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Uncommanded pitch down- Tru Trak

flightlogic

Well Known Member
Patron
Posting for a friend who suddenly went nose down 45 degrees plus enroute home from OSH. RV9A with a new Tru Trak.
Disconnect button did not release the pitch servo. Torque might have been set too tight also, since the pilot reported pulling hard with no effect.
Autopilot power switch released the servo. No trim motor excursion was noted.
Has anyone experienced a similar event? As his instructor, I had previously had his avionics people put a pull-able breaker on trim motor. But this event did not involve pitch trim.
The disconnect button should provide a quick solution to the pitch down.
It did not.
Altitude degraded fast and became a threat.
So, looking for well thought out responses here involving either direct experience or thoughts to diagnose the fault and or prevent another occurrence. Thanks!
 
Forget all the electronics and stuff. Fly the airplane, don't let it fly you. For final safety of flight, never let the autopilot deviate so much without, "Pull harder and shear the (expletive) pin!", so that we can talk about it later.

My well thought out 2C that I've said more than a few times.
 
Scott, I agree. The pilot could not describe to me how much he pulled.
I think the surprise of the angle and time of onset maybe task saturated him.
From his comments, I would have expected the clutch to slip or pin to shear. He says they did not. Tru Trak is looking at torque settings presently.
But, I am interested in the electronics part... since that is how I make my living.
So, anyone else who has data to submit.... please do. Thanks
 
45 degrees down, you would hit vne in couple heart beats. I would be seriously pulling and looking an airport to find the cause.
 
In our aircraft with the Vizion 385 I would wager a solid bet that the servo's don't have enough torque to produce sufficient nose-down pitch.

I've intentionally put the a/p into a mode where the pitch servo had to work hard (against trim forces) and the servo motor lost that battle by a handy margin.

I'm having difficulty conceiving of how the a/p would have to have been configured for the pitch servo to produce enough force to effect that large of a pitch angle excursion.
 
Scott, I agree. The pilot could not describe to me how much he pulled.
......, I would have expected the clutch to slip or pin to shear. s

Trio has a clutch (and a shearable nylon gear); TruTrak has a shear pin.

I?d look at that shear screw, make sure it really is shearable.
 
This is very interesting. When I first bought my RV with TruTrak fitted I wondered how much force would be needed to overcome a seriously out of trim plane. I got the plane level AP off then held my Cooly hat switch full fwd till I got around 45 degs down, that took a few seconds and apart from the speed building up rapidly I left the trim at the position I obtained (45 deg down) and pulled the power off and pulled back the stick to level flight , result? Doable but was not easy to fly.
Mine wasn't obviously in a run away condition as I let the trim switch go but it did prove that the trim motor is slow and although it would be a surprise it's not heart stopping. Why did I do this check you ask? Some years ago I had a runaway trim in a B200 & that took some controlling, I wanted to be ahead of my own plane should I ever have a trim that went screwy on me!
 
Need more data!

All TruTrak servos are torque limited. We specifically design in the smallest possible servo to fly the aircraft in well in normal situations. To pitch an aircraft to 45 degrees down rapidly requires significant torque, which the servos do not have.

I wonder if your friend accidentally ran the trim to the point that the servo slipped and the autopilot was actually no longer flying the aircraft? I know you said no trim deviation was noted, but something doesn’t add up here.

I don’t ever recall a case where the CWS button didn’t release the servos (both roll and pitch), plus we don’t see gyro failures that result in rapid aircraft pitch input (we actually automatically turn off the autopilot these days when a gyro failure is detected)

Please have your friend give me a call about this.

I need to know what mode the autopilot was in, what the display showed when it wouldn’t disconnect, does it have autotrim, there is a lot of information to be gathered.

Thanks!
 
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Similar issues...but different...

In 2007 I installed a TruTrak Digiflight II control head and servos in my RV-7, and flew for close to 400 hours with that configuration, without difficulty.

After 2014 SnF, I decided to upgrade to a Vizion385 controller (new features, GPSS, TRK mode, etc.). Within a few flights I encountered what I would characterize as an uncommanded pitch event, several - the AP would seemingly drive the nose down -- >thunk< >thunk< >thunk< --- etc. Hitting the AP Disc switch took the AP offline and the excursion ceased (your electrical architecture may vary).

Of note - In my installation the trim servos were NOT connected to or controlled by the AP -- The Digi/Vizion has a nice little rolling annunciator telling you if you're out of trim, and I used that to set the trim.

After a few more flights, and consulting with Lucas@TT, it was determined that because the Vizion 385 no longer had pilot adjustable pitch torque settings, I had to move the pitch link position at the servo to achieve the same level of control -- the "Thunk" was the the servo "slipping" at the predetermined torque limit.

During the event, the aircraft was trimmed nose down and the AP was trying to raise the nose to meet requirements of the flight path, attitude, altitude and airspeed. When it hit the torque limit, it would "let go" of the airplane for a brief second and the nose would fall...

Not 45? down, but certainly attention getting :)
 
Andrew at TT

Andrew, I will gather more data and contact you directly.
Anecdotal reports may have a tendency to be exaggerated due to post incident trauma. Not sure if that is the case here.
In any event, this caused both people on board to react and pull the sticks. They both report the same observations. ie. Inability to reduce pitch angle until complete power was interrupted to the autopilot.
I will look further into configuration, wiring, settings as time allows.
I hope to test fly the aircraft as well, in the next few weeks.
Input from members here has already been instructive and I encourage more dialog. We all have a common goal in operating our RV aircraft. This forum is a powerful tool to help all of us build and operate some of the best designs in aviation. Thanks to all!
 
I have had autopilots "surprise" me, i.e.

Century, Piper, S-Tec, Cessna,etc and the aircraft would pitch or roll un-commanded, but I have never had one I could not "out muscle". I have had runaway trim that were more of a challenge! I have learned to have the pullable circuit breaker within easy reach!

I wonder if we might have a near "over-center" condition on the servo?

Ron
 
I had a somewhat similar situation in a RV6. I took out my IPad to use ForeFlight and as I wrote some freqs down I accidentally let it rest on the trim switch running the trim nose up. When the out of trim situation reached the autopilot limits it disengaged and the aircraft did a 2 G pitchup to about 30 degrees nose high. Stick forces were very high to return the aircraft to level flight until the trim situation was corrected. In fact I did a roll to bring the nose back to the horizon rather than fight the stick. My wife was not amused!
 
Trio has a clutch (and a shearable nylon gear); TruTrak has a shear pin.

I?d look at that shear screw, make sure it really is shearable.

Bob, TruTrak told me that they can't say what the shear mechanism is without taking a look at the individual servo. They are EITHER screw or clutch. I had cause to speak to them recently when I thought I sheared a screw by pulling against the A/P.

Related to this I would say I find it difficult to believe that the shear mechanism was able to allow a 45 degree down input against the pilot. Mine took very little effort to break it free.

Also, I think you need to get ALL the settings used during the programming and take a look there. I'm pretty sure my standard rates are not 45 degree or higher in bank. And dive should be controlled by the decent rate entered when a lower altitude is entered during flight. It would seemingly take an input of well over 500 ft/min of decent to reach 45 degrees in my opinion.

Just some thoughts.
 
After a few more flights, and consulting with Lucas@TT, it was determined that because the Vizion 385 no longer had pilot adjustable pitch torque settings, I had to move the pitch link position at the servo to achieve the same level of control -- the "Thunk" was the the servo "slipping" at the predetermined torque limit.

Andrew, thinking on this statement.
Did the original Digiflight II Servos have torque output only controlled by the set up settings in the autopilot?
If Digiflight II autopilot was updated to a Vizion 385 without changing the pitch servo is there no torque control, just max output of the servo? Could this cause a scenario as described by the original poster?
 
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There is simply no way for a Trutrak pitch servo to put a RV9A nose down that far with its relatively high pitch forces. Impossible. An out of trim Trutrak servo will skip steps long before that happens. I have experimented with a Trutrak autopilot pitching down to track the glideslope on an LPV approach in a 9A with an out of trim condition to see what happens and this was at approach speeds. Skipped steps and would not hold the required pitch attitude.
 
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thanks Bob

Bob, thanks for the input. I agree, it does not seem like it could happen.
I will reserve judgement at this point and keep looking for hard evidence.
I have crashed and burned once (actually burned, then went down) and put another in the CA desert when the engine quit. Neither one was solved by NTSB. I just hate mysteries, so I will keep digging. Lucky, this just resulted in a change of pants.
 
I had a somewhat similar situation in a RV6. I took out my IPad to use ForeFlight and as I wrote some freqs down I accidentally let it rest on the trim switch running the trim nose up. When the out of trim situation reached the autopilot limits it disengaged and the aircraft did a 2 G pitchup to about 30 degrees nose high. Stick forces were very high to return the aircraft to level flight until the trim situation was corrected. In fact I did a roll to bring the nose back to the horizon rather than fight the stick. My wife was not amused!

Rolling to the horizon is a common manoeuvre in large transport category jets as well as smaller corporate jets cause bunting over wth high neg G's is not pretty. My last Jet type endo I did involved getting out of a very high nose up attitude and rapidly deteriorating AS. In the real world flying year after year in Ops normal conditions to have the peace of mind to pull that stunt off would take a very switched on driver indeed!
 
With my Dynon setup Dynon recommends to install a toggle switch to supply power to the servos, cheap insurance vs shearing the shear pin.
 
Just for fun, on Monday I went up for a flight in our Vizion 385-equipped airplane. While GPSS coupled and holding a safe altitude I started to run the electric trim servo away from its neutral/level flight position. The a/p pitch servo worked hard to maintain pitch attitude, thus altitude. The scrolling trim indicator on the a/p control head clearly signaled an out-of-trim condition. Then the servo slipped, caught, slipped again. It was only when I punched the a/p disconnect button that I got a significant change in aircraft pitch attitude.

I did this test in both nose-up trim and nose-down trim conditions. Behavior was the same in both. Trim nose-up, a/p tried really hard to hold altitude. Punch off the a/p and the nose immediately bobbed upward to follow the trim input. Just the opposite effect when the aircraft was driven manually to a nose-down out-of-trim condition - when a/p was punched off the nose dropped to follow trim input.

At this point my confidence in the TruTrak Vizion 385 remains solid, based on the solid and as-expected performance of the autopilot when intentionally placed in a runaway trim situation with full deflection of the elevator trim tab.

Your mileage may vary. I'm glad I double-checked and confirmed proper operation.
 
Servo Power

With my Dynon setup Dynon recommends to install a toggle switch to supply power to the servos, cheap insurance vs shearing the shear pin.

Tru-Trak makes a similar recommendation in their installation instructions. I use a DPDT switch to power my AP and servos where the middle position turns on just the head and the top position turns on the head and servos.

Skylor
 
You should never ever install an autopilot with out a master power switch on it!

Opinion vs fact, I'm pretty sure not everyone would agree with the above, myself included (unless you classify a CB as a 'master switch').
The TT AP has been known to have uncommanded excursions, I have never heard of that issue with any of the other EFIS AP's.
 
Thanks everyone

I want to post a shout out to Canadian Joy for his flight test.
The results might be an explanation for the behavior seen by the RV9A pilot.
If the trim was run pitch down inadvertently, the autopilot would have done what it could to hold altitude. Then, when the clutch began to chatter... the pilot might have hit disconnect and felt even further down forces.
Some of the respondents have focused too much on the relayed figure of forty five degrees. I was not there.... but I know a sudden view of a windscreen full of cornfield can lead one to over estimate pitch angle. Just ask a cop how good an eyewitness statement is... versus reality.
Either way, the testing and reflections on Van's members have helped shed light on this topic. Hopefully adding to the knowledge of the owner and others who might be just learning the behavior of their own installation.
I do recall, while teaching synthetic vision systems when they first came out.... that task saturation occurs often and can lead to interpretation errors.
And cockpit voice recording analysis post accident often finds the crew saying, "what is it doing now?" Followed by a long and dreadful silence.
Thanks to all who have contributed! Nick
 
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Yes, either a master power switch on the autopilot or a pullable circuit breaker which accomplishes the same job of removing the power.
 
trim breaker

On my own 9A, I changed out the trim motor breaker to a pullable one. Have suggested that to others, when I have seen a flush breaker.
I will throw this out to the RV community though...

If one inadvertently runs the trim motor (either way)... it is going to travel a bit before the pilot knows it (if altitude hold is in effect)

So, if you reach over and yank the breaker... is there any way to eliminate the control forces? I can't see anyway to neutralize a tab that has either runaway from electrical fault, or pilot fault.

Am I missing something? When I test flew a prototype autopilot in a Baron, that only controlled pitch through the trim... I became aware of how much force is needed to counteract a trim tab in high speed cruise flight. I questioned the inventor and told him my wife (a pilot too) could probably not hold the wheel with one hand.... while simultaneously cranking the trim wheel back into neutral. And that does not account for RV airframes where there is nothing to crank. What say you all?
 
Fair Assesment?

Opinion vs fact, I'm pretty sure not everyone would agree with the above, myself included (unless you classify a CB as a 'master switch').
The TT AP has been known to have uncommanded excursions, I have never heard of that issue with any of the other EFIS AP's.

I don't think this is a fair assessment and seems somewhat anecdotal. Tru-Trak Autopilots were around long before most (if not all) of the experimental EFIS manufacturers even existed. The service history of TT is much longer than the "EFIS Mfg's" AP's and, at least until the last several years, the numbers in service are far greater than than those of the "EFIS AP's". Furthermore "uncommanded" excursions can be caused by several reasons including installation error and operator error. One EFIS manufacturer even cautions against attempting to "capture" the glide slope in approach mode when above the GS because this can result in a sudden pitch down.

The statement "The TT AP has been known to have uncommanded excursions, I have never heard of that issue with any of the other EFIS AP's," is kind of like saying "Van's RV-7's have been known to have inflight break-ups. I've never heard of that issue with the Glastar Sportsman."

Skylor
RV-8
Reno #80
Happy Tru-Trak Owner
 
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I don't think this is a fair assessment and seems somewhat anecdotal. Tru-Trak Autopilots were around long before most (if not all) of the experimental EFIS manufacturers even existed. The service history of TT is much longer than the "EFIS Mfg's" AP's and, at least until the last several years, the numbers in service are far greater than than those of the "EFIS AP's". Furthermore "uncommanded" excursions can be caused by several reasons including installation error and operator error. One EFIS manufacturer even cautions against attempting to "capture" the glide slope in approach mode when above the GS because this can result in a sudden pitch down.

The statement "The TT AP has been known to have uncommanded excursions, I have never heard of that issue with any of the other EFIS AP's," is kind of like saying "Van's RV-7's have been known to have inflight break-ups. I've never hear of that issue with the Glastar Sportsman."

Skylor
RV-8
Reno #80
Happy Tru-Trak Owner

My statement of 'uncommanded excursions' is based on my own personal experience with the digiflight II and Vizion as well as those of friends and customers with various models of the TT AP's over the years. The standard 'fix' for these excursions (can occur in either axis) is to perform the Gyro reset procedure.

I've personally owned and flew with about every model of the TT from the early Digitrak thru the Vizion models so I'm familiar with their systems.

Maybe/hopefully they have resolved this problem in the newer units, I don't know, but it is what it is.

And don't get me wrong, they were the AP pioneers and I respect them immensely, Andrew is a great guy and so are the tech guys I used to deal with (Lucas and others). Great company and I wish them all the success in the world.
 
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With my Dynon setup Dynon recommends to install a toggle switch to supply power to the servos, cheap insurance vs shearing the shear pin.

I've got a big fat toggle switch on mine (GRT Sport SX) to kill the power. A simple circuit that makes for great peace of mind!
 
closing this thread

I got to fly the subject airplane on Sunday. I did so after inspecting the servo rigging to rule out any over center possibility. I noted that the small buttons on the stick for elevator trim are cheaply made and possibly subject to mechanical sticking.
I also noted that the TT installation disables these switches when in altitude hold mode.
I could not create any nose down condition after testing all the functions of the autopilot. It operated smoothly and just as designed. I did three tests from unusual attitudes and then used the LEVEL (save my butt) button. Each time, the aircraft recovered swiftly. I was a little concerned about how quickly it pitched up when invoked from a nose down descending spiral simulation. I told the owner to check and see if the rate at which the system recovers level from nose down could be dialed back a bit. The RV9 series have very low maneuvering speed limits (in my opinion). I am always ready to throttle back quickly if encountering moderate to severe turbulence when at cruise speeds.
So, my thought was.... a quick pitch up when pushing the LEVEL button might impose undue stress and G forces in the recovery.
I did two final demonstrations to the owner how powerful the elevator trim tab is at cruise speed. He was quite surprised how much pull is needed to hold level. And I only deflected the tab partially. About a second or two on the button. A full extension would be severe in my observation.
So, time will tell if the software commanded a pitch change... or the autotrim drove the tab out of neutral in some manner. We did collar the system breaker in red and I suggested a yellow tab on the trim motor breaker as well.
I think if he encounters any more symptoms, they can be quickly mitigated by pulling both side by side breakers quickly.
Thanks to everyone who gave this some thought and replied. NC
 
pitch down

I had the same thing a couple of years ago, my Tru Trak Digiflight II VSVG was acting goofy. Tru Trak said head no longer available, sold me the later head at cost, works perfectly!

Gary Brown
DAR Pacific Northwest
 
When I first got my Vision it did this. The AP worked great on the first few test flights. Then I tired it on my first Cross Country. Since I was going to be going a long way I put in a higher altitude and climb rate. At around 3,000 ft it abruptly pinched down. This happened several times before I abandoned using it. After some on-the-phone troubleshooting with TT (Great people, and great support!). The sent me a new one and I returned mine. No problems with the new one, so I called to find out what they found. - It was a pinched tube internal that lead the AP to think I was approaching stall speed when I was not.
 
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