G1000 / STEC55x Ovation 2 manual electric/autotrim failures

[Timbot]

Hi fellow Mooniacs...

We have had repeated failures of the electric stab trim in our G1000 / STEC 55x ovation 2. It stops working in both manual electric trim modes, and when the STEC commands autotrim.

The first failure traced the fault to the stab trim servo drive power section within the STEC55x, and was repaired. Prior to reinstalling it our avionics AP checked out all the wiring/servo electronics and confirmed nothing was shorted, and functionally it was all working fine on the ground, and in flight for a short while in both modes (manual and AP commanded)

It's now failed again in the same mode (in-flight). Has anyone else seen this before? Would welcome any suggestions/insights on where to start hunting for the root cause...

Cheers

Tim

VH-OVH

Edited October 9, 2020 by Timbot

[carusoam]

Tim,

What is it like when you trim manually?

Are you getting any resistance from the trim wheel?

The challenge may be a simple age of grease in the system increasing the load required to move the system...

Many an MSer has had similar challenges, where cleaning out old grease has been a solution... 

From jack screws to gear boxes...

The latest design of APs for Mooneys have proven how sensitive they are to even dirt on the yoke shaft...

They have sensitivity to the additional friction, that we human pilots don’t notice as much... 

Best regards,

-a-

[Robert C.]

My S-Tec55x problem presented itself somewhat differently but may be related.

a few years ago I had 2 runaway trim incidents a few months apart. I don’t recall if the trim wheel felt differently but do remember that I had my mechanic check out the trim assembly. He didn’t find anything that explained the problem and we went on to trouble shooting the 55x. Bottom line: the wearing of the brushes in the servo units had created dust that caused the problems. After the servos were repaired with new brushes the problem did not recur.

hope this helps, good luck trouble shooting.
 

Robert

[Timbot]

On 10/10/2020 at 2:08 PM, carusoam said:

Tim,

What is it like when you trim manually?

Are you getting any resistance from the trim wheel?

The challenge may be a simple age of grease in the system increasing the load required to move the system...

Many an MSer has had similar challenges, where cleaning out old grease has been a solution... 

From jack screws to gear boxes...

The latest design of APs for Mooneys have proven how sensitive they are to even dirt on the yoke shaft...

They have sensitivity to the additional friction, that we human pilots don’t notice as much... 

Best regards,

-a-

The trim wheel has minimal resistance on the ground and at low IAS.. e.g. during initial climbout. Under aerodynamic loading at higher IAS the wheel is significantly harder to move, and feels like it's binding slightly.

My thinking is that this could explain the failure mode of the stab trim servo drive electronics. After repairing the STEC, it worked fine on the ground and during initial climbout. It was after level off and acceleration that it failed when commanding autotrim. So maybe the increased resistance in the trim system caused the stab trim motor to draw excess current. What I don't understand is why the elev trim switch/breaker didn't trip. It's only a 5A switch/breaker.. surely this should trip before the STEC electronics get fried.

Thanks for the tip on cleaning out the old grease, will probably do that first and then subjectively evaluate trim feel...

Tim

[Timbot]

On 10/10/2020 at 11:23 PM, Robert C. said:

My S-Tec55x problem presented itself somewhat differently but may be related.

a few years ago I had 2 runaway trim incidents a few months apart. I don’t recall if the trim wheel felt differently but do remember that I had my mechanic check out the trim assembly. He didn’t find anything that explained the problem and we went on to trouble shooting the 55x. Bottom line: the wearing of the brushes in the servo units had created dust that caused the problems. After the servos were repaired with new brushes the problem did not recur.

hope this helps, good luck trouble shooting.
 

Robert

Thanks Robert, will keep that in mind if we narrow the fault down to the servo. Best Regards

Tim

[Cruiser]

The S-Tec 55x has a trim fuse mounted on the board inside the computer. Is this the component that failed ? 

This is a well known (common) issue with the 55x

[PaulM]

On 10/9/2020 at 5:25 PM, Timbot said:

It's now failed again in the same mode (in-flight). Has anyone else seen this before? Would welcome any suggestions/insights on where to start hunting for the root cause...

When I bought my Bravo the trim fuse had been replaced at least twice (in the logs) , and during my transition training it failed again.   The local avionics shop was able to again replace the surface mount fuse, and all was good for about 5 years.. we theorized about excess screw force. Made sure the jack screw was well lubricated each annual and run from end to end.

Then 2 years ago it failed again, and the local avionics tech tried to replace the fuse.. checked the servo manually.. worked fine, no overcurrent.  Fuse went again immediately.. Sent the unit back to S-tec, and the power board & perhaps the main board were replaced.  No issues since. 

Others have blamed the servo brushes, but that was not the issue for mine (this time).   If it can be corrected by replacing the fuse, you can have the shop also service the servo.  This sort of issue will be intermittent and may not show up in a straight bench test. 

[Timbot]

On 10/13/2020 at 1:01 AM, PaulM said:

When I bought my Bravo the trim fuse had been replaced at least twice (in the logs) , and during my transition training it failed again.   The local avionics shop was able to again replace the surface mount fuse, and all was good for about 5 years.. we theorized about excess screw force. Made sure the jack screw was well lubricated each annual and run from end to end.

Then 2 years ago it failed again, and the local avionics tech tried to replace the fuse.. checked the servo manually.. worked fine, no overcurrent.  Fuse went again immediately.. Sent the unit back to S-tec, and the power board & perhaps the main board were replaced.  No issues since. 

Others have blamed the servo brushes, but that was not the issue for mine (this time).   If it can be corrected by replacing the fuse, you can have the shop also service the servo.  This sort of issue will be intermittent and may not show up in a straight bench test. 

Thanks Paul. We are looking at the jackscrew, and will have the servo checked when the AP computer goes in for repair. Thanks to everyone that has replied to this thread, the advice has been invaluable.

Tim

VH-OVH