Yoke Position Limit for Trim Switch

[Jeff_S]

Here's a new one, at least new to me. Not sure exactly when, but sometime in the last few months I changed my procedure on when I would set trim for takeoff position, and I noticed that occasionally on the ground the thumb switch would be a little "balky", like it would activate in one direction or not the other briefly. It always ended up working eventually in pre-flight, and NEVER did anything weird in-flight. So figuring I learned to fly with manual trim and this wasn't a dangerous situation, I decided to just keep an eye on it to see if I could detect a pattern.

Lo and behold, today I was just doing database updates and decided to do some tests. With everything at rest and the Master on, pushing the trim switch did NOTHING, either direction, no matter what I tried. But then something clicked in my head and I thought "well, these yokes are at full rest position (meaning all the way forward), which is a totally non-flying position. What happens when I pull them back a more neutral flying position?" 

Voilà, as soon as the yoke was in a more natural flying position, the trim switch worked immediately in both directions, no hesitation, no problemo. So it seems as though there is a limit switch that deactivates the trim button when the yokes are too far forward, but I couldn't find anything about this in the POH. Has anyone ever heard of this? On the long-bodies, the yokes at rest do go all the way to the forward stops, unlike the mid- and short-bodies where there is a mechanism that keeps them in a more neutral position. Perhaps in all my prior pre-flights in two years of owning this airplane I always had the yoke pulled back somewhat which is why I never noticed it.

Or perhaps there is still something wrong that I need to keep investigating.  Any thoughts?

[RLCarter]

not sure if there is limit switch or not, my guess is a bad wire and/or connection

[N201MKTurbo]

Yes, the wires run through the yoke tube. When you move the yoke the wires move and bend. You most likely have a broken wire that works in some positions and doesn't in others. Move the yoke in and out and back and forth while activating the trim. You should be able to find where it works and doesn't work.

[carusoam]

Ahhh, negative captain...

I did not get a reason for an automated cut out of a control system just because the yoke is pushed forwards.

That is worthy of following up on.

At least you have a good starting point for maintenance mechanic to begin with.

It might also be worthy of checking all the wires at the end of the yoke shaft.

The trim switches often get dirty.  They are pairs of switches to keep from accidently having a trim run-away caused by a single stuck switch.

So having a trim switch not operating properly...  to keep from having it turn into a runaway trim...  know how to disable it and /or turn it off.  Label the CB on the panel.  All that stuff.

PP thoughts only, not a mechanic...

Best regards,

-a-

[Jeff_S]

Yes, thanks all...I thought it seemed bizarre. Perhaps this ties in with the IDENT button on the yoke that stopped functioning at about the same time. I suspect some wire crimping could be the culprit. Back to the shop!

[DMJones]

Mine quit working a bit ago, slightly different circumstances, but it was the trim switch.  For the record, I have an older M20K, but I'd bet the switches may be similar if not the same.  I took it to Scotty Collins, Precision Avionics, In Griffin (6A2) and they fixed it.  Cheaply...

[Jeff_S]

Indeed, it seems that the wires in the yoke shaft were the culprit. Those were addressed yesterday so hopefully we're all fixed up. It's always something with an airplane, isn't it!?