and had to manually trim to land.  I had the avionics tech on board this test flight so he is working directly with STEC after witnessing this.

In the past I have climbed out of a busy class B airport area to an assigned altitude.  I set the altitude to 2000, press VS at 700 fpm then Alt and VS to capture the new altitude target.  The plane then trims full down like it is trying to capture a lower altitude.  Fighting the yoke or trim switch on that occasion had no effect.  Embarrassing as ATC ask the Mooney to climb a little faster. Only flipping the A/P and Trim rocker switch off helped.  (G-1000 have no trim fuses on the panel btw).

A couple of months ago I was climbing out of my home airport and set the altitude target, pressed VS and Alt and it trimmed full up.  Turning off the A/P had no affect at the yoke or the main switches.  I forced the trim wheel down as I lost airspeed.  I had the pitch servo rebuilt by Genesis on that one.

Now yesterday.  Is anyone else with STEC 55X seeing trim issues?

Russ

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Russ, my recent issue was probably not the same but sounds vaguely familiar.  I have an sTec 60-2.  Recently had my pitch servo rebuilt by sTec aka Genesis.  They wired it backwards during the rebuild.  My shop installed the rebuilt servo and confirmed it worked but did not confirm it worked in the correct direction.  You know how exciting it can get when your pitch servo runs in the wrong direction?

Genesis had this excuse:  the same servo is used in many aircraft, but needs to be wired (by them) in reverse if it goes in a Mooney.  The knew it was for a Mooney but failed to wire it accordingly.

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33 minutes ago, neilpilot said:

Russ, my recent issue was probably not the same but sounds vaguely familiar.  I have an sTec 60-2.  Recently had my pitch servo rebuilt by sTec aka Genesis.  They wired it backwards during the rebuild.  My shop installed the rebuilt servo and confirmed it worked but did not confirm it worked in the correct direction.  You know how exciting it can get when your pitch servo runs in the wrong direction?

Genesis had this excuse:  the same servo is used in many aircraft, but needs to be wired (by them) in reverse if it goes in a Mooney.  The knew it was for a Mooney but failed to wire it accordingly.

Thanks.  I read this your thread about that.  Wow!  They are sending my pitch servo back to Genesys for determination and I asked them if it is wired backwards.  I am sure I sound like a meddling customer asking that question but heck we have to ask.

Russ

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Did you run through the complete preflight sequence and the manual testing sequences?

http://www.aeroelectric.com/Installation_Data/S-Tec/S-TecServiceManual.pdf

That would go through all of the regimes for flight and tell you if it was trimming in the correct direction.   

I'm going to assume you have this SB already fixed:

http://www.mooney.com/en/sb/M20-291A.pdf

(it also has a ground test procedure)

All I have found with the STEC55x & G1000 is that sometimes it gets into trim osccilations if disturbed by thermals.  Turning off the trim switch and then letting the STEC ask manually for trim (that I can ignore) keeps it from getting out of hand.  This might be a weak pitch servo.  

So, try that as well, trim switch off, put it into a climb mode, let the 55x request trim and trim manually to see if it flies the rest correctly. 

My 55X has something wrong with the CWS hold mode, If you hit CWS it will make a constant right turn, but all other flight modes are fine, so I haven't had it sent in for diagnostics yet.. CWS is the only way to have roll only capture.. the button modes are HDG or NAV/APR. So I assume there is something off with the pure roll capture mode. 

 

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Russ, I fly an Ovation 2GX with the same AP G1000 configuration and have experienced exactly the same run away trim issues you have. The first occasion was when the plane was new to me and was, to say the least, a handful. The problem seems to be resolved now but it has left me hyper vigilant during every clime out.  For a while there I thought that I was doing something wrong but I'm pretty sure that there is some kind of systems issue involved. Grant. 

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Grant, I texted Russ regarding my multiple issues with trim, incorrect roll etc. They replaced my computer board about three times had all the servos cleaned,rebuilt checked and reinstalled, still had issues, I ultimately replaced all the servos with new and wow no problems since, I was told by someone that when the GX's were built a handful of the servos were old and on the shelf for quite awhile, seems probable since the factory was closed for awhile. I assume I had bad servos from the time the plane was built until I guessed and replaced them. I wouldn't be surprised if a few more planes are out there with messed up servos. Just a thought.  Good luck.

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41 minutes ago, Txbyker said:

Genisys wants my rebuilt servo back to inspect it.  I will let you know what they find.  

FYI when they determined that my servo problem was due to reverse wiring as noted in my post above (issue apparently limited to Mooney application of that specific servo), the avionics shop was instructed to simply change the pin location.  A simple field modification.  The servo did not need to be returned to Genesis.

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Russ, as you probably know I had this problem as well and did post on it. Have an update that may or not apply.

My trim would stick and then started running away. After 2 incidents I realized it wasn't a fluke. Local A&P (who is quite good and has worked on a variety of Mooneys) couldn't find anything.

1) Avionics shop found that the pitch servo had worn brushes. Worn brushes = dust in the servo. New brushes/rebuilt servo....runaway trim problem solved.....but...sticky trim still a problem. Trim would roll then stop, often requiring a forceful manual tug to get it going again. No pattern that I could identify.

2) A&P tearing his hair out, can't find anything. Calls out of the blue, he had a brainstorm remembering something from another aircraft years ago. Wires connecting the various yoke switches can chafe and wear. If the electric trim is operating while the yoke is moving the chafed wires then the trim can freeze. He demonstrated it on the ground. Went in and got all the wires out and several were chafed. All were replaced.

3) no more sticky trim!!

This may be a red herring rather than addressing your problem, but if all else has been tried see if this isn't a factor. You never know.

Robert

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1 hour ago, Robert C. said:

My trim would stick and then started running away. After 2 incidents I realized it wasn't a fluke. Local A&P (who is quite good and has worked on a variety of Mooneys) couldn't find anything.

I had the same problem with the King trim switch. I had two cases of runaway trim before it was isolated to the switch. It stuck when trimming for nose down. 

Even though it was fixed a couple of years ago, I developed a new habit of placing my hand over the trim wheel to monitor it.

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1 hour ago, Robert C. said:

Russ, as you probably know I had this problem as well and did post on it. Have an update that may or not apply.

My trim would stick and then started running away. After 2 incidents I realized it wasn't a fluke. Local A&P (who is quite good and has worked on a variety of Mooneys) couldn't find anything.

1) Avionics shop found that the pitch servo had worn brushes. Worn brushes = dust in the servo. New brushes/rebuilt servo....runaway trim problem solved.....but...sticky trim still a problem. Trim would roll then stop, often requiring a forceful manual tug to get it going again. No pattern that I could identify.

2) A&P tearing his hair out, can't find anything. Calls out of the blue, he had a brainstorm remembering something from another aircraft years ago. Wires connecting the various yoke switches can chafe and wear. If the electric trim is operating while the yoke is moving the chafed wires then the trim can freeze. He demonstrated it on the ground. Went in and got all the wires out and several were chafed. All were replaced.

3) no more sticky trim!!

This may be a red herring rather than addressing your problem, but if all else has been tried see if this isn't a factor. You never know.

Robert

Robert, thanks, I am going to ask my avionics shop to check the switches too.

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It seems with our system numerous failed items can/will result with the problems we outlined. Even going back to the thread where he misted inside his wings resulted in the servos slipping. After I pulled out my hair for a few years I ultimately replaced the servos to get the problem solved. During my preflight I at least run the autopilot on the heading bug and hold the yoke still to see if it's starting to slip.

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