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KAP 140 autopilot pitch trim servo clutch slipping?

Posted: Sat Jan 25, 2020 1:09 am
by Diamond_Dan
Hello all - Happy Friday
(2004 G1000 DA40 WAAS KAP 140 A/P with altitude pre-select)
Was working on my instrument proficiency today with an instructor and encountered a problem with the autopilot trim. On a coupled approach, the autopilot would not descend quickly enough with a reasonable power setting leading me to be above glide slope. I somehow figured out that the autopilot was not moving the trim wheel forward. I disengaged the A/P and could not move the trim wheel forward with the pitch trim switch. When I move the trim wheel forward and backward by hand, it moves brilliantly with low friction, no stickiness, and it doesn't slip or creep during flight. For these reasons I am reluctant to adjust the pitch trim friction in any way (e. g. loosening it). I re-engaged the autopilot and found that I could move the trim wheel up and down with no complaining or corrections from the autopilot. :shock:

I had an a-ha moment:
When I disengage the autopilot, the aircraft will often climb. I think it has been more pronounced lately.
I have been flying the airplane for over three years now, and the autopilot has never held glide slope aggressively. I have to carefully manage the power setting or else it will leave the glide slope - reducing power usually fixes the problem tho I may end up slower than ideal. This is my first sophisticated airplane so I thought this was either normal or the KAP 140 is junk.
There has always been a tremendous amount of backlash on the trim wheel when I pitch trim down with the servo.
The autopilot never gave any error or "trim in motion" today.

I am thinking that the pitch trim servo clutch needs adjusting. I found the section in the manual but the instructions call for a slip clutch test stand and they aren't very clear to me with no diagrams. I am guessing we (my mechanic and I) should use the IPC? I would appreciate any insight into my diagnosis and experience making this repair (should I take it to a specialist)?

Re: KAP 140 autopilot pitch trim servo clutch slipping?

Posted: Sat Jan 25, 2020 2:18 am
by Rich
There are various adjustments that are needed in the original KAP140 installation and sometimes in the interim. In fact, some are recommended any time the computer is removed and re-installed. I've attached a troubleshooting guide but the Installation Manual, with lots of information, is too large to attach. You can dig it out from a Google search. Rectifying may require some capabilities not available to you and require a trip to the appropriate shop.

The trim servo clutch sounds like a candidate. But the bad news is that the servos themselves can come to require servicing by Bendix/King.

Re: KAP 140 autopilot pitch trim servo clutch slipping?

Posted: Sat Jan 25, 2020 2:31 am
by Steve
Sure sounds like the trim servo or trim servo clutch. One bit of diagnosis you can do yourself on the ground is to see if the trim wheel moves using the electric trim. On the ground there are no air loads on the system and the only resistance would be friction in the system (which you believe is not excessive). You should also hear the trim servo (under the copilot seat) activate. So, if you hear it it probably is the clutch torque adjustment.

The bad news is that you need a special tool to test the clutch torque. It is likely that you will have to go to a B/K service center to have that done. As Rich states, hope it isn't a bad servo - they are very pricey - I believe that an overhauled KS-272C will run you north of $1200. Good luck, and keep us informed as to what you discover...


Re: KAP 140 autopilot pitch trim servo clutch slipping?

Posted: Sun Jan 26, 2020 8:41 pm
by Diamond_Dan
Thank you very much for the feedback.
I went out to the practice area for about an hour today to get a better understanding of the problem and it doesn't seem as straightforward.
With the autopilot on, the trim wheel would not move when I changed power settings except for 2 or 3 times during the flight. When I disconnect the autopilot, the airplane would usually either climb (once was 1000+ f/m) or descend. I could freely move the trim wheel by hand with the autopilot engaged, and it didn't correct or fault. I was looking for the trim LED to light up on the display but it never did. The trim wheel never failed to move (in the air) when I used the thumb switch on the control stick. I played with it before shutting down on the ground and once it did not move when I pushed the switch. This did happen a couple of times when I was in the air on Friday. Again, no autopilot faults the entire trip.

I plan to call the local Diamond dealer in KASH and a local avionics shop to see who has the most experience with these things. Not sure if Diamond experience or KAP 140 experience will be more helpful.

Thank you again.