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Run away trim on my RV-9

Coryb

Member
I know this has been talked about. In August I was flying across the country and after a takeoff I was leveling off for cruise and my trim went to full down. It would not return to level after trying on both the right trim buttons (buttons are on the sticks) and the left. I slowed down and was able to land without much of a problem. When I was on the ground and playing with the trim buttons it starting working normally. I could not duplicate the problem after it returned to normal function. Since I was almost to my destination I continued on without any problems. (maybe not a good decision but it's the one I made)

I have tried to duplicate the problem every time I fly. After about 25 more hours of flying it happened 2 other times. One time under the same scenario and then yesterday while on the ground. Both times the trim worked normally after playing with it (pushing trim buttons repeatedly). I didn't need to land.

I purchased my RV-9 a year ago with 1100 hrs. I have duel Dynon Skyview screens and Trutrak autopilot. The problem does not seem to be a short, since it only happens when I use the trim buttons. So it must be a relay or the button on the stick.

How do I know? Where would I start? Where can I find the rely for the trim?
 
Look at the -9 assembly drawing for the elevator trim instructions first. The elev trim relay and servo motor are most likely in the elevator where the trim adjust linkage connects to the trim tab. Usually can be accessed/removed via an access panel in the horizontal stabilizer, the servo may be attached to the access panel.

I am going to guess that it may be a bad trim switch in one of the sticks that closes the circuit.
 
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One reason why every RV (and every aircraft) must have a switch or whatever readily available on the panel to disconnect power to the elevator trim.
 
The Dynon HDX can be set up to stop a runaway after several seconds of continuous trim commands. You can take your time at that point removing power.
 
Agree

One reason why every RV (and every aircraft) must have a switch or whatever readily available on the panel to disconnect power to the elevator trim.

I have my trim circuit breaker right in front of the pilot near the center so I can pull the breaker with my right hand; I figure with run away trim, my left hand would be busy trying to keep from crashing.
 
Another Option

The late model Cessna 182 (G1000 era) has TWO trim switches in the pilot yoke. The idea is that BOTH have to be pressed in order to activate the trim. Being side by side that's easy enough. Point is, if one switch sticks or shorts, you don't get a trim run away.

Preflight procedure is to activate each individually and confirm that when using only one switch the trim doesn't move.

Easy, simple. No trying to spot the right breaker while fighting the plane. What I did in my stick.
 
The Dynon HDX can be set up to stop a runaway after several seconds of continuous trim commands. You can take your time at that point removing power.

Not in this install as the OP is not using the Dynon Autopilot servos or Autopilot control module.

What is unknown is what trim relay switching is in the OPs plane. This would be the first thing I’d look at. If it is a roll your own, then I suggest replacing it with the nice Safety Trim product.

Carl
 
One reason why every RV (and every aircraft) must have a switch or whatever readily available on the panel to disconnect power to the elevator trim.

I disagree that it "must" be present.

All you need to do to fly an RV with a completely nose-up or nose-down trim is *slow down*. It's not ideal, but it need not be a major issue, either (and you should have tested this during Phase I or when you acquired the aircraft).
 
Limit Travel?

My trim goes about 23 degrees up and 25 degrees down. I have never used it more than maybe 5 or 6. I would like to limit the travel. I have it set to only run 3 sec on my G3x per push but I see no reason I would ever need full trim either direction. I also have the VPX and going to the menu to shut power is not a quick action.

Any ideas on how to limit the travel?
 
The late model Cessna 182 (G1000 era) has TWO trim switches in the pilot yoke. The idea is that BOTH have to be pressed in order to activate the trim. Being side by side that's easy enough. Point is, if one switch sticks or shorts, you don't get a trim run away.

Preflight procedure is to activate each individually and confirm that when using only one switch the trim doesn't move.

Easy, simple. No trying to spot the right breaker while fighting the plane. What I did in my stick.

Only $420….
Not really necessary IMO if you install an high quality Otto switch, good relay deck and most importantly good wiring!
 

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My trim goes about 23 degrees up and 25 degrees down. I have never used it more than maybe 5 or 6. I would like to limit the travel. I have it set to only run 3 sec on my G3x per push but I see no reason I would ever need full trim either direction. I also have the VPX and going to the menu to shut power is not a quick action.

Any ideas on how to limit the travel?
The short answer - don’t.

Van’s established the trim tab travel range for a reason.

Carl
 
Trim

I just can't stand having a spare switch. Had to fill it with something, so...:cool:
 

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Safety trim anyone?

There is also this device. I have it installed in my 10.
It does several things to keep what happened to your trim from happening.
Limits trim activation to 2 seconds.
Via a switch in the panel, you can either turn trim on or off or reverse it.
From what I see your Dynon might already have some of these features, like
selective time limit on trim actuation.

https://sarasotaavionics.com/avionics/safety-trim
 
“I also have the VPX and going to the menu to shut power is not a quick action“

VPX has an override built in. Opposite input stops the trim and at that point you can use the soft keys on the G3X or just SLOW down and you can land the airframe full trim up or down. (Slowly take it up a few thousand feet (Also slows the airframe while gaining safety)) Vans designed it this way. Practice this both full trim up and down. It actually flies well and since you are focused probably makes a better landing. Watch your numbers.
 
Runaway trim protection only works with the Dynon AP Panel

The Dynon HDX can be set up to stop a runaway after several seconds of continuous trim commands. You can take your time at that point removing power.

Since you have the SkyView system, I would recommend you change your servos over to the Dynon servos, the mounts are interchangeable with the Dynon mounts, and add the AP Panel, that way you will have protection from a runaway trim, and also gain auto trim inflight.

Brian
 
I like the idea of replacing my servos and AP with Dynon. I think that will be my next upgrade. I don't like the idea of pulling the breaker or have a cutoff switch. I think by the time you recognize the problem you would have full trim and no way to correct it. I have been able to correct it 3 out of 3 times. That's a good percentage. I think I will try to locate the relay and check it. From what I understand from jeffw@sc47 the relay is most likely in the horizontal stabilizer.
 
I. I think I will try to locate the relay and check it. From what I understand from jeffw@sc47 the relay is most likely in the horizontal stabilizer.

Trim relay is usually underneath the seat pan or behind the panel, never seen one in the horizontal.
Running trim switch wires and power to the tail would make no sense.
 
I like the idea of replacing my servos and AP with Dynon. I think that will be my next upgrade. I don't like the idea of pulling the breaker or have a cutoff switch. I think by the time you recognize the problem you would have full trim and no way to correct it. I have been able to correct it 3 out of 3 times. That's a good percentage. I think I will try to locate the relay and check it. From what I understand from jeffw@sc47 the relay is most likely in the horizontal stabilizer.

Cory,
I recently upgraded to the Dynon with Dynon servos and the A/P Panel and I love it. I haven’t gotten it calibrated perfectly so it’s a little more jittery than my old TruTrak but I’m confident I will get it there. Love the setup though.

The other thing I’ll add is strictly technique. Trim should be actuated in clicks. Don’t push and hold the trim button to trim the airplane. Put the airplane where you want it and trim off the pressure. Don’t use trim to fly the plane. Again, technique.

Rest of luck
 
trim full up on takeoff

I disagree that it "must" be present.

All you need to do to fly an RV with a completely nose-up or nose-down trim is *slow down*. It's not ideal, but it need not be a major issue, either (and you should have tested this during Phase I or when you acquired the aircraft).

Try a takeoff with full up trim and tell us what it is like. wow!!! Ask me how I know. Dale F. Field
 
Try a takeoff with full up trim and tell us what it is like. wow!!! Ask me how I know. Dale F. Field

I know. Because I tested full up and down trim simulated (at altitude) take-offs.

The huge force you have to exert on a full nose-up trimmed RV as it accelerates ought to tell you something. Like, abort, maybe.

In any case, what I wouldn't be doing in such a case is to start fumbling with breakers or fuses or switches to try to disconnect the servo. It's *already* run to the hard stop, what good would cutting power to it do?
 
up trim takeoff

I have had a full up trim on takeoff ,caused by me waiting for a training pilot and instructor to finish a delayed takeoff discussion. I was number 2 for departure and impatiently waiting rested my forearm on the stick. causing the trim to go full up from a neutral setting and as I lifted off the nose went about 60 degrees skyward. Needless to say, but I was greatly surprised in the lively performance of my steed! Roughly 50-100 lbs of force to regain control of the
bird. You can not remember the exact pressure one needs to do this task, but I can assure you it gets your attention. What to do? Fly the plane. I glanced at
the trim gauge while forcing the stick ahead and pressing the down trim and at the time it seemed about an hour went by before the pressure was reduced
to normal takeoff conditions. Do not try this for fun unless you have another
pair of underwear with you. Thank you for your attention.
 
Maybe add this item to the end-of-the-runway final check, whichever version you use. "Trim set for take-off", and a visual verification of the trim indications on the EFIS.
 
Both the Garmin and Dynon systems as I mentioned before can be set to lock out the trim after several seconds of continuous operation. It’s a excellent safety feature and would prevent most of the situations discussed here.
 
trim up

My point is full trim up is a major deal. been there and done that. Stick forces are barely felt until you rotate and then there you are, luckily some people never make an err.
 
"I have had a full up trim on takeoff ,caused by me waiting for a training pilot and instructor to finish a delayed takeoff discussion. I was number 2 for departure and impatiently waiting rested my forearm on the stick. causing the trim to go full up from a neutral setting and as I lifted off the nose went about 60 degrees skyward. Needless to say, but I was greatly surprised in the lively performance of my steed! Roughly 50-100 lbs of force to regain control of the
bird."

Exactly the reason I do not have trim on the stick! I've been there and done that....
 
The Dynon HDX can be set up to stop a runaway after several seconds of continuous trim commands. You can take your time at that point removing power.

That sounds great if the trim runaway isn't a software fault that also prevents that feature from working.

(the thing about software is that once a bug has been triggered, the entire system is in an indeterminate state, which means you can only hope that the rest of it works the way it's supposed to. Which gets you to the hallowed ancient SRE proverb, "`Hope,' is not a strategy.")

Software protection is nice, but give me a switch which physically disconnects the servo any day.

- mark
 
Both the Garmin and Dynon systems as I mentioned before can be set to lock out the trim after several seconds of continuous operation. It’s a excellent safety feature and would prevent most of the situations discussed here.

However, the pilot must remember this important point you pointed out during a stressful situation. In other words, when the trim goes crazy the pilot will tend to hold the trim switch opposite direction to counteract stick forces, but in "holding" the button more than a couple of seconds the runaway timeout safety will disable trim. As the panicked pilot sits there with a death grip on the trim switch expecting something to happen nothing does since the actuation has timed out.
If instead the pilot were to pulse the trim switch it would respond and work properly. But in that permanent panicked death grip switch activation it puts things in a tight spot with no trim whatsoever. I have personal experience during an post-installation flight in an RV-8 where the trim was configured by the shop for opposite than normal operation. I was in the rear seat and the pilot in front was panicked and holding the trim switch instead of pulsing it and thereby disabling the trim servo completely. It took both of us pushing forward on the stick to over-ride the trim force to return to the airport and make a non-standard yet successful landing. That's when the post-flight investigation found not only the installation error of opposite trim actuation but more importantly the important point about the runaway trim time out disabling the best efforts to solve the situation.
 
One reason why every RV (and every aircraft) must have a switch or whatever readily available on the panel to disconnect power to the elevator trim.

Previous owner had this switch installed: Trim master On/ Off / Reverse.
Never have had to use it, but it could come in handy!
 

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The Dynon SkyView AP system has a 5 second trim control time-out feature. 5 seconds is too long in this situation, at least in my RV-8. I'd rather have a 3 second time-out based on testing it.

But note that the Trim Motor Control Safety Features, including the trim control deactivation after 5 seconds, is only available if the SV-AP-PANEL is installed.

Ref: Page 18-5, SkyView System Installation Guide – Revision AJ.

"Trim Motor Control Safety Features
...
• If a trim control button is pushed for more than five seconds (or longer, such as a stuck button), trim control on that axis will be temporarily inactivated until the button is first released.
..."

 
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I have not tested the feature in the 8 but in the F1 Rocket the aircraft remains controllable after the time out especially at TO and landing speeds where it’s almost a non event.
In the case of the reversed trim that’s a bit of a unusual situation that should have been caught on the preflight after trim system maintenance. In a runaway if the feature activated you would be able to retrim back to neutral with one activation. Pulsing the trim is always a good idea.
 
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