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Date: Sun, 29 Jun 2008 00:06:35 -0400
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From: Tim Olson <Tim@5000feet.com>
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Subject: Re: [LML] Runaway Trim
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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