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Crazy full-up elevator issue...

markserbu

Active Member
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 whatever...is 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!
 
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.
 
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.
 
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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!
 
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.
 
That’s a pretty serious issue. I would strongly suggest grounding the airplane until it’s resolved.
 
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.

Finn
 
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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.
 
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.
 
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.
 
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.
Pete
 
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.:D 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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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...
 
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.
 
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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.

I’m 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.
 
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)
 
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.
 
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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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.
 
Mark’s plane has an xCruse and the electric elevator trim is not controlled by the AP. After he recovers he finds the elevator trim is in the full up position. Not sure why it’s happening but maybe it has to do with what was mentioned regarding the problem with the blackhawks. Is it possibly 5G interference which I think came online earlier this year?
 
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There is an interesting Lockheed facility in the vicinity. I don’t know what it is. Probably just a coincidence thing. It’s ‘pointed’ at DEARY…
 

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Mark’s plane has an xCruse and the electric elevator trim is not controlled by the AP. After he recovers he finds the elevator trim is in the full up position.

OK, so the A/P is on, 2-axis mode, holding altitude via the pitch servo, right?

So what is he "recovering" from?

ETA: Never mind, I think I get it...something ran his pitch trim full nose up while on A/P, the A/P couldn't hold it anymore and disconnected, at which point the plane shot upwards, as one would expect.

The original post was just poorly written, that's all...I had trouble figuring out how many times this happened, what was going on with the A/P, etc. I see it now.

As to *why* the trim tab was run to full or nearly full nose down, heck, I dunno...your guess is as good as mine, but I'd look for simple things first: chafed wire or bad switch.
 
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OK, so the A/P is on, 2-axis mode, holding altitude via the pitch servo, right?

So what is he "recovering" from?

He is recovering from a runaway trim condition. The trim in some aircraft can overcome the strength of the autopilot servos and cause them to disengage. We don't have a full enough description of symptoms to know if this is the case, but we do know the pitch trim ends up being in the full-nose-up position. Since the autopilot and trim systems are not interconnected in any way we can remove the autopilot from our consideration of potential causes.

I also have an XCruze - or rather Trutrak Vizion 385 autopilot and Ray Allen electric trim motor in my aircraft. I have very carefully wired my aircraft, paying particular attention to shielding, BUT the 5-wire trim cable is not shielded, thus I am very, very wary of any potential for externally-induced trim movement.
 
He is recovering from a runaway trim condition. The trim in some aircraft can overcome the strength of the autopilot servos and cause them to disengage. We don't have a full enough description of symptoms to know if this is the case, but we do know the pitch trim ends up being in the full-nose-up position. Since the autopilot and trim systems are not interconnected in any way we can remove the autopilot from our consideration of potential causes.

I also have an XCruze - or rather Trutrak Vizion 385 autopilot and Ray Allen electric trim motor in my aircraft. I have very carefully wired my aircraft, paying particular attention to shielding, BUT the 5-wire trim cable is not shielded, thus I am very, very wary of any potential for externally-induced trim movement.

Yeah, I got it...it was just a badly written description...even confused some others when he talked about pulling the trim motor CB...it read like he pulled it *before* the next event, and that had no effect. (see post #13 para 2).
 
There is an interesting Lockheed facility in the vicinity. I don’t know what it is. Probably just a coincidence thing. It’s ‘pointed’ at DEARY…

That's an odd-looking building and I can't find any information on it. As much as I love a good conspiracy theory, I think that building is shaped more as a firing range than some odd antenna. Next time I fly back from Melbourne I'll definitely have video rolling and figure out everything I can about the "event". At least pinpoint the location.
 
I'm going to make another observation here that is going to seem harsh, but is not intended to be, so bear with me.

It seems that the OP wasn't adequately knowledgeable about his A/P and trim systems. He didn't know that the A/P had no autotrim function, so doesn't "know" anything about the trim settings, and he didn't know that his trim system used a RAC motor with a pushrod but instead thought there was a spring. And, he probably didn't realize the amount of force necessary in an RV to overcome a full pitch up or down trim setting at cruise speeds, nor the proper recovery from such a scenario.

All of this points, IMO, to a lack of training in the type of aircraft, the specific aircraft, and the aircraft systems.

I don't mean to bag on anybody, but I've been thinking about this a bit. His trim went to full nose up for whatever reason TBD while the A/P was on during cruise, and the A/P did precisely what it was designed to do, which is to disconnect the pitch servo when it exceeds the torque limit. The resulting nose up could have been easily corrected by simply trimming back to level flight (perhaps after slowing down), but instead he fought the system for some time, even going so far as to pull the breaker which then *removes* the ability to restore trim (it had already run away to the limit, it can't go any further, so pulling the breaker is ineffective at best).

Granted, it was a new airplane to him, but I suggest some time with an instructor and some serious digging into the avionics manuals is in order, along with diagnosing the root cause here.

Sorry if this sounds harsh...as I said, it's meant to be helpful, not critical.
 
Antenna test range.

I assume you're just kidding around? I mean, the "event" has happened to me 3 or 4 times in the same location so it's obviously not a coincidence. On the other hand, it could be a time thing...climbing Westbound out of Melbourne for a given amount of time would tend to put me in the same area. But then why doesn't it happen climbing out of every other airport?
 
5G comms are way too low power to induce currents powerful enough to run a trim motor, even if the wires were tuned to the right length to receive the signals.

Well of course not run the motor itself, but possibly induce current in the circuitry along the way? Looking at the data for the Ray Allen data sheet it's pretty basic stuff outside the servos...just wires and switches. The system uses a 1 amp fuse, though, and that tells me that it's not a very high current system outside the box. Maybe here's where our 5G or other RF can wreak some havoc?
 
5G, from what I read, is around -70 dBm. Seems like a pretty big reach to your conclusion.

You need a schematic of the circuit. Then try basic troubleshooting first...check all the wires for chafing, check the switch. Chances are something is grounding, thus completing the circuit and activating the servo.

Think horses, not zebras.
 
Well of course not run the motor itself, but possibly induce current in the circuitry along the way? Looking at the data for the Ray Allen data sheet it's pretty basic stuff outside the servos...just wires and switches. The system uses a 1 amp fuse, though, and that tells me that it's not a very high current system outside the box. Maybe here's where our 5G or other RF can wreak some havoc?

The only possible way that an RF signal could affect the trim motor would be if you have a speed control board between the trim switch(es) and the motor.

Have you looked at the schematic? Is there such a board? The board would have an airspeed input (pitot & static lines) or at least a flap position input. The idea of trim motor speed control is that it takes much less trim change input at high airspeeds than at low airspeeds. So much that at cruise speeds I just flick the trim switch whereas at low speeds I can actually press the switch for a second or two.

Finn
 
The only possible way that an RF signal could affect the trim motor would be if you have a speed control board between the trim switch(es) and the motor.

Have you looked at the schematic? Is there such a board? The board would have an airspeed input (pitot & static lines) or at least a flap position input. The idea of trim motor speed control is that it takes much less trim change input at high airspeeds than at low airspeeds. So much that at cruise speeds I just flick the trim switch whereas at low speeds I can actually press the switch for a second or two.

Finn

The plane doesn't act like it's got a speed control board and there isn't one on the schematics. I'm starting to discount the RF theory anyway. I like the theory of it failing at the same elapsed time into the flight each time, though it doesn't explain why it always happens on the return trip from KMLB.
 
Well of course not run the motor itself, but possibly induce current in the circuitry along the way? Looking at the data for the Ray Allen data sheet it's pretty basic stuff outside the servos...just wires and switches. The system uses a 1 amp fuse, though, and that tells me that it's not a very high current system outside the box. Maybe here's where our 5G or other RF can wreak some havoc?

Ok. It takes about 1/2 an amp @12V to move the motor Show me a method for inducing 1/2 amp of current into a wire from one mile away. That is a seriously strong signal I am quite confident that cell phone transmissions won't do that. Otherwise all sorts of motors and relays would constantly be activating in your car as you drove down the road. The relay would take about 30-50 milliamps to activate and send power to trim motor. A much easier task, but still a serious amount of energy to induce from thin air. I promise you that 5G wouldn't exist if it could induce enough current to activate a relay from a mile away. Just imagine the havoc to the world at large.

I am based at a Chicago airport about 4 miles from the WGN AM radio antenna. It is 700' tall and broadcasts at 50,000 watts and can be picked up in Iowa and some days Nebraska. Never had any uncommanded motor or relay activation. That antenna is 6 miles from the worlds busiest airport (ok, sometimes second busiest) and directly under the landing path. The landing jets fly directly over it at only about 1000' separation. Cells phone towers transmit at around 5 watts.

Larry
 
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The plane doesn't act like it's got a speed control board and there isn't one on the schematics. I'm starting to discount the RF theory anyway. I like the theory of it failing at the same elapsed time into the flight each time, though it doesn't explain why it always happens on the return trip from KMLB.

Have you actually examined all of the wiring and switches? I mean, pulled inspection panels and bulkheads and whatever and gotten your hands on the wires themselves, looked carefully at them, inspected the servo motor, checked the connections, done pull tests on the crimped terminals, etc.?

If not, it sure sounds like it's time to do that before hypothesizing unlikely causes.
 
A similar thing happened to me many years ago. I was flying along using a Sectional Map trying to look for landmarks etc, when I was suddenly pointing up in the air. The map that I was using had rested on the stick mounted trim switch, causing the abrupt climb.
Is it possible that you were using an iPad that contacted the trim switch? It really does not take much effort.
 
I was thinking along the lines with Tom as I rest my hand on top of the Ray Allen grip with trim. On occasion I hit the trim switch.
Then I remembered that if you hit the trim it turns off the AP and sends a tone. At least it does with my avionics.
 
I assume you're just kidding around? I mean, the "event" has happened to me 3 or 4 times in the same location so it's obviously not a coincidence. On the other hand, it could be a time thing...climbing Westbound out of Melbourne for a given amount of time would tend to put me in the same area. But then why doesn't it happen climbing out of every other airport?

At your home airport, set up your pitch trim in flight to be neutral for cruise flight.
Then land with no trim changes. Disconnect the wires at the trim servo.

Then run leads back to the front end from the leads you just undid.

Fly over the same area at the same altitude and monitor for voltage with a micro tester.

To be extra OCD, Buy a spare trim servo of the same make and put it in the cockpit beside you and see if it moves with no input on the same flight.

To be completely OCD, Run leads from the servo on the elevator to the front end with a second microtester, monitor for energy harvested by the servo in the elevator.

If there IS radio wave energy that strong in that location do you really want to expose your AC and yourself over and over?
 
A similar thing happened to me many years ago. I was flying along using a Sectional Map trying to look for landmarks etc, when I was suddenly pointing up in the air. The map that I was using had rested on the stick mounted trim switch, causing the abrupt climb.
Is it possible that you were using an iPad that contacted the trim switch? It really does not take much effort.

Nah. My iPad is strapped to my leg and I never have anything in the cockpit in front of me that would rest on the stick. Also, this happens FAST...not like simply pushing the trim button. Anyway, over the weekend I made it to Petit Jean from Tampa with a stop on the way, and I did a couple of flights while up there. Stopped 3 different places on the way home. The whole time I had the elevator trim breaker pulled, only engaging it briefly to set the trim a few times. Not a single incident. I will start doing some troubleshooting of the system as I get time.
 
The whole time I had the elevator trim breaker pulled, only engaging it briefly to set the trim a few times. Not a single incident. I will start doing some troubleshooting of the system as I get time.

I'm not sure what you learned from this, though. That an electrical device won't operate at all without electricity?

Can you post the schematic for your system?
 
Nah. My iPad is strapped to my leg and I never have anything in the cockpit in front of me that would rest on the stick. Also, this happens FAST...not like simply pushing the trim button.
The pitch change in this failure mode isn't because the trim is moving, I agree that would be too slow and you'd catch it. The pitch change is because the trim moves and the autopilot corrects for it... Right up until it can't correct anymore and disconnects. When it disconnects, you get the sudden pitch change.

It's easy to test on your next flight. Engage autopilot, and then slowly increase your nose up trim and see if the autopilot corrects for it, then continue until the autopilot disconnects.
 
The pitch change in this failure mode isn't because the trim is moving, I agree that would be too slow and you'd catch it. The pitch change is because the trim moves and the autopilot corrects for it... Right up until it can't correct anymore and disconnects. When it disconnects, you get the sudden pitch change.

It's easy to test on your next flight. Engage autopilot, and then slowly increase your nose up trim and see if the autopilot corrects for it, then continue until the autopilot disconnects.

Ah, excellent point! I will definitely try that, thanks. And I'll have a video camera pointed at the autopilot the next time I climb out of KMLB.
 
the "event" has happened to me 3 or 4 times in the same location so it's obviously not a coincidence.

Maybe the location is a red herring. Is it possible that the "event" takes place after a set amount of time? If you take the same route over and over, that would explain why it would seem to happen at the same place.

In any case, I am very surprised that this plane is still deemed airworthy. Surely there are laws in US that would stop people from flying a dangerous aircraft? If there were to be an accident (God forbid), would an insurance cover anything once they realize the plane has flown whilst there was such a known issue?
 
I don't have an auto-pilot, but I do have a question.... How can you trim with the AP engaged? It would be like you, the pilot, trimming while the co-pilot is trying to keep the aircraft level.

Not knowing what I am talking about, I might think that disabling the trim power during the time the AP is active (and vise versa) would be no brainer.
 
Maybe the location is a red herring. Is it possible that the "event" takes place after a set amount of time? If you take the same route over and over, that would explain why it would seem to happen at the same place.

Years ago when I was commuting to work, almost every day as I passed this house heading out of town I would hear something hit the underside of my car.

It took a while, but I figured it out..... I live on a gravel driveway and the tire tread was prefect for a rock to stick and not release till about 40 mph.
 
Basic fixed stab, trim tab on elevator, autopilot servo on elevator pushrod- autopilot tells you to trim nose up or down to offload long period servo torque.

Exceed or don't heed or shear screw/clutch releases- pitch excursion.
 
My dad had a gremlin in a pickup truck that seemed to be geographic based.
His engine would stutter....only when approaching Southbound on a certain overpass bridge.
It happened many times over a long period of time....

it did not seem to be related to length or time of his drive.... I was thinking maybe it happens x number of minutes into the drive and his route was common by habit. Nope.
It did not seem to be related to slope of the road since it never happened at other bridges.
not weather related
never happened anyplace else
etc.

We spent a lot of time trying to think of less obvious variables other that that particular place.
There is a Corning factory there, and THAT was the only thing we kept coming back to.

If I recall correctly it turned out to be a fusible link but I forget in which circuit it was. Nothing to do with Corning, or that road, or anything about that place.

Spooky.
 
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