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Trim went to full up during run-up?

RV7A Flyer

Well Known Member
Patron
This has happened to me only a couple of times in about 130 hours now...

Sometime after engine start, during taxi or run-up, my pitch trim has run to full up, despite not having touched the trim button on the stick *at all*. I noticed it after run-up (during which I keep the stick full aft), because it's a checklist item (pitch/roll trim set).

It's a Ray Allen trim servo, and Ray Allen stick grip with push-button trim buttons, wired just as diagramed by RAC.

It's disconcerting, has never happened at any other time though...and I have no idea if I can reproduce it.

Ideas?

My initial (ridiculous) thought was the the aerodynamic forces from run-up are moving the tab, but that can't be, can it???

ETA: Trim position is indicated via Dynon Skyview, wired as diagrammed/per Dynon installation manual.
 
Yeah, that would be disconcerting. :eek: I've got the same electric trim and there's no way to over-power the drive motor by pushing on the trim tab. You'd simply break or bend the trim tab first. My guess is that you have something shorting to one of your trim wires.

Also, It's a stretch to think you could drive that motor by picking up inductance voltage from some other circuit, like strobes or something...meaning, if the wires from the trim tab are running next to the wires for your strobe, maybe (unlikely...but still) the trim motor is getting inductance voltage from the strobe.
 
Not the strobe...I have Aveo strobes, which require only a +12V power wire (no external high voltage line a la Whelen). So I don't believe that's it...and it has only happened on the ground, *sometime* during taxi and run-up.

I'm a little mystified, but I will pull the stick grip off and examine the switches inside it carefully, of course.

The switch is wired such that it is normally open (actually, connected to ground on the other pole), and closing it connects the trim to +12V, btw. The trim wires are all inside a single, shielded bundle, as well (IIRC, it's a 5-wire bundle).
 
I would look for a place where the wire from your hat switch has the insulation scraped off, and is contacting the airframe or the stick when the stick is full aft. Maybe near the base of the stick?

Edit: this is only if the trim hat switch goes to ground. Does not explain if the switch goes to 12 volts as stated, unless full aft pulls on the wires and shorts them out at the switch.
 
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I'll look there...but two things make me think that isn't the issue.

The wires from the stick exit and are enclosed in clear vinyl tubing, and if it hit were to ground to the airframe, then it's just...grounded, which can't cause the trim motor to move. It requires power to be applied, and the only way to do that is to close the switch.

I'm a bit stumped, other than an *extremely* intermittent short to +12V within the grip/switches somehow.

All other connections are well insulated, utilizing D-sub pins/sockets and connectors (a CPC below the stick) and multiple layers of heatshrink, plus vinyl tubing at any potential chafe points (into the elevator, e.g.).
 
I would look for a place where the wire from your hat switch has the insulation scraped off, and is contacting the airframe or the stick when the stick is full aft. Maybe near the base of the stick?

Edit: this is only if the trim hat switch goes to ground. Does not explain if the switch goes to 12 volts as stated, unless full aft pulls on the wires and shorts them out at the switch.

I've pulled the stick full aft a gazillion times with the system powered up, and nuthin'...but it's a good thought.
 
I've pulled the stick full aft a gazillion times with the system powered up, and nuthin'...but it's a good thought.

Perhaps the addition of engine vibration is what does the trick. And are you SURE it wasn't your thumb or some other object nudging the switch while you did your run-up checks?
 
That was actually my first thought :) ...that I fat-fingered the stick and fumbled around and caused it to actuate.

I *hope* that's what it was...at least, if that was it, I wasn't holding down the "Smoke" button :).
 
I once managed to apply nose up trim by inadvertently contacting a trim button at full back stick during a loop. It was, um, disconcerting to find myself pushing in the last quarter of the loop.
 
I have similar setup and have some issues with the switches in the stick grip not always engaging when I push them. Maybe yours have a problem of making intermittent contact? I like my stick grip but am disappointed in the action of the switches.
 
Over the years like Dan I had a couple of instances where up trim was dialed in at cruise while reading a chart. And that will get your attention real quick.

Trim button in my Rocket is on the throttle because of this.
 
I have "fat fingered" mine

As others have said, I too have fat fingered mine, and the buttons are pretty sensitive...gets your attention fast ,especially during acro. I have my flaps on the stick also, and have bumped trim while working flaps. with stick being held full aft, its possible your belt/harness or something may be bumping it?
 
Disconnect

I leave mine trimmed for cruise. Dual infinity grips with trim on both sticks.
I have a toggle switch before the relay box to de energize the trim system for safety.

After a couple of landings trimmed for cruise, you barely notice the nose heavy stick force at landing or take off.
 
That was actually my first thought :) ...that I fat-fingered the stick and fumbled around and caused it to actuate.

I *hope* that's what it was...at least, if that was it, I wasn't holding down the "Smoke" button :).

i hope u aren't flying with out solving this problem! have u ever put full trim in an airplane you were flying? this is a very dangerous situation!
 
i hope u aren't flying with out solving this problem! have u ever put full trim in an airplane you were flying? this is a very dangerous situation!

As I read the post on this subject I was thinking this same thing. Just in case please do not fly this aircraft until you have a hard find and it's fixed. As far as the issue sounds to me like its making ground while the engine is causing a vibration. Most likely shorting in the stick grip but could be anyplace.

Pat
 
As I read the post on this subject I was thinking this same thing. Just in case please do not fly this aircraft until you have a hard find and it's fixed. As far as the issue sounds to me like its making ground while the engine is causing a vibration. Most likely shorting in the stick grip but could be anyplace.

Pat

we were on a rescue mission back in he 60's in a KC130 tanker when we took off our rudder trim went full throw. what a ride that was!!!!! a C130 has a massive rudder and a rather large trim tab. rudder has lots of throw on the ground but as soon as u rotate, gear extend they activate a micro switch which cuts most of the rudder throw for obvious reasons. this lead to a tremendous amount of rudder throw to stay in the air. 2nd problem, like i said we were a fully loaded tanker and coun't land until we dumped tons of fuel. long story short we landed w/o incident. changed our pants and flew another day! try that in an RV with full elevator trim and the ending might not be the same.
 
This story sent a shiver up my spine as I had the elevator trim on my -36 model Bonanza freeze up on me last December in a slightly nose down angle during final approach into a relatively short field. The landing wasn't pretty, in fact it was darned ugly, but as they say, I was able to re-use the plane afterwards.

I've always been concerned about the powered trim on the RVs without a manual backup for this reason, but I've always rationalized it based on how light the plane and control forces are. This post re-opens my concerns.
 
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Just a guess, but maybe a dirty stuck switch, or perhaps some moisture such as sweat getting into the switch. Beyond that, a bad switch. Perhaps a short in the wiring, most likely at an intersection where the parts move and stretch the wires or connections.
 
Short or intermittent contact...

These servos need a 12VDC at a reasonable current to run the motor (that's all they are .. a DC motor and gear train) so any inductive / capacitive pickup would not have nearly enough energy to power the motor.

It's a short, a fat-finger, or possibly vibration closing the contacts to drive the motor. I haven't seen the inside of the stick grips, but I'd imagine that the switches are small and the contacts fairly lightweight and close together. A resonance driven by the proper engine RPM could cause the contacts to bang together ... possibly.
 
So let me reiterate, and add to this thread.

It can't be a *short* in the wiring harness after the stick, because there's *no power* in any of the wires unless the switch is closed. The trim servo wires are inside of a 5-wire shielded bundle, which is chafe protected at all points, and even if it *did* short, it would short to *ground*, not to power (+12V).

Thus, it *must* be the switch in the stick. Whether an intermittent short there, fat-fingering (and I checked the aft throw against seatbelts, harnesses, etc., and it doesn't seem possible for anything to contact it), is unknown.

This has happened twice in >125 hours, both times on the ground, never in flight.

I looked through the data from the Dynon last night, and it occurred *prior* to runup. I can see the ELEV POS (i.e., elevator trim position) going from -3 to +100 % over about 8 seconds, but none of the other parameters (RPM, MAP, volts, amps, nothing) changed...sitting at about 1050 RPM idling, just *prior* to going to 1800 RPM for run-up.

ETA: Re-checking my schematics...the ONLY connections to +12V in the stick grip at all are on the 4 trim switches. Everything else pulls some circuit to ground, rather than applying 12V.

Figure two events, 8s each, out of ~125 hours...that's 1.8 * 10^-5. That's an incredibly small failure rate. I'd expect an intermittent switch shorting issue to be *much* more common than that.
 
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One of the Reasons for 40 Hours

During my Navy flying career I had several incidents of runaway trim in the A-4 and A-7. I actually installed separate Aileron and elevator trim cut off switches in my -8A. After I had progressed most of the way through the 40 hour Phase 1 I practiced full trim nose up / nose down flight, takeoffs and approaches. I found that I could fairly easily handle any trim position with stick force and airspeed control. I have a forward CG so it might be a unique flying quality of my plane.

My most serious incident was first flight after painting reassembly - elevator trim was wired backwards. Should have caught it but didn't. Overrode the trim on takeoff then did the funky chicken PIO drill for a few seconds til I figured it out. Never considered myself to be in danger. Point is that - at least for my -8A - the trim can be overridden.
 
Think

That I'd consult with Ray Allen Co in addition to the above. If guessing, I'd change out the switch on the stick. If that didn't fix it, I'd remove all wiring that connects the stick and servo and replace it. If that didn't fix it, I'd replace the servo. Yeah, time and money but what's your life worth?
 
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Spoke w/ RAC...he concurs that no fault in the wiring after the stick can cause this, nor can any fault in the servo. Suggests (which I had planned on doing) taking the grip off and doing an inspection of the inside for loose wire strand, solder ball, etc. Also concurs that no RF or EM interference could cause this.

Gotta be something inside the grip...will take it off tomorrow or Friday and do an inspection, probably vacuum out the inside of it as well, check for wire motion during stick throw, etc.

Then, I'll probably just have to wait until it happens again to see if I can capture precisely when it is occurring.
 
No relays, only the pilot side grip has trim buttons. No enable/disable or trim reverse switches, or speed controllers, either.
 
Have you verified that the trim tab has actually moved as indicated or is it possible to be an indicator only problem?
 
Ray Allen

I have had the trim run on me several times on Ray Allen Grips. I found I had touched the button on the grip while doing aerobatics! I would come out of a roll with the trim off.
I no longer use Ray Allen Stick grips. The buttons are too small and too easy to touch inadvertently. I have a different kind in my RV8.
 
I had this happen to me today. During my run up I noticed both the pitch and roll trim were pegged. The pitch trim was full aft, nose up. The role trim was full right. I never use the roll trim. I just don't need it in my RV 8. My first thought was that I had an indicator problem. For a minute, I debated about going flying anyway. Fortunately I came to my senses and taxied back to the hangar. I really thought I had an indicator problem. So, I checked the fuse and the circuit breaker that supply power to the trim system. They were fine. Then, I noticed the elevator trim tab fully down. So I added power and ran the trim both pitch and roll back to neutral. Everything looked fine.

I have no idea how this happened. I think I would have noticed the elevator trim tab during my pre flight. But, I can't say for sure. My best guess is that it happened sometime between startup and right before I perform my mag check.

I have the CHT grips with a hat switch. No passenger today. I do not think I could have fat-fingered it long enough to run the roll trim full right.

I am not sure how to track this one down. I plan to spend a lot of time on the ground taxing around, wiggling the stick tomorrow to see if I can reproduce the problem. Not sure how long it will take before I feel comfortable flying again if I do not find the specific cause. I do have a trim CB I can pull if I catch the problem quickly enough in flight.
 
Maybe I don't see anything here about breakers because mine was the only one that had a non pull able trim breaker. The day after I purchased it I yanked that out and replaced with PWR that I could kill..
Just something to think about as you track down the fault in the switch.
 
I've experienced nose-up excess trim, ostensibly happening during runup, while flying my friend's RV-8 a few times. Turned out that I was inadvertently fat-fingering the buttons on top of the stick (Ray Allen #205 stick grip with 4 trim buttons, the rear one for nose up is quite touchy) while holding the stick back, when I could've sworn that I wasn't touching any of the buttons... but it turned out that I was. I changed my technique of how I held the stick back during runup and never had the problem again. Taking off with too much nose-up trim can really get your attention when the plane suddenly jumps off the runway and the nose points skyward way before you're expecting it to :eek:
 
Just to add to this newly updated thread that I started...I have not seen this problem since I reported it here, which would be about 60-70 hours of flight since that time.

I can only conclude that I must have inadvertently been hitting and holding the button during taxi with the stick held full aft. Being attentive to how I'm holding the stick grip during taxi seems to have "fixed" the problem.
 
i hope your problem is just because of of a fat finger.

for what its worth,when my servo finally bit the dust the trim went to full up or down position. dont remember which way though. :)
 
We had this happen on our -6 as well. We figured we must've fat fingered it while holding the stick back during the landing roll out. Since this has happened, we added "trim to neutral" on the After Start Checklist and again on the Line up/Final Items Checklist.
 
I will have a Safety Trim system for sale shortly as the new avionics panel going in has it integrated.
 
Well, I am going with the "fat finger" theory. I played with the trim on the ground for about 20 minutes including taxing around and doing a run up or two. Never could repeat the problem.

About a month ago I had not flown in a while and knew that it would be even longer before I got a chance to go out. So, I decided to take the plane out and taxi it around and do a couple of run ups. The winds were blowing way beyond my personal limits that day. So much so, in fact, I felt the need to position the ailerons and elevator accordingly; in this case I held the stick in the 4:30 position. I remember my arm getting tired and that I moved my hand around on the stick quite a lot; including placing it on top of the stick at times. My theory is that when I did this I was unknowingly activating the hat switch. If you think about holding the stick in the 4:30 position, if you did hit the hat switch you would be calling for nose up trim and right aileron. That is exactly where I found the trims set. So, I think all is fine; but I will keep a close eye on this. As part of my preflight, I always take specific note of where the autopilot and trim circuit breakers are. I'll pay even more attention to this part of the checklist in the future; even practice reaching quickly to pull the trim CB.

I appreciate all of the replies and suggestions!
 
Late to the party, Tony, but I had the exact same issue when I first began flying my 7A. Cruising along and all of a sudden a huge nose pitch up. I swore my trim was running away. Low and behold, it hasn't happened in the last year - once I trained my hand to grip the stick as not to interfere with the hat switch. Checking trim is one of my last before take-off checklist items, as on the ground I hold the stick from the top and have a tendecy to bump the switch.
 
One reason my pitch trim rocker is on the panel beside throttle, right beside a push to enable button. No power goes to trim servo with switch failure or accidentally pressing rocker switch. I think I adjust trim on takeoff, after level off, descent and landing. PTT and AP disc on the stick, 4 wires. Simple.
 
reverse switch

In my RV-10, the elevator trim is hard to overcome in case of a runaway trim. I was always thinking about this possible fatal failure.
Last year, i installed a double pole switch with 3 position ( normal- off- reverse) for the direction of the servo. So if i have a runaway trim, i can reverse it and put it to off when the trim is on a manageable position.
I sleep better now. It only cost the switch and some wire.
 
Safety Trim system

Safety Trim is an excellent system to prevent any of the scenarios mentioned
previously.
It features a 2 second limit on actuation, even if fat fingers accidentally actuate trim, it'll stop trim travel.
It also features an on/off/reverse button where you can disable trim or reverse
to neutral in case of runaway trim.
It also comes with an optional 2 speed trim sensor built into the pitot/Static
system and will automatically reduce trim speed at a preset airspeed for more responsive trim at low speed and reduced trim speed at cruise.
It addresses all the problems that posters have mentioned except faulty wiring
of course.
 
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