I used to fly Canberras, which had a powerful electric trim motor that changed the horizontal stab angle of attack. There were several fatal accidents before it was found that the trim could run away, and the elevator did not have enough authority to overcome the hardover trim. We then fitted two microswitches that had to be pressed simultaneously, which fixed it. As we have heard, it can happen.
The obvious thing to check is that you can overcome a runaway trim motor in flight. If you cannot, then reduce the trim authority, most easily by reducing the size of the trim tab. Beyond that, I am installing a circuit breaker near the trim controller so that I can pull it and cut off all power.
Runaway trim frightens the hell out of me. It can be very hard, or impossible, to manage. We need to make sure that we can deal with it by checking it out in flight and making it controllable. Just my opinion.
Jerry Fisher
On Sat, Jun 28, 2008 at 5:42 PM, < marv@lancair.net> wrote:
Posted for "Abe Gaskins" < abe@mail.mgmindustries.com>: In my Legacy I had autotrim installed. I am empirical proof that a runaway
trim condition can happen. Upon attempting to reach Sun-n-Fun this year I was over flying Chattanooga and I had a progressive runaway trim pitch up condition. Trim went to the stops. I found the condition managable though I
did have to push forward with heavy input in order to maintain control. OK in VFR but scary and potentially leathal in IMC on a dicey approach. Abe Gaskins N272AG --
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