Inadvertent Servo Engagement

I've had 2 incidents recently that really got my attention, I thought something very wrong with the elevator control system. On both occasions it happened on landing.

First time was about 2 weeks ago, I could not flare the airplane normally, it took lots of back pressure and finally just felt like a ratcheting relief as the stick reluctantly came aft. I checked it over after landing and all was perfectly normal with feel and movement of the elevator.

Last Saturday it happened again in traffic pattern, this time half of usual nose up trim kept the airplane in trim and all felt normal - but I know something was not right and cautiously continued the approach and landing. During flare same thing, I could not get the nose to come up like usual and then it just broke free. The nose pitched up but was abled to salvage a fairly decent landing with a little power.

I knew instantly the auto pilot had been engaged!!

In talking with Trio yesterday we concluded control wheel steering engaged sometime just before landing. How could that happen? I con't have the disconnect circuit hooked up.

Well, the culprit has not yet been located but we are reasonable certain the unprotected wires are making ground and engaging control wheel steering. The wires are somewhere under the floor going aft to the servo.

Its not proof positive but Trio engineering and I are in agreement that is what has happened. All the evidence is there I just need to find the wires and shield them or hook them up to a switch.

Control pwr to servos

I have switches to power off my servos prior to landing because I have small control wheel switches on the stick which I could easily bump; and cause a similar problem.

I also have a "Autopilot Power" switch that I leave off until established in the climb and turn back off in the pattern. I know an inadvertent activation is rare but I don't want to be fighting the autopilot servos close to the ground.

The only problem this causes is an audible "WARNING" from the Skyview when I switch power off. I have gotten used to advising passengers of this behavior prior to cutting power.

the unwanted co-pilot on the stick

It would be prudent to add a couple of items to your checklist. Killing the main power to your autopilot prior to entering the pattern would be one.
In my RV, there are two separate systems. One for pitch and one for roll.
The pitch needs to initialize when sitting still, so that power switch comes on prior to start up. The Trio comes on anytime I want, since it does not seem sensitive to motion for the internal gyro.
We all know that an out of trim setting can give us a surprise on takeoff, so we have it on our checklist. I tried to rotate in a Cessna 421 one time after another pilot ran the pitch trim all the way nose down while doing some tests. I could not even get it off the ground. Good thing Boise had 10K feet of runway.
Anyway, any interference to the normally light control forces in an RV can lead to damage upon landing or takeoff. Since you can't control the wind, you already have enough surprises in store while getting down. Switching all autopilot master switches off can help. Another thing to have memorized is the location of the pullable breaker for pitch trim motors. If one of them runs away on you... the forces get heavy fast. Have fun out there !!!

Panel design flaw

In certified systems (trim or autopilot) there is always a means by which power can be cut off, and for good reasons, as you have discovered. In my plane the electric trim can be controlled manually, thru two switches, or by the autopilot. So I have pullable breakers for both, side by side, directly in front of me. If the trim runs away I need to pull both; then figure out the problem (on the ground).

David, I was flying in a friend's RV when we had a similar experience. I believe he had the same AutoPilot you have, I forget. Any how. His AP engaged but in the landing flare it disengaged any we about lost control. It was ugly. When he pulled his servo and sent it in they told him water had gotten into the servo and then possibly frozen during cruise and then re-thawed down low. This was after the Sun-n-Fun tornado storm. Might be something you want to look at.

apkp777 said:

David, I was flying in a friend's RV when we had a similar experience. I believe he had the same AutoPilot you have, I forget. Any how. His AP engaged but in the landing flare it disengaged any we about lost control. It was ugly. When he pulled his servo and sent it in they told him water had gotten into the servo and then possibly frozen during cruise and then re-thawed down low. This was after the Sun-n-Fun tornado storm. Might be something you want to look at.

Click to expand...

Thank you for sharing that experience, it is spooky when it happens. There has been no water near the servo's so that probably is not the cause.

So far I have identified the two wires in question coming from the control unit on the panel, one for pitch and one for roll. Both are buried in a wire bundle headed into and down the left gear tower. Removed front seat and floor panel yesterday but they are not with the servo wires there so must be terminated forward, probably in the left gear tower. I will locate the ends today. If either is bare and touches the airframe, control wheel steering will engage.

The on-off switch on the control unit kills power completely including that to servos. I may just leave it off for take off and landing but I believe locating those two wire ends will tell the story. They need be properly shielded or connected to a switch. Not very bright not having taken care of that earlier.

My AP will come on and do crazy things when I power down the two 11" Dynon panels. This happens when I cut the master and the right screen shuts down but the left screen stays on because of the backup battery.... This also happened one time several months ago on take off after I installed a cowl flap switch and left the Right screen power connection terminal loose on the switch. 300' off the ground and it started feeling like I had a elevator control problem! Having only about 30 hrs on the plane in Phase 1 it surely got my attention! It was caused by the right screen booting down then trying to reboot when the loose terminal would make connection again. I finally turned the right screen switch off and after Effis 1 realized Effis 2 was offline, everything was fine. After I landed, I found the loose connection.

Terms?

Isn't control wheel steering a method of temporarily disconnecting the servo, so that one can manually control? Then upon release of control wheel steering, the a/p re-engages. Seems that the term CWS is being used differently from typical. My (ancient now...) Century 2000 has a control wheel steering button on the stick. It disconnects the clutch on the servos when depressed, but the a/p is still active.

I just thought clarity on this might be important...

AlexPeterson said:

Isn't control wheel steering a method of temporarily disconnecting the servo, so that one can manually control? Then upon release of control wheel steering, the a/p re-engages. Seems that the term CWS is being used differently from typical. My (ancient now...) Century 2000 has a control wheel steering button on the stick. It disconnects the clutch on the servos when depressed, but the a/p is still active.

I just thought clarity on this might be important...

Click to expand...

Good point Alex.

With Trio the term is Pilot Command Steering (PCS). It is activated with the same button used to disconnect the servo, only difference is hold the button at least 5 seconds.

Theory right now is the wire is causing servo to reengage as it intermittently contacts ground. Seems a bit far fetched but the wire end has to located and secured as a start to resolve what has happened.

David-aviator said:

Good point Alex.

With Trio the term is Pilot Command Steering (PCS). It is activated with the same button used to disconnect the servo, only difference is hold the button at least 5 seconds.

Theory right now is the wire is causing servo to reengage as it intermittently contacts ground, Seems a bit far fetched but the wire end has to located and secured as a start to resolve what has happened.

Click to expand...

This is where I'm having some trouble understanding how this circuit is the root of your problem. CWS, or PCS I guess in Trio's case, only affects an A/P that is on and engaged. It should have absolutely no effect if the A/P is powered off or on but not engaged. IOW grounding that circuit should not cause the A/P to engage the servos because that's the exact opposite of what CWS does.. Granted I know nothing or Trio A/Ps as I have a TruTrak, but my understating is the concept is the same regardless of make. In my own system the A/P disconnect and CWS is the same switch -- press it momentarily and the A/P disconnects. Press and hold and CWS is activated releasing the servos until the button is released.

Auburntsts said:

This is where I'm having some trouble understanding how this circuit is the root of your problem. CWS, or PCS I guess in Trio's case, only affects an A/P that is on and engaged. It should have absolutely no effect if the A/P is powered off or on but not engaged.

Click to expand...

Some autopilot systems (Skyview for sure, probably others) have the ability to activate/engage the autopilot by holding the CWS button for a length of time, so long as power is available to the servos. This feature can also be disabled in the Skyview setup menus.

krw5927 said:

Some autopilot systems (Skyview for sure, probably others) have the ability to activate/engage the autopilot by holding the CWS button for a length of time, so long as power is available to the servos. This feature can also be disabled in the Skyview setup menus.

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Ahh so. I get it then.... Not sure if I like that setup though. YMMV....

Auburntsts said:

Ahh so. I get it then.... Not sure if I like that setup though. YMMV....

Click to expand...

Agreed. I want my pushbutton to do one thing only - disconnect the autopilot. I'm not flying yet but I'm configuring my Skyview screens, and I believe I'll disable that little "feature".

I located the 2 servo disconnect wires, I HAD properly insulated the ends there is no way they grounded to anything.
Back to the starting gate, the pitch servo inadvertently engaged twice just before landing, how come?
At this point I do not have a clew so I am going flying to see if it occurres again. If it does I will send the control unit to Trio and have them check it out.

...

airguy said:

Agreed. I want my pushbutton to do one thing only - disconnect the autopilot. I'm not flying yet but I'm configuring my Skyview screens, and I believe I'll disable that little "feature".

Click to expand...

Might want to explore that furhter before deciding, although I understand the inclination...

The G3X CWS breaks down like this...
Mash CWS Button wildly = disconnect
Hit button once= disconnect
Press and hold down for what I count to be at least 3 mississippi's, engages when released.

Still plenty peach of mind for the "button mash" approach to ensuring autopilot disengagement prior to TO.

To clarify. On the Trio with the servos engaged if you push the momentary switch on the stick it will disconnect the servo/servos. If the switch is held down for 3 seconds or more and then released and the current vertical rate is more than +- 200 fpm then the pitch servo will re-engage and fly the current vertical rate. Five seconds for the roll servo and it will re-engage and fly the current course.

David,
This is a long shot but I suggest you cut the servo disconnect wires off at the plug that goes into the control head to completely isolate them from the system.
Years ago I had issues with these wires that I had installed in the stick to a switch at the top of the stick. Often when I pushed the PTT switch (also on the stick) the AP would disconnect. Sounds weird but it seems there was some sort of interference between the servo disconnect wires and the PTT wire. Tried everything including shielding the wires with no success. Ended up having to remove the servo disconnect wires from the stick and running them well away from the PTT wire to a relay activated by the stick switch.

Fin
9A

David-aviator said:

I located the 2 servo disconnect wires, I HAD properly insulated the ends there is no way they grounded to anything.
Back to the starting gate, the pitch servo inadvertently engaged twice just before landing, how come?
At this point I do not have a clew so I am going flying to see if it occurres again. If it does I will send the control unit to Trio and have them check it out.

Click to expand...

David, go ahead and install a disconnect push button switch. It is very handy to be able to disengage both pitch and roll with one press of the button.

My switch is located on the panel just above the throttle where I can hit it with one finger while my hand is on the throttle. As others have stated, the switch will also give you PCS which is a nice feature.

Finley Atherton said:

To clarify. On the Trio with the servos engaged if you push the momentary switch on the stick it will disconnect the servo/servos. If the switch is held down for 3 seconds or more and then released and the current vertical rate is more than +- 200 fpm then the pitch servo will re-engage and fly the current vertical rate. Five seconds for the roll servo and it will re-engage and fly the current course.

David,
This is a long shot but I suggest you cut the servo disconnect wires off at the plug that goes into the control head to completely isolate them from the system.
Years ago I had issues with these wires that I had installed in the stick to a switch at the top of the stick. Often when I pushed the PTT switch (also on the stick) the AP would disconnect. Sounds weird but it seems there was some sort of interference between the servo disconnect wires and the PTT wire. Tried everything including shielding the wires with no success. Ended up having to remove the servo disconnect wires from the stick and running them well away from the PTT wire to a relay activated by the stick switch.

Fin
9A

Click to expand...

Good info Fin. The 2 wires were wrapped up tight with many others and who knows, gremlins may have been leaping about.

Sam Buchanan said:

David, go ahead and install a disconnect push button switch. It is very handy to be able to disengage both pitch and roll with one press of the button.

My switch is located on the panel just above the throttle where I can hit it with one finger while my hand is on the throttle. As others have stated, the switch will also give you PCS which is a nice feature.

Click to expand...

Good point about locating the switch, it does not have to be on the stick.

David-aviator said:

Good point about locating the switch, it does not have to be on the stick.

Click to expand...

It's that way on a couple of our planes Dave - retrifitting the autopilot was a lot easier than changing the stick that had no button, so we just mounted it near the throttle. Easy to use.

AlexPeterson said:

Isn't control wheel steering a method of temporarily disconnecting the servo, so that one can manually control? Then upon release of control wheel steering, the a/p re-engages. Seems that the term CWS is being used differently from typical. My (ancient now...) Century 2000 has a control wheel steering button on the stick. It disconnects the clutch on the servos when depressed, but the a/p is still active.

I just thought clarity on this might be important...

Click to expand...

In some models/brand the CWS will also activate the auto pilot if it is held for more than a second or two. It is another method of engaging your A/P

Got it all back together today including a push button switch to disengage servos or hold it in and go into PCS (Pilot Command Steering).

Flew with it and all was normal. Not much else on this subject, if it self engages again - will report it.

I must say, now that I am getting into the manual, this Trio Pro Pilot has more features than I will ever use and more than were available on commercial aircraft in a previous life. It is amazing what comes in a small package these days.

Amaze what comes in a cheap package too.

I had a TruTrak installed about a year ago. From memory, acquiring and paying someone to install it cost less than $3500 Australian.

I've spoken to a Cessna-owning friend about the fact that I'd installed it, and he whistled through his teeth and made an exclamation about how rich I must be. He couldn't believe it when I told him the price; In certified world, he expects to pay upwards of $10k for the features provided by the TruTrak.

But who'd fly certified, eh?

- mark