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Runaway Trim

dweyant

Well Known Member
Had a really scary experience yesterday.

Was flying from central Texas to Tennessee. putting along with a nice 30 knot tail wind when all of a sudden my Autopilot kicked off.

I was having just a little bit of turbulence, so my first thought was it was more than the AP could handle. However by the time I grabbed the stick and started to mess with reengaging the AP I realized something was very wrong.

My elevator trim had gone crazy on me, going full down deflection.

As you can imagine it is really hard to control an airplane in such a situation. I did (obviously) manage to get the plane under control, but I was unable to get the trim to move back to the proper position.

I wound up declaring an emergency (amazing how easy that is to do when you really need one). The controller was great and found me an airport close by so that I could shoot the RNAV approach, by hand, down to almost minimums.

Fortunately (very) I got the plane on the ground in one piece. Once on the ground I went about troubleshooting the problem.

I was able to unstick the trim, and run it to full stops numerous times with no issues, however I had to do it from the soft keys on the G3X touch. The buttons on my stick not only aren't working properly, but are performing the wrong function. the trim down button now works as right alieron....

Beyond that, and sending Garmin the logs they requested I haven't tried much troubleshooting yet.

Any ideas?

-Dan
 
Won´t help much on your troubleshooting, but maybe "food for thoughts" for people still building.

For exactly that reason, i did not put in the electric trim Option in my -8. Runaway-trim used to be a BOLD FACE procedure (immediate Action required you had to know by heart, no time for checklist) at my former workplace. Runaway-trim ain´t no fun! And if a system can fail, it will.

Furthermore, flying my -8 now, i find the trim super-super-sensitive. I would even doubt that you find the perfect trim by using the electric motor operated by a switch. You may get close, maybe even very close, but i doubt that you get it "just there", hands-off-level-flight.

For me it ended up (elec. trim) beeing another system that could fail with overall doubtfull use.

Please, anybody teach me wrong if i am.

If i hear your story, i am glad that i installed manual trim ( i know. Can fail too!)

I am glad to hear that the whole event went by with no injuries or a bent or wrecked aircraft. Runaway-trim is surely no fun to deal with.
Declaring emergency was the right thing to do, in my opinion.
Shooting an IFR Approach in IMC (i assume) with an "out-of-trim" aircraft is, even if you have a flight director, nothing easy to do.

Well done!!!
 
ok-----I probably should have left my comment to myself---and turn this over to the avionics guru's. Didnt know you had a VPX magic box.

Tom
 
I have resisted the urge to put an altitude servo on my elevator and especially anything that could affect elevator trim except the basic Pat Hatch relay board. runaway trim or autopilot on my elevator is a risk I want to avoid. talk to Boeing about it. I know where my trim circuit breaker is and I can find it blindfolded. glad you made it down safely.

a challenge for you...... apply full trim at cruise speed, up or down, and see how fast you can find the trim circuit breaker and pull it.
 
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TCW Safety Trim would have taken care of the issue.

In fact, most likely would have prevented the problem in the first place, as the VPX would not have been in the picture---the servo and trim switch are wired directly to the Safety Trim unit.
 
TCW Safety Trim would have taken care of the issue.

In fact, most likely would have prevented the problem in the first place, as the VPX would not have been in the picture---the servo and trim switch are wired directly to the Safety Trim unit.

solution, more electronics?
 
solution, more electronics?

Actually less electronics as I see it. In prior posts it was mentioned that both a VPX and a G3touch were involved. The Safety Trim will work without either of these boxes. Both the VPX and the G3 are doing a lot more that just the trim, so way less electronics in the whole.

For those who choose an electrical trim, certainly a choice.

For those who choose manual trim---not needed.

I happen to like having the trim on the stick, so I fall into the first group.

Always hated the manual trim in the Cessna.
 
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No hat switches for me!

I see lots of people with electric trim using the all in one "hat" switch. That's a single point of failure. What happens if it sticks or shorts internally?

I followed the Cessna 182T trim design - two rocker switches. BOTH have to be pressed up or down to make the trim move. If either one sticks on the trim doesn't move. Oh and there's a preflight checklist item to confirm no trim movement if you only actuate one switch.
 
Dan,

The common thing to all the switches in the stick would be the ground as there is only 1. You may lost your ground all together or it is shorted to another wire in the stick. There are several places under the stick for wires to get pinched. That is at least a starting point.

Jay
 
I see lots of people with electric trim using the all in one "hat" switch. That's a single point of failure. What happens if it sticks or shorts internally?

I followed the Cessna 182T trim design - two rocker switches. BOTH have to be pressed up or down to make the trim move. If either one sticks on the trim doesn't move. Oh and there's a preflight checklist item to confirm no trim movement if you only actuate one switch.

yep, the hat switch is the highest potential trim failure point for me. especially when climbing in and out the tight cockpit it could become damaged. at least I try to protect it with a hard cover.

IMG_20201121_170833.jpg
 
Practice runaway trim!

Flying one day in my 9A it suddenly and unexpectedly pointed to the ground.
My electric trim switch is near my throttle so I can use my index finger to reach up and touch it when needed.
After slowing down go get the back pressure manageable, I applied a bunch of up trim to get things back in trim... but it took a lot longer than I would have liked before I realized that's what the issue was.

The only thing I can figure is that my hand was on the throttle and finger was next to the trim switch, and without realizing it, my finger hit and held the switch. I can imagine this would be even easier to do with a hat switch on the stick.

I encourage everyone to practice what it feels like to have a runaway electric trim so you know in advance what it feels like, and what to do about it.

I also asked GRT to add a pop-up warning on the screen if the trim goes out of normal range - that would have helped identify the problem faster. I haven't seen them add the feature yet though.
 
Curious to know from OP.... Was your G3X set up for reduced trim motor speeds at cruise and/or maximum trim time before shutoff? I have my G3X set up for 25% trim motor speed at cruise and max of 2 seconds before “timing out”. I had hoped these settings get me some margin of safety in event of a runaway trim. Curious to know if I have a false sense of security?
 
Long ways away from flying my 10, but planning on using advanced panel with their vpx type cbs. In my 8 I have electric trim, with a trutrak in the middle for autotrim. Every prestart check when I check circuit breakers in I put my fingers on the trim circuit breaker. I might add an switch on my 10 to be able to interrupt up after seeing this. Need to do some research. Thanks for sharing, got me thinking.
 
I was able to unstick the trim, and run it to full stops numerous times with no issues, however I had to do it from the soft keys on the G3X touch.

-Dan

Dan - Where are the G3X Touch soft keys for trim located? I wasn't aware of these.

Also, as described by another poster, I have my trim controlled by my G3X system and set up with a maximum 2 second run time for the elevator trim, which should prevent a runaway due to a stuck trim switch. (My max run time for the roll trim is longer, as that trim isn't as effective and a longer run doesn't introduce as much out-of-trim.) Perhaps if you have your trim through a VPX system this isn't an option? I also have my trim breaker marked with a yellow breaker cap so I can find it quickly if needed.

Glad you were able to maintain control and get safely down.

Dave
 
Trim Switch Wiring

Had a really scary experience yesterday.

Was flying from central Texas to Tennessee. putting along with a nice 30 knot tail wind when all of a sudden my Autopilot kicked off.

I was having just a little bit of turbulence, so my first thought was it was more than the AP could handle. However by the time I grabbed the stick and started to mess with reengaging the AP I realized something was very wrong.

My elevator trim had gone crazy on me, going full down deflection.

As you can imagine it is really hard to control an airplane in such a situation. I did (obviously) manage to get the plane under control, but I was unable to get the trim to move back to the proper position.

I wound up declaring an emergency (amazing how easy that is to do when you really need one). The controller was great and found me an airport close by so that I could shoot the RNAV approach, by hand, down to almost minimums.

Fortunately (very) I got the plane on the ground in one piece. Once on the ground I went about troubleshooting the problem.

I was able to unstick the trim, and run it to full stops numerous times with no issues, however I had to do it from the soft keys on the G3X touch. The buttons on my stick not only aren't working properly, but are performing the wrong function. the trim down button now works as right alieron....

Beyond that, and sending Garmin the logs they requested I haven't tried much troubleshooting yet.

Any ideas?

-Dan

The trim switch wiring is probably a good place to start. If you put the G3X Touch into configuration mode, you can activate the physical trim switches and see which servo input is active. This should assist in identifying any wiring problems.

The maximum trim run time setting was implemented to limit the affect of a trim switch wiring issue. It is worth reviewing this setting and double-checking that it is programmed to a value that will protect against a potential switch wiring problem.

We will keep an eye of the datalogs from the G3X Touch to see what information can be gleaned from them as well.

Thanks,

Justin
 
Since this doesn’t actually seem to be a model-specific problem, I suggest that this thread be moved to another forum such as Safety, General Discussion, or Ongoing Maintenance to better catch the attention of the general membership.
 
I Love Our Manual Trim

There sure seems to be a lot of discussion around electric trim systems. Yeah, manual trim can fail, too. It could have a push-pull bit come adrift, and it can get stuck. But it cannot run away.
 
Every prestart check when I check circuit breakers in I put my fingers on the trim circuit breaker. I might add an switch on my 10 to be able to interrupt up after seeing this. Need to do some research. Thanks for sharing, got me thinking.

I put my trim and autopilot pullable CBs right in front of the pilot, marked with a line around them. (Since the Trio can do auto-trim, in a runaway I need to pull both breakers immediately, then sort out the problem.)
Right seat has its trim switch on the panel. Too many stories about passengers resting a book on their stick trim switch.
 
I have the Dynon autopilot panel which also controls the elevator trim. Even when flying manual the trim switch actuates whatever is in the box and so drives the trim motor. There is a safety feature which allows only 3 sec of trim at a time. But I don´t see how this thing is build and thought trim runaway could be possible.
So I installed a guarded switch which selects power to the trim motor from the autopilot/trim panel or - when the guard is open and the switch activated - to a second springloaded alternate trim switch (no electronics inbetween). I believe this reduces the chance of trim runaway considerably.
I also have a collar on the Trim C/B to be able to quickly locate and pull it
 
I am glad to see this issue getting some attention. I am also glad to hear that the OP was able to control his AC in a very extremis situation. I can add nothing to the suggestions already made here regarding fixing his problem. I can add my opinion about the system and what I did to mitigate runaway trim risk

My opinion is that the typical RV trim system in general has more trim authority than necessary. There is no option that I know of to limit the travel of the typical Ray Allen trim servo. During Phase 1 while simulating run away trim, it became very obvious that stick forces could potentially build to disastrous levels pretty quickly in some flight regimes.

My approach was to add a TCW system set to a very short trim cycle (about three seconds) and run the trim at very slow speeds.

For those who are relying on reaching a circuit breaker to stop a runaway, I suggest that you run your trim donward at max cruise speed for about three seconds and then see if you would like to remove one of your two hands that will be fighting the forward stick pressure to pull that breaker. Not saying it is undoable, just probably harder than you imagine.

In addition. I "biased" the trim servo so that it can run the servo in nose down trim only the amount necessary to trim for level flight at max cruise speed. this leaves a lot of trim authority in the nose up direction, but my thinking is that a nose up runaway is much easier to handle than a nose down one. As the nose goes up, with a timely power reduction, airspeed will bleed off quickly to a point where reduced trim tab authority could be overcome by stick pressure and balanced flight (or at least manageable forward stick pressure) could be achieved without disaster.

Looking back at it, I would probably install manual trim. With the inclusion of auto pilots that drive the trim tab, there is a bunch of trust and complexity built into a system that should be kept very simple.

All IMHO of course.
 
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I'm pretty sure most RV guys would find this overkill/to costly but this is the 'old' way to avoid trim issues. You have to push both switch sections to activate trim, one switch is for trim and the other is for trim power. So power is shut off to the system unless you operate both the power and trim switch simultaneously. I've considered installing this myself as I don't use aileron trim anyway.

Switch quality is another factor, check out the Otto switches and compare them to what some other grip manufacturers choose to use, you get get what you pay for. Check out the Otto grips vs another popular 'military' grip maker, the price difference is mostly the quality of switches used.

In reality I think good wiring practices along with good hat switches and the Garmin limit function is a pretty safe bet, and no way I'm giving up 'autotrim'.

Slowing down is the first thing to do with out of trim situations, much less force required at slower speeds.

11-07354a.jpg


Links to switches and grips:

https://www.aircraftspruce.com/catalog/elpages/ottopistolgrips.php

https://www.aircraftspruce.com/catalog/elpages/trimSwitches2.php

https://www.aircraftspruce.com/catalog/elpages/trimswitch11-15410.php

https://www.aircraftspruce.com/catalog/elpages/4wayforb8gripswitch.php?clickkey=7529516

https://www.aircraftspruce.com/catalog/elpages/infinitygripaccess5.php?clickkey=13068
 
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Dan,
I have the same setup in my 8 and as others have suggested, I would suspect a problem with either the switch or a short somewhere in the wiring. Since you were able to control the trim with the soft keys I would suspect the former. Just curious, but did the VPX present a fault message when the trim ran away? In addition to the advice from the G3X experts, it might be worthwhile to run the VPX configuration routine outlined in the installation manual to verify you have it programmed the way you want it.

Good job getting on the ground safely, especially with this problem on an approach down to minimums.

J. Baker
 
I've seen posts in this thread and others about having circuit breakers on the trim servos so that you can pull them in the event such as this. I'm actually at the point in my panel layout were I'm locating some of the breakers and in fact on this iteration I have breakers for the pitch and roll trim to the GAD27 power inputs. My question is how does this solve the problem if the servo has run to an extreme limit?
 
I realize a trim runaway is disconcerting, but it may not be something worth adding a lot of backup/safeties/support systems, which often bring new issues into play. The aircraft is controllable with trim at the limits. Stuff not installed has a perfect reliability record.
 
Lesson from the 737Max

I just started building my fuselage when the 737Max double crashes happened. I didn't know these big birds have the build in trim cutout switches to prevent the runaway trim. Unfortunately, the pilots of the doom planes didn't use them.

From that lesson, I now have the trim cutoff switch and the flap cutoff switch mounted directly in the front panel. These two switches will be normally on all the time to provide power, except for the case I think the trim motor is misbehaving. Then it is a simple step to flip the switch to off and cut off power the motor. This action is much faster than pulling circuit breaker or fuse. They can be pulled at anytime afterward.

I have the flap cutoff switch for the same reason. I purchased the Flap Positioning System from Vans, which included a flap controller. I discovered during the installation that if the guide-rod isn't properly aligned, the flap motor won't shut itself off. So that is the reason to have another cutoff switch.
 
I'm pretty sure most RV guys would find this overkill/to costly but this is the 'old' way to avoid trim issues. You have to push both switch sections to activate trim, one switch is for trim and the other is for trim power. So power is shut off to the system unless you operate both the power and trim switch simultaneously. I've considered installing this myself as I don't use aileron trim anyway.

Switch quality is another factor, check out the Otto switches and compare them to what some other grip manufacturers choose to use, you get get what you pay for. Check out the Otto grips vs another popular 'military' grip maker, the price difference is mostly the quality of switches used.

In reality I think good wiring practices along with good hat switches and the Garmin limit function is a pretty safe bet, and no way I'm giving up 'autotrim'.

Slowing down is the first thing to do with out of trim situations, much less force required at slower speeds.

11-07354a.jpg


Links to switches and grips:

https://www.aircraftspruce.com/catalog/elpages/ottopistolgrips.php

https://www.aircraftspruce.com/catalog/elpages/trimSwitches2.php

https://www.aircraftspruce.com/catalog/elpages/trimswitch11-15410.php

https://www.aircraftspruce.com/catalog/elpages/4wayforb8gripswitch.php?clickkey=7529516

https://www.aircraftspruce.com/catalog/elpages/infinitygripaccess5.php?clickkey=13068

Though not as handy as this high dollar switch, I put a simple trim power toggle switch on the panel. I switch it on and trim with the stick grip hat switch, then turn off the power. Runaway trim scares the heck out of me.
 
Trim runaway cutoff switches? By the time a pilot realizes what is happening and flips the switch, the trim will already be nearing the limits. Even less time if the autopilot was engaged.

As Walt said, the runaway trim response is to simply slow down. Next flight, back it off to 100 knots and run the trim to the limits, because chances are that's where it will be, even with a cutoff switch. KIS.
 
I appreciate some of the responses, but a number of the suggestions are a bit absurd.

It takes about 3-4 seconds for the trim to go from roughly neutral to full deflection.

The AP attempted to hold level until it couldn't costing me at least a couple seconds. By the time I grabbed the stick and processed that something was very wrong the trim was already at (past?) the stops. Pulling a breaker, etc would have done no good, and only distracted from the immediate emergency.

With the benefit of hindsight I'd change a couple things I did, but not much.

First, it takes some amount of time to process going from a nice pleasant flight to something being horribly wrong. Some of you are better pilots than me and might do it faster, but I doubt much.

First reaction was to apply up trim. It took several seconds to realize it was doing nothing.

Looking back at the logs this is the point that I lost about 1500 feet of altitude and managed to get the plane into a "unusual attitude" fortunately I was at 11,000, and as soon as I realized the plane was getting away from me I stopped messing with the trim and just flew the airplane.

About then ATC (nicely) asked me what was going on. I'm sure they had noticed I was off track and had lost altitude. I told him I had a runaway trim issue. He offered me a block altitude. I thought about that for about half a second and declared an emergency.

The one mistake I made at that point was not slowing the plane down significantly. For whatever reason it didn't occur to me for several minutes that a slower airplane would have been easier to control.

Fortunately adrenaline worked as designed, and my daughter did an incredible job of staying calm and doing what I asked her. ATC was also great. I didn't have time to brief an approach, or even find an airport. I told him find me something with an RNAV and give my vectors. He did , quickly and talked me through the fixes and read me a lot of the plate so I wouldn't have to take my eyes off of the panel. I only wish I had gotten his name so I could have thanked him directly.

I'm anxiously awaiting Garmin to get back to me with what they find (if anything) in the logs. I also talked with Chad at VP and he had a couple suggestions of things to look at. My guess is it is a wire (or two?) that is shorting out. That combined with a corner case that somehow got my trim motor stuck. Honestly the stuck trim motor is the most disturbing. I ran it to full deflection several times on the ground and couldn't get it to stick again.

I'm a huge fan of accident analysis. I welcome any constructive criticism. If there is a training solution that would have made this go better, please let me know. We will never build an airplane that is 100% reliable so being able to deal with emergencies will always be critical.

Looking back on this two days later, trying to be objective, the only two things I "might" have done differently. Slow the plane down sooner. Would have saved my strength. Fortunately didn't matter, but could have. Second, the AP had kicked off about 30 minutes before. "assumed" that my daughter who sleeping had touched a trim switch, etc. so just reset it and didn't think much about it.

-Dan
 
Trim runaway cutoff switches? By the time a pilot realizes what is happening and flips the switch, the trim will already be nearing the limits. Even less time if the autopilot was engaged.

As Walt said, the runaway trim response is to simply slow down. Next flight, back it off to 100 knots and run the trim to the limits, because chances are that's where it will be, even with a cutoff switch. KIS.

You are right on there, but with a trim power ON/OFF switch an accidental hat switch activation by pilot or passenger will do nothing.
 
Dan, I think that your time of response would be pretty typical. Trimming back would be a very natural response that would not be helping, wasting valuable response time. That is why I said “ trim down for three seconds” to see what it might feel like at cruise. A three second reaction/recognition period in this instance is probably optimistic.

How much stick pressure did you have to overcome? When I did it in Phase 1 the stick loading became very uncomfortable very quickly - to the point I was concerned about damaging / bending flight controls.

I also have a trim power switch. Never thought about turning it off during cruise to minimize the time that trim could potentially be flying the airplane.

Thanks for posting. This kind of experience is extremely valuable education to the community
 
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I also have a trim power switch.

Yep, A/P power also. Both right above the throttle. If the plane decides to fly itself somewhat strange, I can kill both with a single motion.

Never thought about turning it off during cruise to minimize the time that trim could potentially be flying the airplane.

Same here, but not that it was brought up, seems like a really good idea.
 

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Mike, it does sound like a good idea. I spend hours bombing around on trips with no need to ever touch the trim. Sounds like a new SOP.

Dan H, for sake of clarity, when you say “controllable With trim at the limits” are you talking “ultimately” controllable? What I mean is; yes, I think I could fly around at the trim extremes with comfortable stick forces at 65 knots, but I don’t think so at 180 knots

Knowing your thoroughness, I suspect that is not a cavalier statement and that you have tested. Can you clarify?
 
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Dan H, for sake of clarity, when you say “controllable With trim at the limits” are you talking “ultimately” controllable? What I mean is; yes, I think I could fly around at the trim extremes with comfortable stick forces at 65 knots, but I don’t think so at 180 knots

I have not tested in a controlled manner, just dumb stuff like accidently triggering full up trim while flying a loop (not my airplane, different grip). I promise to confirm trim limit forces at cruise speed next flight, ceiling permitting.
 
Folks talk about 'runaway' trim as if it some kinda mysterious voodoo PFM event.
Trim doesn't just go off on it's own, very clearly the failure modes are controllable, namely good parts and good wiring are the key components to a reliable system.
 
Agreed Walt. One of my points is that the typical Ray Allen servo, although seemingly very reliable (no reported failures or accident cause factors), is not exactly a Mil Spec piece of equipment, nor is the typically used control switch/coolie hat. Many driven by autopilots. All connected to a trim tab that has lots of authority (originally designed for manual actuation).

Not trying to create histerics or a call to ground the fleet, just saying that this is a system that many take for granted as virtually foolproof. It deserves some prudent consideration towards risk mitigation.

Be interesting to see what Dan H comes back with. Might surprise many people.
 
During the build, I had a lot of discussions with the gang of fellow builders here about runaway pitch trim. I considered installing a reversing switch, but never did. Schematic is attached, two different views of the same principle. This would allow you to remove power from the servo, and then after getting the plane slowed and leveled off and taking some time to think things through, try reversing the polarity to run the pitch back to neutral and then cutting power.

I did test full nose up and nose down trim and yeah, that's a metric f-ton of force, so the lesson I took away was *slow the f**k down immediately* to get things under control.

Just a thought...don't know if anyone has any experience with such a switch, either with or without an actual run-away.
 

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As previously mentioned, TCW Safety Trim system has a three way switch as part of the package for this very reason. Normal, Off, Reverse. Simple and elegant. Excellent attributes for important systems.

Bevan

During the build, I had a lot of discussions with the gang of fellow builders here about runaway pitch trim. I considered installing a reversing switch, but never did. Schematic is attached, two different views of the same principle. This would allow you to remove power from the servo, and then after getting the plane slowed and leveled off and taking some time to think things through, try reversing the polarity to run the pitch back to neutral and then cutting power.

I did test full nose up and nose down trim and yeah, that's a metric f-ton of force, so the lesson I took away was *slow the f**k down immediately* to get things under control.

Just a thought...don't know if anyone has any experience with such a switch, either with or without an actual run-away.
 
What about a standby trim switch? Put it somewhere out of the way with the intent of only using it if the primary trim switch on the control stick fails or runs away. In practice, it wouldn't have to work well, just relieve some pressure off the controls to make the aircraft easier to fly.
 
Agreed Walt. One of my points is that the typical Ray Allen servo, although seemingly very reliable (no reported failures or accident cause factors), is not exactly a Mil Spec piece of equipment, nor is the typically used control switch/coolie hat. Many driven by autopilots. All connected to a trim tab that has lots of authority (originally designed for manual actuation).

Not trying to create histerics or a call to ground the fleet, just saying that this is a system that many take for granted as virtually foolproof. It deserves some prudent consideration towards risk mitigation.

Be interesting to see what Dan H comes back with. Might surprise many people.

I've never seen a motor run without power or a trim relay stick in a closed position.

I have seen plenty of bad wiring/connections and cheap switches fail.

Removing power from the system is one way to alleviate the risk, but no way I'm giving up autotrim.

And most importantly the airplane is 100% controllable if you just slow down a little!
 
I understood from a fellow-10 builder that the trim was very powerful. I put a “trim power” switch in my bank of seven switches that must be up ( on) for takeoff and landing. (7 up). In my 40 hour test I ran trim full up an down at forward and aft CG. It is controllable but requires a major stick force. I would hate to land it at any full trim position.

I put a TCW trim controller in the ship after about 10 hours to resolve the problem with the trim speed at high speed and to add its trim limiting function.

Belt and suspenders but issue solved.
 
I see lots of people with electric trim using the all in one "hat" switch. That's a single point of failure. What happens if it sticks or shorts internally?

I followed the Cessna 182T trim design - two rocker switches. BOTH have to be pressed up or down to make the trim move. If either one sticks on the trim doesn't move. Oh and there's a preflight checklist item to confirm no trim movement if you only actuate one switch.

I have a throttle grip momentary switch. Press it up and it configures the flaps and pitch trim to takeoff positions. Press it down and it configures the flaps and trim in the landing positions (below a certain airspeed). After takeoff, I put on the autopilot which engages the autotrim.

I can do a complete flight without using the stick hat switch. If I do trim manually, that shuts down the auto-config and autotrim modes.

I trust the hardware and software for this, since I did it all, but it does add complexity to a simple function.
 
Dan I want to commend you for declaring an emergency so quickly. We ATCers don't always understand the nature or severity of what's going on in the cockpit. It's very helpful for us to know early that there are problems because we can start getting resources together. I'm happy to hear ATC was so helpful and glad you got down safely.
 
Trim runaway

Is there a airplane made that the pilot is physically strong enough to counter full deflection pitch trim at cruise speed? I would bet not many. Trim runaway is a real emergency. Ideally there is a way to quickly remove power from the trim system at the first hint of trouble. After that slow down as much as possible.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Dan I want to commend you for declaring an emergency so quickly. We ATCers don't always understand the nature or severity of what's going on in the cockpit. It's very helpful for us to know early that there are problems because we can start getting resources together. I'm happy to hear ATC was so helpful and glad you got down safely.

Do you work at cedar rapids approach? I have flown to Ames about 40 times from Chicago and fly right over Cedar Rapids and use either FF or IFR clearances. They are always very helpfull and one time even helped me find a hole to get under a BKN that almost turned OVC deck in icing Wx. Yes, it was well BKN 30 miles behind me, so not dangerous.

I also bought my previously started RV10 kit from a guy in Cedar Rapids.

Larry
 
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Is there a airplane made that the pilot is physically strong enough to counter full deflection pitch trim at cruise speed?

Yes, at least one RV-8.

Pretty evening here, so I set up 180 KTAS at 7500 with a digital scale in hand. The scale location below was too low (interference with certain prized anatomy), but I was able to make a pull measurement with the scale hooked at grip trigger level.

Note the following is an RV-8 at about 79", near the forward CG limit (IO-390, metal Hartzell, solo).

Full nose up trim: The push feels quite heavy, manageable with one hand, easier with two. I could not get a measurement, as I could not read the scale up under the panel. Slowing reduces required force, eventually reversing to a required pull. At this CG I normally land solo with full up trim, so slowing to 80 knots or so makes a locked aft trim a non-issue, and less requires a small (normal) pull.

Full nose down trim: At 180 knots true, the one hand pull is manageable, two hands flyable. The scale reading was 26 lbs. A surprise autopilot release would generate negative G and float everything in the cockpit. A long flight to an airport might be tiring. In my case a bit of experimentation found that positioning my hand over the end of the stick and placing my knuckles against the panel relieved the strain (photo below). In the interest of science I flew it back to 08A in this way, at about 100 knots, and landed using the usual "70, 65 across the numbers". Adding flaps increased the pull requirement, as expected, but it wasn't a problem.

Go fly your airplane and explore the trim limits at speed. It is worth knowing what to expect.
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Full nose down trim: At 180 knots true, the one hand pull is manageable, two hands flyable. The scale reading was 26 lbs. A surprise autopilot release would generate negative G and float everything in the cockpit. A long flight to an airport might be tiring.
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I assume you have your stick the length in the RV8 plans. You have not cut it off to be shorter. Do you think it would be doable if you had a 2” shorter than stick? I guess easy to do the math on what the load would be.

I have landed my 6A with full nose down trim. Happened during phase 1. I did not have the grip cross drilled to the stick and during a negative G push over the grip came off and trim ran almost full nose down. Flew about 10 miles back to airport and landed.
 
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I assume you have your stick the length in the RV8 plans.

I really don't remember. I can tell you it is about 7.75" from the pivot to the top of the bushing...measured it this evening so I could do the math if needed.

I did not have the grip cross drilled to the stick and during a negative G push over the grip came off and trim ran almost full nose down.

Quick note...the Tosten grip comes with a bushing, the OD of which is assumed to match the ID of the stick tubing. Retention of the bushing and the shaft of the grip is done with a single allen screw and locknut.

Mine had a wee bit of clearance at the bushing OD, not much, but enough to foresee some bending of that single screw when grip force is high....like today. I didn't like that idea, so I drilled and tapped from each side and the front, and installed shortened screws in addition to the set screw. An alternate plan might be to fire up the lathe and make a new bushing 2x or 3x as long. Please excuse the fuzzy photo.
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