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If the trim motor fails it will certainly just stop moving - can't move without power. But moving back upstream, if one of the relays fail closed there is a runaway condition, and a relay can stick either open or closed. Further, the stick switch can fail open or closed. Granted, it is more likely for one of these components to fail open than closed, but the severity of the failure is far, far worse if closed than open. In my case, if a stick switch fails closed I have to recognize this and switch to the other stick immediately. If the relay fails closed I have to find and pull the breaker. It would be nice to have a "master" trim on-off switch. In my case there are coolie-hat switches and I worry that something could fall under one of the switches, jamming it in position. On my list of things to modify are, in order of priority: 1. Reduce the travel of at least the elevator trim tab so that it will produce only the minimum-required down trim. Full up-trim is probably manageable, but full down-trim might not. 2. Eliminate the relay array by wiring the two sticks in parallel. The only disadvantage of this is that if the two pilots trim in opposite direction simultaneously the breaker will pop (at least settling the disagreement :-). 3. Going to a two-switch arrangement as mentioned below by Robert. The certificated planes i have flown have this arrangement for exactly this reason: If one switch sticks closed power will be interrupted by the other switch when released. If the opposite trim direction is then selected the breaker will blow. Intuitive and fail-safe. 4 And I've also thought about putting 4 switches on the stick. Instead of a pair of SPDT momentary-contact switches use 4 SPST momentary-contact switches. The two on the front of the stick are to trim down and the up-trim switches are on the back. My coolie hat is on the back and I've had a couple of times when doing a go-around that I was unable to simultaneously push hard enough and operate the trim switch so I had to operate the trim switch with the other hand. The theory is that when pushing forward the index finger is free and when pulling back the thumb is free to move the appropriate switch. Gary Casey
From: Dennis Johnson [mailto:pinetownd@volcano.net]
Sent: Thursday, June 26, 2008 9:51 PM
To: lml@lancaironline.net
Subject: Runaway Trim
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.)
A runaway trim seems unlikely to me too. There was one GlaStar incident a few years back that was attributed to such a failure while in the pattern. That incident resulted in two fatalities, and a design change to the GlaStar trim authority. Remembering that accident and having experienced the muscle required to overcome the mis-trim on my ES-P even when slowed to 90 kt, I figured I could either install a mechanical backup (not practical) or try an electrical approach. To eliminate runaway some installations have two buttons that must be pressed simultaneously to activate either direction. That eliminates runaway but not single-direction failure. In my failure mode it was not a runaway failure: I was able to trim nose-up but not nose-down. This new controller should allow me to reverse the previous direction bypassing the control stick buttons. So if after climb out I again could not trim nose-down I would be able to press the reverse button and reduce stick pressure. At least - that's the theory. Robert M. Simon ES-P N301ES
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