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  #11  
Old 09-13-2022, 06:32 AM
Ezburton Ezburton is offline
 
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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.
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  #12  
Old 09-13-2022, 07:53 AM
Southern Pete Southern Pete is offline
 
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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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  #13  
Old 09-13-2022, 08:17 AM
lr172 lr172 is offline
 
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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.

larry
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Last edited by lr172 : 09-13-2022 at 09:00 AM.
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  #14  
Old 09-13-2022, 08:35 AM
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Snowflake Snowflake is offline
 
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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...
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  #15  
Old 09-13-2022, 08:48 AM
F1R F1R is offline
 
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Assuming it is not a fault in the trim switches & AP plumbing as a few posts have outlined above.

One method to hunt / ID a ground based EMI source. Set up your pitch trim for neutral in cruise flight. Then land at your home airport and disconnect the leads from the trim servo.

Run wires all the way back to the front end-from the leads that come from just before the trim servo and hook up a micro tester.

Then fly over the area in question and monitor for voltage.

EMI received can be very specific to the wiring in one airframe.

A great video presentation of some various issues considered in one EMI study in the F/A 18 can be seen here:

https://youtu.be/SmamWpTCuW8 At the 3 Min 45 second point there is a decent illustration of why certain wavelengths may effect specific airframes.


I am not sure I would go hunting for the energy source in flight. It might be something the FAA has better and safer methods to locate.

Last edited by F1R : 09-13-2022 at 08:57 AM.
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  #16  
Old 09-13-2022, 09:50 AM
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dpansier dpansier is offline
 
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As mentioned above by F1R, it is likely RFI. Ground based antennas are usually grouped on higher terrain near large cities to provide maximum coverage. Flying near these antennas expose your aircraft circuitry to very high levels of RF energy sometimes in excess of 100 kilowatts, levels like this can overpower the low signal levels used, for example, in the autopilot devices.

Iím sure there are several here on VAF that are involved as Defense Contractors that could go into detail on how easy it is to take control of unhardened circuitry with powerful levels of RF energy.
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  #17  
Old 09-13-2022, 10:45 AM
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vlittle vlittle is offline
 
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Quote:
Originally Posted by Snowflake View Post
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...
... That sometimes happens when the passenger is reading a book and the pax stick has a trim button on it. The stick is perfect for holding the book in a comfortable position.

My new airplane is putting the copilot trim on the panel for this reason. It can also be useful to have a copilot trim disable switch.

VV (A friend of Rob's)
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  #18  
Old 09-13-2022, 12:20 PM
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markserbu markserbu is offline
 
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Some very good discussion here! Ok, mistake #1 was my assumption that the elevator trim was handled by the autopilot servo. Now that I know it has a Ray Allen servo system I will start my troubleshooting there. It does seem that there's some interaction with xCruze autopilot, and so far the issue only occurs when the autopilot is on. EXCEPT for when it happened after I did an aileron roll, but I've just figured that out. When the plane came out of its condition inspection around a month ago I noticed that they'd turned on the AEP (Automatic Envelope Protection) that automatically engages the autopilot when the plane banks more than 40 degrees. I'm going to turn that back off. And now when I fly with the autopilot engaged and I'm trimmed out, I will pull the elevator trim breaker. In the meantime I will look for the electrical gremlins.
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  #19  
Old 09-13-2022, 12:46 PM
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RV7A Flyer RV7A Flyer is offline
 
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Can't you just turn pitch axis autopilot controls off, independently of roll?

I'm also a bit confused here, reading your first post. Are you saying this happens with the autopilot *on*, or off? And if on, why are you pushing the stick, as you stated?

I just don't understand what configuration your aircraft and systems are in when you say this happens...
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Last edited by RV7A Flyer : 09-13-2022 at 12:48 PM.
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  #20  
Old 09-13-2022, 04:30 PM
Canadian_JOY Canadian_JOY is offline
 
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Quote:
Originally Posted by lr172 View Post
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.
larry
Something like 30+ years ago we had a problem with the UH-60 Blackhawk fleet. Every once in a while one of them would be flying nap-of-the-earth and would suddenly dive straight into the ground, usually with the loss of the entire crew.

Investigations into the root cause took a long time... more crews were lost. Then it was discovered that radiated energy from microwave communications towers was causing the stabilator control go run to its full nose-down position. No way a Blackhawk in cruise flight could be recovered from this condition - no pilot would be strong enough to overcome the nose-down pitching force.

Sikorsky undertook an aggressive EMI-hardening program that required extensive rewiring with particular care paid to shielding and continuity of shielding. That was an expensive fix...but not nearly as costly as the value of the lives lost.
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