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  #41  
Old 09-19-2022, 02:33 PM
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markserbu markserbu is offline
 
Join Date: Apr 2018
Location: Tampa, Florida
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Quote:
Originally Posted by Tom Martin View Post
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.
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  #42  
Old 09-19-2022, 08:38 PM
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RV7A Flyer RV7A Flyer is offline
 
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Location: US
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Quote:
Originally Posted by markserbu View Post
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?
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  #43  
Old 09-20-2022, 08:37 AM
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Snowflake Snowflake is offline
 
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Quote:
Originally Posted by markserbu View Post
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.
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  #44  
Old 09-20-2022, 11:17 AM
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markserbu markserbu is offline
 
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Quote:
Originally Posted by Snowflake View Post
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.
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  #45  
Old 09-20-2022, 02:59 PM
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RV7A Flyer RV7A Flyer is offline
 
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Originally Posted by markserbu View Post
Ah, excellent point!
And by my count, the 5th or 6th time in this thread that you've been told that.
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  #46  
Old 09-20-2022, 03:25 PM
Mistfarer Mistfarer is offline
 
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Quote:
Originally Posted by markserbu View Post
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?
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  #47  
Old 09-20-2022, 03:37 PM
gasman gasman is online now
 
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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.
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  #48  
Old 09-20-2022, 03:48 PM
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Originally Posted by Mistfarer View Post
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.
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  #49  
Old 09-20-2022, 03:48 PM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Louisville, KY
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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.
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  #50  
Old 09-20-2022, 03:49 PM
Blw2 Blw2 is offline
 
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Location: Saint Johns, FL
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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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