[roee]

I noticed something potentially dangerous in the design of the electric aileron trim system.

The geometry is such that the C-606 linkages that connect the springs to the control column will experience rotational forces (i.e. torque) any time the stick is not centered laterally. There's nothing to oppose the rotational forces other than the friction/tightness of the one bolt that connects the C-606 linkage to the control column (that also connects the F-665 pushrod). So there's potential here for the bolt to loosen and the linkage to start rotating. Or worse, the bolt could eventually come completely loose and the F-665 pushrod would disconnect from the control column. Not a robust design, and failure here could be catastrophic (loss of aileron control).

I raised this issue to Van's. Initially they acknowledged my concern, but qualified it with the usual "[my paraphrase] but we've got hundreds flying and we haven't heard of it being a problem". Still, tech support passed it on to engineering. Next time I corresponded with them however, their message was CYA lawyer-ese, basically "no comment".

To cover my own "A", I won't make any recommendations here on how to fix it. But what I plan to do on my airplane is to redesign the C-606 linkage so that it connects to the control column through both bolts. That way the shear strength of the bolts opposes rotation, rather than relying on friction/tightness of one bolt.

I don't know if Van's is taking this issue seriously or not. But if they don't issue a service bulletin, I would suggest that anyone installing the electric aileron trim should study and evaluate the safety of the design for themselves.

 

[dan]

If you're really worried about that, ditch the nylok nuts (AN365) and go with AN310 castle nuts and cotter pins!

 

[roee]

That's an improvement...

Dan, that's a good suggestion. A castle nut and cotter pin would at least keep it from threading out. In fact, I'd say it would be wise to use a castle nut here even without the aileron trim issue. With the aileron pushrods, the rod end bearings should normally take the rotation, but if they ever get tight or seize up then they'll start rotating around the bolt. So as far as I'm concerned, castle nuts here for sure.

Still. Even with castle nuts, the aileron trim linkage could start rotating, causing wear and (highly unlikely but possibly) jamming on something. Again, unlikely I agree, but I prefer to keep Murphy away from my primary flight controls. Call me paranoid.

 

[dan]

For what it's worth, 888 hours with the stock setup (AN365 nuts) and nothing has come loose...and I don't believe it ever will. There is so little tension on those trim springs, and even when the tension is highest (stick fully to one side), the tension is nearly aligned with the "axis" of the little attach bar (depends on how you rig it).

Given that I only exercise the ailerons to the stops during the pre-takeoff check (and occasionally during an aileron roll...most of the time my rolls are only about half to 3/4 stick displacement), it's not like those bars/nuts are seeing frequent force.

Don't let me talk you out of anything that makes you feel better about it, but realistically it's among the least of my worries.

 

[roee]

Fair enough. I totally agree it's a matter of personal comfort level. I do appreciate your input though.

I'll post more info after I've completed this part.