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  #1  
Old 12-16-2020, 08:27 AM
dweyant dweyant is offline
 
Join Date: Sep 2012
Location: 07TS
Posts: 487
Default Runaway Trim

Had a really scary experience yesterday.

Was flying from central Texas to Tennessee. putting along with a nice 30 knot tail wind when all of a sudden my Autopilot kicked off.

I was having just a little bit of turbulence, so my first thought was it was more than the AP could handle. However by the time I grabbed the stick and started to mess with reengaging the AP I realized something was very wrong.

My elevator trim had gone crazy on me, going full down deflection.

As you can imagine it is really hard to control an airplane in such a situation. I did (obviously) manage to get the plane under control, but I was unable to get the trim to move back to the proper position.

I wound up declaring an emergency (amazing how easy that is to do when you really need one). The controller was great and found me an airport close by so that I could shoot the RNAV approach, by hand, down to almost minimums.

Fortunately (very) I got the plane on the ground in one piece. Once on the ground I went about troubleshooting the problem.

I was able to unstick the trim, and run it to full stops numerous times with no issues, however I had to do it from the soft keys on the G3X touch. The buttons on my stick not only aren't working properly, but are performing the wrong function. the trim down button now works as right alieron....

Beyond that, and sending Garmin the logs they requested I haven't tried much troubleshooting yet.

Any ideas?

-Dan
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  #2  
Old 12-16-2020, 08:50 AM
TS Flightlines TS Flightlines is offline
 
Join Date: Apr 2010
Location: Ridgeland, SC
Posts: 2,713
Default

Stuck trim relay?
Talk to Pat Hatch about his system.
Tom
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Joint Venture with Aircraft Specialty
Teflon Hose Assemblies for Experimentals
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  #3  
Old 12-16-2020, 08:55 AM
dweyant dweyant is offline
 
Join Date: Sep 2012
Location: 07TS
Posts: 487
Default

Quote:
Originally Posted by TS Flightlines View Post
Stuck trim relay?
Talk to Pat Hatch about his system.
Tom
I don't think so. the VPX handles the trim input and output so I don't think I have a trim relay per se.

-Dan
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  #4  
Old 12-16-2020, 09:29 AM
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Daida Daida is offline
 
Join Date: May 2005
Location: Hannover, Germany
Posts: 151
Default

Wonīt help much on your troubleshooting, but maybe "food for thoughts" for people still building.

For exactly that reason, i did not put in the electric trim Option in my -8. Runaway-trim used to be a BOLD FACE procedure (immediate Action required you had to know by heart, no time for checklist) at my former workplace. Runaway-trim ainīt no fun! And if a system can fail, it will.

Furthermore, flying my -8 now, i find the trim super-super-sensitive. I would even doubt that you find the perfect trim by using the electric motor operated by a switch. You may get close, maybe even very close, but i doubt that you get it "just there", hands-off-level-flight.

For me it ended up (elec. trim) beeing another system that could fail with overall doubtfull use.

Please, anybody teach me wrong if i am.

If i hear your story, i am glad that i installed manual trim ( i know. Can fail too!)

I am glad to hear that the whole event went by with no injuries or a bent or wrecked aircraft. Runaway-trim is surely no fun to deal with.
Declaring emergency was the right thing to do, in my opinion.
Shooting an IFR Approach in IMC (i assume) with an "out-of-trim" aircraft is, even if you have a flight director, nothing easy to do.

Well done!!!
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  #5  
Old 12-16-2020, 09:55 AM
TS Flightlines TS Flightlines is offline
 
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Location: Ridgeland, SC
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Default

ok-----I probably should have left my comment to myself---and turn this over to the avionics guru's. Didnt know you had a VPX magic box.

Tom
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Teflon Hose Assemblies for Experimentals
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RV7 Tail Kit Completed, Fuse started-Pay as I go Plan
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  #6  
Old 12-16-2020, 10:14 AM
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Steve Melton Steve Melton is offline
 
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 3,034
Default

I have resisted the urge to put an altitude servo on my elevator and especially anything that could affect elevator trim except the basic Pat Hatch relay board. runaway trim or autopilot on my elevator is a risk I want to avoid. talk to Boeing about it. I know where my trim circuit breaker is and I can find it blindfolded. glad you made it down safely.

a challenge for you...... apply full trim at cruise speed, up or down, and see how fast you can find the trim circuit breaker and pull it.
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Simplicity is the art in design.
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My Artwork is freely given and published and cannot be patented.
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Last edited by Steve Melton : 12-16-2020 at 10:19 AM.
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  #7  
Old 12-16-2020, 10:23 AM
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Mike S Mike S is offline
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,702
Default

TCW Safety Trim would have taken care of the issue.

In fact, most likely would have prevented the problem in the first place, as the VPX would not have been in the picture---the servo and trim switch are wired directly to the Safety Trim unit.
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Flying as of 12/4/2010

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"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #8  
Old 12-16-2020, 10:26 AM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 3,034
Default

Quote:
Originally Posted by Mike S View Post
TCW Safety Trim would have taken care of the issue.

In fact, most likely would have prevented the problem in the first place, as the VPX would not have been in the picture---the servo and trim switch are wired directly to the Safety Trim unit.
solution, more electronics?
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Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 800+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com
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  #9  
Old 12-16-2020, 10:31 AM
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Mike S Mike S is offline
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Quote:
Originally Posted by Steve Melton View Post
solution, more electronics?
Actually less electronics as I see it. In prior posts it was mentioned that both a VPX and a G3touch were involved. The Safety Trim will work without either of these boxes. Both the VPX and the G3 are doing a lot more that just the trim, so way less electronics in the whole.

For those who choose an electrical trim, certainly a choice.

For those who choose manual trim---not needed.

I happen to like having the trim on the stick, so I fall into the first group.

Always hated the manual trim in the Cessna.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."

Last edited by Mike S : 12-16-2020 at 10:37 AM.
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  #10  
Old 12-16-2020, 10:40 AM
blaplante blaplante is offline
 
Join Date: Jan 2010
Location: Southern California
Posts: 168
Default No hat switches for me!

I see lots of people with electric trim using the all in one "hat" switch. That's a single point of failure. What happens if it sticks or shorts internally?

I followed the Cessna 182T trim design - two rocker switches. BOTH have to be pressed up or down to make the trim move. If either one sticks on the trim doesn't move. Oh and there's a preflight checklist item to confirm no trim movement if you only actuate one switch.
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