Electric Trim Problems

[BiplaneKim]

I originally posted this at the end of the thread  titled "Trim Jackscrew Binding started by PeytonM a few days ago but I thought it might be more appropriate to create a new topic. I do think my Jackscrew needs some maintenance attention and that topic was full of fantastic information but I'm not sure it is the cause of my electric trim problems so I'm trying to reach anyone who has had to deal with similar electric trim problems. Manually, in flight, my trim wheel may feel a bit stiff but I've never considered it a problem and always assumed I had an electric trim problem.

Sorry if I'm overlapping the mechanical trim system with the electric trim system but I'm thinking both must be considered.

I have an intermittent electric trim problem with my 1980 Mooney M20K (231) which is equipped with a Bendix/King KFC-200 autopilot/flight director. Before asking some questions, I'll give you some background info.

Through the course of troubleshooting this problem, I ran across a MooneySpace thread which really explained a lot about Trim Jack Screw Binding. I've always thought I had an electrical problem but after reading through this thread, there's a chance it could be a trim jack screw binding problem. I believe that my Jack Screw system needs some attention (including removal, inspection, cleaning, lubrication, and re-installation) but I still suspect an electrical problem.

I usually use the electric trim and the intermittent problem has become more frequent. Often when using the split trim switch on the yoke, it trips the breaker, sounds an alarm and the electric trim is inoperable. Sometimes I reset the breaker but that generally does not work and I use the manual trim wheel. This problem usually does not occur on the ground and often may not occur on the next flight.

I thought I’d start by making sure it is being maintained correctly, such as lubrication, so I looked through my aircraft service manual which does not provide a lot of information. After talking to the shop that has done my avionics work, I found out that the electric trim system service & maintenance is covered more thoroughly in some later Mooney manuals for the M20R and Bendix/King manuals including Honeywell Service Bulletin 292. They were kind enough to copy the related pages.

From the pages they gave me, I discovered that there was an STC for the addition of a carrier bearing located on the trim shaft directly below the trim servo. From inspection, I can see that bearing has been installed on my Mooney. The pages also say there is a maintenance decal that is supposed to be installed at the same time to provide Annual inspection and lubrication instructions but my plane does not have that decal. I do not believe that this bearing has been inspected and possibly not serviced or lubricated in the 18 years that I have owned the aircraft. For about the first 10 years I owned the plane, I had a highly qualified Mooney specific IA doing my annuals and I worked with him and I do not recall ever doing anything with this bearing. He has passed away, and I do not think my current IA is aware of this bearing inspection or maintenance either.

A few days ago, my friend (A&P) and I spent the day troubleshooting and planned to inspect the bearing (below the trim servo). I started by asking him to operate the trim wheel to see how it felt to him and he immediately thought that it operated too stiff. He noticed that there are smooth areas and stiff areas. As I worked the trim wheel, as he instructed, while he observed and felt the trim shaft from the avionics bay opening to the Jack Screw, he concluded that everything worked freely until it got to the Jack Screw system. At that point we stopped for the day and I did some on-line investigation and found the jackscrew binding thread. He and I have gone through all the comments and photos of that and believe that my Jack Screw system is not operating as freely as it should and needs some attention and could possibly be causing the problem with the electric trim.

There is a number of weeks before we plan to remove the jack screw system so we decided to throw out some questions to get any help or ideas any of you have.

1. Do you think Trim Jack Screw Binding is a possible cause of my electric trim problem?

2. I saw some comments in that topic that their trim got so stiff that it "jammed" the electric trim.  Has anyone's trim failure been like mine (where it trips the circuit breaker) or does it just not operate.

3. Do any of you with a KFC-200 system have any specific thoughts on trim problems that I should investigate? Have you been inspecting and lubricating the Carrier Bearing below the Trim Servo as I mentioned? It appears to be a difficult job due to access and the screws being behind the servo chain sprocket. Any thoughts?

4. Through all these comments, we didn't see anyone talk about replacing the jack screw boot. My boot looks just like all the photos in that topic. It seems well worn and disintegrating on the ends such that tiny threads and pieces come loose and are in the jack screw threads and grease which could be causing binding. We plan to replace mine and wondered if you all have replaced yours when doing this work? Where is a good place to purchase the boot?

Any thoughts and ideas would be appreciated!

Thanks for your help!

[carusoam]

Welcome out into the posting open, Kim! You have been quiet for years!

I saw your post the other day.

I’m glad you started a new thread.  This challenge is worth looking into separately...

1) Trim switches are pretty notorious for needing some tiny switch replacements... pretty easy to find and do...

2) cleaning switches maybe all that it takes... Find the right spray electronics cleaner to do that.  Fast and easy...

3) the rest of the clean and re-grease for all Mooneys is pretty universal... the parts may be different, but the challenges are the same...

It is important to be familiar with all things trim... and all the things that work to adjust yoke loads... power vs. flaps...

We lost a K around here a while back... the trim stuck... Find Amelia’s Writing regarding Marvin K Mooney...

Fortunately, Amelia is still with us... Marvin regretfully, has been recycled... happily replaced by an Eagle... 

Much of this testing can be run on the ground... mechanically, by hand, run the trim full up and full down a couple of times.... any sticky ness in spots?   Trim stops before being full up or full down?  (Know where full up and full down are first... often the stickiness stops the travel before reaching the end....)

PP thoughts only, I am not a mechanic...

Best regards,

-a-

 

[PT20J]

Sounds like some basic troubleshooting is in order. Separate mechanical issues from electrical. Two most likely issues are mechanism so stiff that trim servo stalls, or weak trim servo.

With electric trim off, can you manually move the trim wheel easily from stop to stop? If not, find the source of the problem. Clean and lube easy to get to stuff first. The trim carrier bearing is not that hard to get to and should be serviced but is not likely the cause of the problem. Most heavily loaded parts are the jackscrew and  associated thrust bearings. 

Does alt hold work without tripping trim switch/breaker? Put in alt hold and vary airspeed with power and this will exercise autotrim. If this works, you know that the trim servo is good and the problem is in the manual electric trim either in wiring, switch or the KFC 200 flight computer. 

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