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Old 09-08-2022, 03:24 PM
markserbu's Avatar
markserbu markserbu is offline
Join Date: Apr 2018
Location: Tampa, Florida
Posts: 27
Default Crazy full-up elevator issue...

The day I bought my RV-8A and flew it home (March, 2021) I first encountered this problem. I was heading home to KVDF from visiting a buddy at KMLB, on autopilot, and somewhere just East of KORL's Bravo airspace the plane started heading for the sky! I pushed the stick forward...back to neutral, but it was like fighting a demon. Fairly strong; it felt like I was pushing against a 15+ pound spring. Maybe more. I was a bit freaked out the first time and I don't know how the problem was solved, but it was. I visit my buddy at least several times a year and without fail the plane does the same thing around that same area as I head home!! I figure it's some sort of electric gremlin but what on earth could trigger it in a fairly specific location? I finally got smart and was ready to pull the elevator trim circuit breaker when it happened, and I did. Didn't do anything. And of course turning off the autopilot was always the first thing I did, with no result. I finally know exactly what it's doing...electronic waves, Satan, random chance, or setting full-up elevator trim. The indicator shows it. Solving it just takes pushing the trim down button and setting it back to neutral. No idea why it happens at that location (somewhere near DEARY), unless the intersection of two victor airways somehow creates enough electrical interference to cause it. Doesn't seem likely. But the other day I noticed that it "did it" after an aileron roll, and nowhere near the "usual spot". This would possibly point to a loose connection, I'd think. I have yet to dig through all the paperwork and see how the trim motor is connected to the control...I assume it's through a fairly powerful spring? Any thoughts on this crazy topic are appreciated!
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Old 09-08-2022, 04:04 PM
Mile High Relic Mile High Relic is offline
Join Date: May 2010
Location: Arvada, Colorado
Posts: 386

Could it be a stuck trim button?

I've experienced full aileron trim deflection, and while it got my attention it was more like a push versus "a demon."

If I ever had to fight a demon in a plane I'm pretty sure I'd figure out exactly what the issue was before leaving the ground again.

A victor airway or an intersection is just a fix in 2D space, and exists only because someone defined that point and gave it a name. There isn't any special "electrical" effect at that point, so don't assume the coincidence in location has anything to do with the actual issue.
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Old 09-08-2022, 04:12 PM
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BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,906

Depending on your panel, the data recorded might include the requested position, the actual position, and the torque to achieve position.

All of these parameters are available on the Garmin boxes and were extremely helpful in diagnosing a similar event on a friends 10 where the nose headed for the dirt suddenly.

Not sure what other vendors to but would guess they have a data stream recording feature. 1 HZ is a good starting point if that is selectable. It could be as simple as trim set for takeoff, not adjusted for speed, then the AP pitch servo reaches its torque limit at high speed cruise and shuts off. Up you go. Get data.

None record supernatural parameters.

Last edited by BillL : 09-13-2022 at 06:17 PM.
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Old 09-08-2022, 04:29 PM
sailvi767 sailvi767 is offline
Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,527

I had something similar happen once. It was my IPad that bumped the trim switch as I was selecting music with it in my hand!
RV-6 sold
F-1 Rocket
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Old 09-12-2022, 04:14 PM
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markserbu markserbu is offline
Join Date: Apr 2018
Location: Tampa, Florida
Posts: 27

I appreciate the responses, thanks. I dug into the plane a little bit over the weekend and didn't find anything unusual, as is common with electrical gremlins. The trim motor installation was as it was shown in the schematics for the plane. No spring to be found, so obviously I was fighting a software "spring", pushing back with a particular "K" value. The oddest thing about all this is that usually when the elevator trim is set too far one way or another, the autopilot tells you to trim up or trim down. When this happens the autopilot seems to be blissfully unaware of the full-up elevator trim. That would seem to indicate that the trim motor is commanded to full-up trim without "reporting back", yet the indicator correctly shows full-up, so the system knows about it. Would love to see if I can record the trim position parameters, but I suspect my 20-year-old Dynon D100 may not be up to the task.
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Old 09-12-2022, 06:45 PM
RV74ME RV74ME is offline
Join Date: Dec 2015
Location: Auburn, AL
Posts: 264

Thatís a pretty serious issue. I would strongly suggest grounding the airplane until itís resolved.
Steve T.
RV-7, Lyc IO360
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Old 09-12-2022, 07:18 PM
FinnFlyer FinnFlyer is offline
Join Date: Aug 2018
Location: Bell, FL
Posts: 705

There is no "spring".

Trim servo (motor) directly sets the elevator trim tab via a push/pull rod. So you're fighting the trim tab.

Assuming your trim motor is the RC Allen (?) brand there really is only one way the motor moves: you apply + or - 12 volts to it.

You need to examine the trim switch and wires from it back to the motor.

If you have a servo speed or relay board between the switch and the motor, that could be the problem.

N214FL RV-4 Mazda 13B Renesis First flight 20 Feb 2021
N46AZ RV-3B Mazda 13B EFI -- Bought -- Flying
N993FL RV-3A Mazda 13B NA 575 hours

Last edited by FinnFlyer : 09-12-2022 at 07:23 PM.
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Old 09-12-2022, 07:25 PM
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greghughespdx greghughespdx is offline
Join Date: Nov 2011
Location: Aurora, OR
Posts: 1,293

Originally Posted by RV74ME View Post
Thatís a pretty serious issue. I would strongly suggest grounding the airplane until itís resolved.
I agree with this point.
Greg Hughes - Van's Aircraft
Van's web site | Instagram | Facebook
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Building RV-8A since Sept 2014
Dual AFS 5600, Avidyne IFD 440, Whirlwind 74RV, Superior XP IO-360
VAF build thread - Flickr photo album - Project Facebook page
Aurora, OR (EAA Chapter 105)

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Old 09-12-2022, 10:23 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 10,085

I agree as well.

One possible cause that I have seen previously is if you have an EFIS system that has auto trim enabled and the trim direction hasn’t been properly configured in the set up. The auto pilot decides it needs pitch trim in a particular direction but when it activated actual it gets trim in the opposite direction so it keeps trying. If nothing has been changed on the airplane recently before you bought it I would think this would not be the cause because it should have been detected previously.

Another potential cause is an intermittent short in the wiring from the stick or trim switch on the panel, depending on how your airplane is configured. The activation of most trim control circuits is by grounding a wire through a switch. Because of this, if there is a wire rubbing somewhere in its path through the fuselage and intermittently contacting ground that is enough to activate the trim. To troubleshoot this you should follow the wires from every switch through their entire path and look for any evidence of rubbing. Particularly where they pass sharp edges or corners of aircraft structure.
Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips,
ideas, etc. obtained from any post I have made in this forum.

Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR, EAA Technical Councelor
RV-6A (aka "Junkyard Special ")
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Old 09-12-2022, 10:58 PM
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boom3 boom3 is offline
Join Date: Dec 2006
Location: Sumner, WA
Posts: 734

I agree that it should be investigated thoroughly.

However, I will share a quick story about interference with my D120 EMS with capacitance fuel tank sensors. When I fly under the Charlie airspace on the West side of KPDX, between the foothills and downtown, I receive some radio interference that causes my left fuel tank fuel level indication to go to zero. The first time it happened years ago, it scared the bejesus out of me. Boost pump on! Switch fuel tanks! What the heck just happened? In less than a mile it went away. Pretty scary as there really isn't a great place to land.

After landing, I immediately checked everything, but wasn't able to find anything wrong. Flew the plane again, no problems, ever, well until I flew the same route in the same spot.

It still does it today, in fact it just happened again the other night after a burrito run to Mulino. (In the same location as usual.)

1464 hours and it has never happened anywhere else.
Jeff Bloomquist
Sumner, WA

1480 Hours
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