runaway trim
 

OK, I was all but sold on the idea of using electric trim since I'll be putting in an autopilot allowing for autotrim. Then I read that some people have encountered runaway that can (potentially) overwhelm manual control.

When I review detail of the failure mode for the RAC servos they appear to lock in place on power loss. I would have expected them to disengage. So effectively you cut power to runaway trim and you are stuck fighting it to the ground.

When I look at the 9a preview plans I see the servo/cable placement makes it such that it appears to be one or the other. Does anyone make a servo that allows manual control pass-through so that when you disengage the electric trim the servo releases and allows a cable to takeover. Of course the vernier control must be modified to be unlocked during electric control but this would take away runaway angst and servo locking into position.

Sorry for the mouthful, and hope it makes sense. In GA I believe the AP servo still gives access to the trim wheel. Maybe someone else has already addressed this but I don't see it anywhere.

Thanks.


MIchael B.

Michael,

You may consider this product to minimize the risk of runaway trim. It also makes wiring the trim much easier in the first place. See

http://www.tcwtech.com/Safety-Trim%20Page.htm

I am not flying with it yet but really like the features and ease of install considering I didn't have to mount relays.

Bevan

I always worried
 

about the electric trim in our -10 and -8A --- all of the certified aircraft I ever flew had manual trim wheels, or cranks to "reset" runaway trim (which I had a couple of times) ---- my -6A has manual pitch and aileron trim and I am quite happy with them.

Runaway Trim
 

Don't know about the -9, but for my -8A full nose up/ full nose down trim can be overridden with reasonable muscle power at normal cruise speeds. Not very comfortable - but doable. Also, the runaway will not occur instantaneously - you'll have a short time to react. I've included both elevator and aileron trim cutoff switches in my electrical design.

I installed the TCW Safety-Trim controller in my -10 but I also have an ON/OFF toggle switch for power to the trim servo's just to be safe. I very rarely change the elevator trim and have left the aileron trim in its neutral position since day one. I find my elevator trim very sensitive so if it needs a little change up or down, I flip the trim power toggle switch ON, trim the plane, then turn it back off.

Here's what I think is a better approach for trim control.

The A or AT version of this device minimizes the electomechanical failure points inherent in other designs and provides stuck trim switch detection and correction. Can use a Garmin or Dynon serial stream for speed scheduling and the AT version provides SkyView autotrim.

Of course, no trim controller can correct a failed trim motor. Fortunately, that's rare.

My Garmin G3X has a setting that allows only 2 (setup allows this number to be changed as desired) seconds of trim operation in case of a stuck switch.
I think the possibility of runaway trim is a risk I will take.
I love the auto trim function on my servos and the trim speed control through the G3X.

I think it's all starting to sound like a development opportunity for a fail safe trim mechanism. I didn't realize the G3X was now also offering a safety cut out. I was familiar with the others. Making me lean toward manual still. I'm very comfortable with manual and like the feel. Not to happy with the idea of being locked into a wrong trim for a long trip down from altitude.

Thanks.

The trim can't run away without electrical power, so the only way to get that is for the electrical side of the trim controller to fail and run away. This can be due to broken switches, shorts in wires, or failures in the controller itself. As mentioned here, lots of the electronic trim controllers work around a lot of those failures. The trim controller in the Dynon system will time out a single stuck switch and still allow you to use the opposite direction switch to return the trim to neutral after a failure, or still use the buttons on the other stick.

However, if this really worries you, install a secondary switch. Normal position is for your normal trim controller, but it also has an up and a down position. This would allow you to re-position the trim after your controller fails for any reason.

Thankfully the trim control motors actually don't go free when they have no electricity. This would be a heck of a recipe for flutter!

Search AO Frog's posts. I seem to remember he designed a dual trim system by putting an electric motor that can turn the manual trim knob.