Anybody had this elevator trim symptom before?

[Yooper Rocketman]

I've had this pesky elevator trim symptom for about a year and would like to address it during my annual right now.  Just randomly I will be adjusting the trim with the yoke mounted switch and it will stop moving.  If I reach down to "assist it", the trim wheel is locked solid until I let go of the trim switch, then it will manually adjust with no more than normal resistance.  I initially lubed the jack screw very well, with no effect (other than easier to trim manually).  I had my local avionics shop look at it one day when I had just arrived at my home airport from a trip (didn't even shut down the airplane) and he had me engage the autopilot after using the yoke switch and it was stopping during yoke trim inputs.  Adjusting the flight director and watching the A/P make trim adjustments, it ran through the full trim range with no issue.  We did this several times and it would stop (and lock) the trim wheel with the yoke switch, yet run through the full range with the A/P?????

The tech said he would call Bendix King for advice on the symptom, but after several calls to him later, he was never able to get in touch with B.K.  His response was let's just pull the pitch servo off and send in for inspection and OH.  I'm thinking, wait a minute; it works fine with the A/P, how does the servo know whether the command to roll in pitch is coming from the A/P or the yoke switch?  It doesn't seem it could be the yoke switch because when the trim stops adjusting while the switch is activated, the trim wheel is locked solid (in other words, it must be still getting the command to adjust trim) and letting up on the switch the trim wheel becomes "adjustable" again.

Anyone else ever had this symptom and if so, what did you ultimately find?

Tom

[BorealOne]

Just got my bird back from the shop. I had similar symptoms, which developed slowly over time. The yoke switch would run fine on the ground, but would intermittently fail in the air, usually on approach. The flight director would run the trim just fine, but then started to blow through ALT holds. Turned out to be the clutch on the pitch servo. The forces in certain flight configurations would overcome the clutch, while the motor remained engaged. Rebuild was required.

[carusoam]

This sounds unlikely, but...

Are there two separate servos that move the elevator?

One that handles the trim as we know it... and a separate one that adjusts on the fly...?

The description of this failure sounds like two different motors may be involved...

The BK manual for the AP probably shows each motor in the diagram...

Which version of the BK AP is installed... KFC150?

Best regards,

-a-

 

[peevee]

My thought would be a faulty autopilot disconnect switch based on limited experience and a lot of research on our own problems. 

Second thought would be a failing servo motor. I could be way off on both.

[N201MKTurbo]

The trim servo has two functions. One is to engage the servo, which usually is a solenoid that meshes two gears coupling the servo motor to the torque tube through the slip clutch. The other function is to run the servo motor. 

It sounds like the engagement solenoid is working but the motor isn't working. You will have to catch it while it is not working to troubleshoot, but it is easy at that point. There are 4 wires going to he servo. Two for the engagement solenoid and two for he motor. If you see voltage at the servo connector motor wires and the motor doesn't turn then your servonis broke. If you don't see any voltage the problem is elsewhere. My experance is the problem is likely in the autopilot disconnect relay.

[Bartman]

I had a similar issue last year.  The yoke mounted trim switch would engage, but not turn the motor. Siting in the hangar I could hear it click to engage. After releasing the switch I could then turn the manual wheel, but it was like I had to overcome that engagement before it would turn. The wheel turned easily until I used the trim switch, then had to overcome some resistance before it would turn again. That's the best way I can describe it. 

It turned out to be the trim switch.  They are expensive, but my IA likes to repair rather than replace. He ordered 3 new micro switches and it works great now for about 1/3 the cost of a new switch.  May not be your problem, but hope this helps. 

[Yooper Rocketman]

6 hours ago, carusoam said:

This sounds unlikely, but...

Are there two separate servos that move the elevator?

One that handles the trim as we know it... and a separate one that adjusts on the fly...?

The description of this failure sounds like two different motors may be involved...

The BK manual for the AP probably shows each motor in the diagram...

Which version of the BK AP is installed... KFC150?

Best regards,

-a-

 

Yes, KFC 150

[carusoam]

Reading the pilots operating manual there are two separate servo motors that seem to operate the elevator...

1) A pitch trim servo - KS179

2) A pitch servo - KS177

http://www.bendixking.com/HWL/media/Pilot-Guides/006-08377-0001_1.pdf

I have included the manual to see if that is of any help...

It sounds like one may be working and covering for or hiding the other's misbehavior...

Best regards,

-a-

[N201MKTurbo]

2 hours ago, carusoam said:

Reading the pilots operating manual there are two separate servo motors that seem to operate the elevator...

1) A pitch trim servo - KS179

2) A pitch servo - KS177

http://www.bendixking.com/HWL/media/Pilot-Guides/006-08377-0001_1.pdf

I have included the manual to see if that is of any help...

It sounds like one may be working and covering for or hiding the other's misbehavior...

Best regards,

-a-

The problem would be with the pitch trim servo.

[Yooper Rocketman]

Found Problem!!!!

https://mooneyspace.com/topic/22795-found-the-elevator-electric-trim-problem/

 

[peevee]

I like easy fixes.

Thanks for the update.