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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: 390

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: 6,921

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: 1,531

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 09-13-2022, 06:32 AM
Ezburton Ezburton is offline
Join Date: May 2017
Location: MD
Posts: 123

A few thoughts...

I'd contact the previous owner and ask if it ever happened to him. If it hadn't, maybe something was touched during the pre-buy and might help narrow your search.

Pressing the trim switch should have disconnected the autopilot. If you still had to pull the breaker, then something might be wrong with the servo or relay.

If you can't find anything on the ground, you may want to change the setting so that the autopilot doesn't control trim. You'll have to trim when it tells you, but it won't be commanding unexpectedly. If it happens again, then it's the trim circuit and not the autopilot.
Velocity SEFG
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Old 09-13-2022, 07:53 AM
Southern Pete Southern Pete is offline
Join Date: Jul 2015
Location: England
Posts: 234

If this happens at the same location I would look into electromagnetic interference with the trim system. Is there a large emitter nearby that could induce sufficient current in either the autopilot or trim systems to cause this behaviour? It would have to be a powerful transmitter - several kW - probably at a low to mid MHz frequency. You may have to completely re-wire your trim system.
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Old 09-13-2022, 08:17 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 8,080

While the symptoms certainly sound like some kind of ground interference, I am struggling to see how that can happen via trim, as the trim system is dead simple. 12V through relays to a dc motor. That kind of stuff is mostly immune from anything but VERY STRONG magnetic interference. I don't believe the D100 has auto trim, so rules out that. Do you possibly have an AP different from the D100? Trutrak has an auto trim module. Are you positive it is not the autoplilot commanding the up elevator? It is more plausible for microprocessor based systems to strangely react to this kind of interference.

You mention that pulling the trim CB had no effect and that kind of rules out the trim system as the culprit, especially if the problem resolves without having to re-adjust the trim. I am geussing that the AP servos are commanding the up ele. However, it is probably not the D100 itself doing it, as you said disconnecting the AP did not solve it. Those servos have microprocessors with their own software and rely upon electronically transmitted instructions from the D100, so not immune from interference. The dynon does not use shielded cables for their AP system and therefore a bit more vulnerable than other systems that do. If a message got garbled due to interference, the message may have looked like a ele up message and the servo responded as it thought it should. This further explains why the issue magically resolves itself without any pilot input. Next time pull the CB for the servos and see if it goes away. They cant command anything without power. You can also probably lower the torque setting on the servos to make it easier to over power the servos. The manuals give instructions for setting torque to be just enough to control the plane in turbulence but not enough that you cannot over power it. Most don't do this and instead set it to a high value, not realizing the consequences. Americans fully subscribe to the more is better mantra. Remember that these servos are universal and need to be powerfull enough for large planes and without proper setup can be too strong for little planes. That is why they give you variable torque configuration.

This kind of interference is very real. Decades ago in the telecom industry, we struggled with strange, mostly unrepeatable problems at a customer who made pool liners with some kind of ultrasonic welders. They were under the Ohare landing path and having constant battles with the FAA. We put ferrite chokes on everything and still couldn't make it work reliably. They had to go back to analog stuff. They had a faraday cage surrounding half of the plant. Remember hospitals banning cell phones? While they weren't the cause it was known to be some kind of interference affecting microprocessor based equipment.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 09-13-2022 at 09:00 AM.
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Old 09-13-2022, 08:35 AM
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Snowflake Snowflake is offline
Join Date: Oct 2009
Location: Sidney, BC, Canada
Posts: 4,420

I'm aware of a similar situation a friend experienced... While looking at a paper map, it was resting on the hat switch on the stick, and slowly trimming nose up. As this was happening the autopilot was correcting with nose down input to maintain altitude. Eventually the trim outran the ability of the Autopilot to keep up, and the Autopilot self-disconnected... Leaving the elevator free, but with significant nose-up trim. Guess what happened.

In that case, trimming nose down re-centered everything and re-engaging the autopilot put them back on their way... with a little more diligence given to positioning their paper map...
Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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