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Dennis,
While a runaway trim may be a rare thing, stuck trim on RAC relays
seems to be pretty common. 3 of us flying RV-10's had issues
during OSH last year at the same time. Replaced the relays and
it was better, but it got me thinking...
Ran into Bob Newman, maker of Safety-Trim, See this link:
http://www.tcwtech.com/Aviation%20Products.htm
and ended up coming to the conclusion that there are DEFINITELY
better ways to do it than the standard relays.
Here's a write-up I did on it.
http://www.myrv10.com/N104CD/upgrades/20071027/index.html
The controller here handles dual-speed trim and dual-axis as
well, and there is a lot more benefit in that little box in the
way of other features and ease of wiring. It's an easy
retrofit as well.
Anyway, the write-up tells it all.
The problem I had was that the trim stuck in one direction, and
reversing the servo was impossible with the stuck relay. Attempting
a couple of quick back and forths with the trim switch just ended
up making it worse...then soon you're stuck having to land or
fly in a less-than-comfortable condition...which isn't all that safe.
In the end, for many reasons, I'm much happier and glad I made
the effort to retrofit.
Tim Olson
Dennis Johnson wrote:
Hi Robert,
My understanding from talking with the RAC guys, who make the trim components I installed in my Legacy, is that stuck (runaway) trim is unlikely. (Actually, they said it was impossible, but I find it hard to use that word.)
The trim servo must receive power to operate. A runaway can only occur if either the trim switch or the trim relay is stuck in the "on" position. If that happens, moving the trim switch to the opposite position should almost certainly cause the switch or relay to open, clearing the problem. (Wiring faults could also cause runaway trim, but I was careful to protect the wiring when I installed it and therefore do not consider this cause to be a realistic possibility during the time I own my Legacy.) I've given this some thought and have decided that runaway trim is possible, but unlikely. I've decided that the additional parts count and complexity to reduce the chance of an already unlikely runaway trim isn't justified. I've practiced this and believe I can control the airplane with full trim if I slow it down, although my arm would get tired after a while.
Best,
Dennis Johnson
Legacy, 140 hours
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