KAP 200 autopilot fails with electric trim on (was: failed with altitude hold)

[MooneyAcolyte]

1 minute ago, Arthur said:

I had the exact same problem in November. Pitch servo has been in avionics shop (well known for King Auto pilot repair) since Feb. They can’t seem to find the problem. Servo works but under load draws too much current and overheats. My servo is the KS270-065-0027-04 Not the upgraded K207A.

Difficult to find a servo under $3000

Wow! That's steep. Now with @Jake@BevanAviation's help I am 90% sure it's the trim servo KS179 (P/N 65-0052-04), I will talk to the avionics shop here in Midden-Zeeland (EHMZ). They are good, but VERY busy and not too customer-oriented. I'll share my journey here.

[ilovecornfields]

59 minutes ago, Arthur said:

Difficult to find a servo under $3000

Ouch! How much does a GFC500 cost?

[MooneyAcolyte]

Ouch! How much does a GFC500 cost?

15k$ I heard, installed. Don't quote me.

[MooneyAcolyte]

On 5/19/2023 at 7:09 PM, Jake@BevanAviation said:

See attached for interconnect.

KFC200 M20J-K.pdf 80.58 kB · 3 downloads

After studying the correct wiring diagram sent by @Jake@BevanAviation, I am now convinced the problem is with the PITCH servo (KS-177), not with the trim servo. As you can see, the PITCH servo has the "AUTO TRIM UP SENSOR" and "AUTO TRIM DN SENSOR" lines connected. Given the symptoms, the most likely root cause is a false  trim sensing. The only other option is that the flight computer (KC 295) misinterprets correct sensor input. The procedure proposed by @Jake@BevanAviation is indeed designed to distinguish the two cases. For me, the next step is to perform this procedure. I'll share my results here.

[MooneyAcolyte]

Latest update: I have now an appointment with the avionics shop in Straubing (near Munich) in August for repair of the pitch servo. I'll keep you posted.

[Fly Boomer]

9 hours ago, MooneyAcolyte said:

Latest update: I have now an appointment with the avionics shop in Straubing (near Munich) in August for repair of the pitch servo. I'll keep you posted.

Good luck to you.

[MooneyAcolyte]

It's fixed!

 

Thanks to all of you helping to diagnose this fault. As it turns out, we were spot on. Indeed, the microswitch inside the pitch servo (KS179) had failed. I went to Avionik Straubing (https://www.avionik.de), who did an EXCELLENT job. They took apart the KS179 (see attached) photo, fixed the micro switch, overhauled the motor (so I get another couple of years out of it), and lubricated the mechanical part. Due to the clear diagnosis, they were able to focus on the work that was actually needed. As a consequence, the repair was still rather affordable (€1100). I flew PH-FAR back from Straubing to Rotterdam and tested the AP extensively. PERFECT!

 

Thanks again to everyone involved. This case is another demonstration of the value of mooneyspace. Signing off on this topic - Mooney Acolyte

[Fly Boomer]

35 minutes ago, MooneyAcolyte said:

Thanks again to everyone involved. This case is another demonstration of the value of mooneyspace.

Great story!  Thanks for closing the loop.  I love it when a component can be brought back to life.  These days, mechanics of all stripes (cars, airplanes, plumbers, etc.) are wont to just rip out the old, and stick in new.  Not so easy with 30-50-year-old airplanes.

[Jake@BevanAviation]

Good to hear the issue has been resolve and at a decent repair price.  If you have any additional issues in the future just let me know.