Uncommanded pitch down- Tru Trak

[RONSIM]

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

[sailvi767]

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!

[AndyRV7]

Originally Posted by BobTurner

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.

[Ted RV8]

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?

[rocketbob]

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.

[flightlogic]

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.

[RV8JD]

Originally Posted by sailvi767

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!

This technique bears repeating. In a uncontrolled pitch up where it's hard to get the nose down by just pushing the stick forward and you need to get the nose down quickly, the technique of rolling the airplane to some large bank angle to bring the nose down to the horizon could save one's bacon someday. It's a technique that's been discussed and used successfully more than a few times over the decades.

[Capt]

Originally Posted by sailvi767

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!

[DHRYCAUK]

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

[Canadian_JOY]

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.