Well, it certainly sounds like real-world experience, even in our own little fleet of Lancairs, has proven me wrong when I said runaway trim should be rare.  It sounds like the main culprits are busted switches (or perhaps more accurately, stuck switches) and broken relays. 

When we were developing the Javelin it became clear to me that modern Part 23 certified airplanes require a split switch, with both halves operating together before the trim motor will run.  But I don't recall anything about relays, and that's probably because we had a multiple-channel computer running the trim motor (which was also used for Mach trim compensation, which is why the computer had to be involved, but that's beyond the scope of our Lancairs).  There was also a thumb-button disconnect on the stick to disable the trim temporarily (same button as autopilot disconnect) and a permanent shutoff switch on the panel.  This was a certifiable configuration in today's Part 23 world. 

Yes, I have relays in my airplane.  They are the little MAC (RAC?) ones, which are presently sitting on my bench, not yet installed.  I have Columbia teak grips, not the MAC grip, with a 4-way coolie hat.  I'll probably put those in, verify that the airplane is controllable with full runaway trim, and the first time they fail I'll yank them and put in more robust ones.  Although a better soluion would be to have a switch which was capable of handling full motor current and getting rid of the relays entirely.

Or doing what Grayhawk does, and use the mechanical trim wheel.  I don't imagine that could ever run away.

- Rob Wolf