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Runaway Trim Yesterday in 14A

samgaddis

Member
Some context: I am a relatively new pilot - just 270 hours, but about 260 of those hours were over a dozen years ago. I bought an RV-14A last month and did 5 hours of instruction on the way home and another 5 back here in Texas before getting signed off to fly my own bird solo.

So all week I've been shooting touch-and-gos and was doing the same yesterday. My procedure once I touch down is to quickly clean up the airplane - flaps up (3 momentary clicks) and hold the hat switch up to send the trim from nearly full up back to neutral. I visually confirm the flaps are up, check the trim indicator and then hit the throttle to go again if the trim is anywhere near neutral.

I'm not sure exactly when the runaway trim started (probably the motor just kept going after I let off the switch), but somewhere around 50ft AGL I noticed I was having to apply what I would call extreme back pressure to climb. For a second I felt I couldn't climb. It caught my attention immediately.

After the initial shock of it, I took a breath and focused on flying. I noticed that the trim indicator showed full down but toggling the hat switch had no effect. I did not even think to go into the VPX settings to try to adjust from there, but honestly I probably wouldn't have been able to reach over to the MFD and handle that given the focus and physical strength it took to fly the airplane.

Having just got back into aviation after a long hiatus, the term runaway trim wasn't even in my lexicon, and although I saw the indicator stuck on full up, a part of my brain was convinced there was some structural problem with the elevator that could get worse any moment. For this reason I decided to stay in the pattern and land as soon as possible. In hindsight, I should've left the pattern to stabilize the airplane and troubleshoot. At a minimum, I could've experimented with pitch control and better understood the limitations before attempting a landing.

But I was in a hurry, so I radioed the 3 other planes in the pattern to let them know I had an elevator problem and asked for them to clear the way. Prior to this point I was hitting the numbers at 65 every single landing, but in my haste to get on the ground this time the approach was hot and I crossed the threshold probably around 80. There was a lot of bouncing and I used 3500 of the 4000 feet available. When I got to the ramp and turned every thing off and back on, the trim functioned normally.

This topic has been discussed extensively in another thread, so the technical considerations perhaps belong there. But after researching it, I'll be doing a few things:

1. I'll be working with my A&P to diagnose exactly where the failure occurred. I'm not a builder so I need help there.

2. I'll be practicing (at altitude) runaway trim scenarios. The airplane is in fact controllable, especially as slow speed in the pattern, but my arm was sore after fighting it for just a few minutes and it wasn't a pleasant experience.

3. I'll be considering solutions from the market for this, like the TCW Safety Trim module, with the understanding that this in one sense just adds more electrical complexity.

I certainly welcome any feedback from the community as well.
 
I suugest you start with the micro molex connectors in the empennage. that connect the trim motor in the elevator to the wiring harness (assuming original builder used Van's standard harnesses and connectors). However, they would be more likely to fail in a way that would result in trim not responding rather than runaway trim. Many people have replaced these connectors with better ones or with d-sub pins (see this video: https://youtu.be/f8DCvBV2V4M). The connectors (2 sets) are in the elevator and on the aft deck. To inspect these remove trim motor inspection panel on elevator and the empennage fairings.

The stick base also uses micro molex connectors, another place to look (again, more likely to fail with non-responding trim as opposed to runaway).

If you have G3X and some other systems they can be programmed to only allow trim motor to run for 5 seconds without actively re-engaging the trim switch on the stick or panel. You can also program trim motor speed based on airspeed to make trim less sensitive in cruise.
 
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Have u read thru the VPX Installation & Operation manuals. I think your problem and solution may be there. Very important u know how it works. Been a while, but I believe it handles run away trim when setup properly.
 
Had a customer with similar trim “problem”, which turned out to be operator error. (14A w/Garmin G3X)

He inadvertently held the AP disconnect switch (which was the button above the PTT on the Tosten grip) which also engages CWS if enabled, but the fact that he was pulling back on the stick with the AP now engaged it started auto trimming nose down.

Bottom line in AP setup mode be sure CWS is either set to off or only available when AP is ‘on’.
Auto trim doesn’t know the difference between actual out of trim, or someone pushing/pulling on the controls.
 
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Had a customer with similar trim “problem”, which turned out to be operator error. (14A w/Garmin G3X)

He inadvertently held the AP disconnect switch (which was the button above the PTT on the Tosten grip) which also engages CWS if enabled, but the fact that he was pulling back on the stick with the AP now engaged it started auto trimming nose down.

Bottom line in AP setup mode be sure CWS is either set to off or only available when AP is ‘on’.
Auto trim doesn’t know the difference between actual out of trim, or someone pushing/pulling on the controls.

Much more likely scenario given that wiring issues typically interrupt power somewhere along the line as opposed to applying power to a trim servo...
 
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My only runaway trim in a RV was self induced by letting a IPad I was holding rest on the trim switch. My current setup shuts the trim off after 3 seconds of trimming. I think it’s a excellent safety feature. I also practiced flying the aircraft with the trim at the stops to insure it was controllable.
 
I suggest that you download the flight data off your efis’ sd card to analyze if in fact a trim command was given. Assuming you have a modern efis with a storage card this should be relatively easy. You would then have a better idea what you are dealing with.
 
3. I'll be considering solutions from the market for this, like the TCW Safety Trim module, with the understanding that this in one sense just adds more electrical complexity.

I certainly welcome any feedback from the community as well.

The safety trim product is very good, and I recommend it IF you do not have a modern EFIS system like SkyView that integrates Auto Trim. I also note the safety trim product eliminates all the servo switch relays so it reduces complexity, not increases it. The SkyView Autopilot module has the full trim servo control system along with the 3 second time out, as well as the auto trim function when George is flying.

Side note - as you work your POH for trim runaway, figure out how to label the trim breaker so you can pull it as an immediate action.

Carl
 
Well, good job flying the plane. Lack of panic is important! I'm sure you will get to the bottom of this.

Cheers
 
The SkyView Autopilot module has the full trim servo control system along with the 3 second time out, as well as the auto trim function when George is flying.

The SkyView AP system actually has a 5 second time-out feature. 5 seconds is rather long in this situation. I'd rather have a 3 second time out.

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.
..."



EDIT:

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.
 
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Side note - as you work your POH for trim runaway, figure out how to label the trim breaker so you can pull it as an immediate action.

Carl

I put my trim CB and autopilot CB side by side, in front of the pilot, with a distinctive line around them, for quick access. I included the AP since it can directly drive the servos in my set up.
 
I think in the original post a VPX was mentioned. If you have an EFIS and a VPX the trim could have been wired to either one based on the builders choice.

Either one has trim runaway protection which just has to be configured properly to work.

Adding another system will not increase your level of protection but will increase the complexity and likelihood of failure.

On the other hand if it was operator fault as Walt suggests neither one will fix it :) .


Oliver
 
Autopilot Configuration

Sam,

This is the autopilot configuration Walt mentioned.

AutopilotConfigurationPage.png

If you have your system configured to "Engage AP Via CWS Input", you might want to disable that.

I have mine disabled since I always use the nice dedicated AP button on the GMC 507 to engage/disengage the autopilot.

AM-JKLUf_viUWxLdKqpZ-fLuW4WGeHsWSbGKX3Bl3J7QJ_WhkCs1GVLUtdvqIGfqdtQGPNmOeVySIfWSh78Pb3gIN_o_L4f_0icQarb3tgQmAz4H9xLIWjv7LC3Qo8Sve6MgIVHQBXgfVUFhXKt248hINbne=w640


Steve
 
Runaway trim

If you have no wiring problems before, I doubt you have a wiring, or micro pin problem.

I would check your hat switch to see that it is loose or tight depending upon how the switch has been attached. Pull up slightly on the hat switch so it has a little more play. If it was extremely tight on down trim it could have stuck in down position.
 
What is installed system?

It would be helpful if the OP would describe the installed system. We could be more focused in our responses.
 
It’s been a busy week, and I waited until I could get back to the airplane to download the flight logs. Anyway, here are my responses.

@mturnerb, I will check the micro molex connectors and consider upgrading them (both in the empennage and the stick).

@pilotkms, great idea. I've familiarized myself with the VPX manual. I learned in the future I can simply hold the reverse trim input to stop the motor and then control it from the VPX soft buttons on the MFD.

@Walt, I don't think this particular operator error applies to me as AP I don't have AP disconnect on my stick and it was off at the switch throughout the flight. I also don't have auto-trim.

@Av8rRob great idea. Attached is the trim data (with the blue line showing altitude). Here you can see I did a normal touch and go with trim gradually moving to full up, as I typically do. However, on the 2nd T&G where the incident occurred, I intentionally wanted to see what it felt like to land with less trim (I remembered this only after viewing the data). And that meant that when I took off, my muscle memory move to hold trim down moved it through trim neutral to full down. This doesn't explain why I couldn't get the trim to come back to neutral, but it does further the plot.
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@spatsch, I believe in my case the trim was wired to the VPX since I am able to make adjustments to it there. I do not see much to adjust related to runaway protection in the VPX configuration (when connected to ethernet) or in the VPX manual. But I do plan to find an expert on this locally to discuss more. Thanks for the heads up.

@Allan Stern, I almost thought the problem was be as simple as a stuck switch, but if that were the case, wouldn't toggling the switch the opposite way, as I did during this flight, reset it. I wonder why I had to get on the ground and restart the system to regain control of the electric trim?

In any case, thank you all for the thoughtful comments. This gives me additional avenues to explore. Above all, I'll be practicing high up in weird trim configurations.
 

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