Trutrack Pictorial Pilot/ Trutrack Altrack Altitude Hold - Uncommand Operation

[tevbax]

So thankfully, this happened on the ground yesterday. Started the airplane, taxi to the ramp, and go through my typical "stir the pot" control check. The stick refused to move like the stabilator was locked. Shut the airplane down, make sure there wasnt an animal wedged in it, pull the inspection hood to look at the cables..... everything swings just fine. So, back in the plane, restart, finish my taxi to the ramp. Here we go again, the control stick is in full bind. This time, the brain kicks in and immediately disable the autopilot via the breaker. All returns to normal. 

Trying to look through the limited wiring diagrams I have to try and run this issue down. To my understanding, the AP should only engage when commanded by the button on the control stick. The AP took it upon itself to not only power on (which is normal, 10 second stand still for gyro) but to COMMAND itself on. If this would have been in flight, things would have had the opportunity to go very wrong very quickly. 

 

Really lost on this one, as I have never dug into the AP system yet. Where do I start? 

[Tom Baker]

I would start by checking the switch on the stick, and then the wiring from the stick to the AP.

[Warmi]

Regardless how your autopilot was engaged you should be able to overpower it unless it went into over-center which is not supposed to happen ever and is a function of physical aileron/elevator stops and installation - worth checking that out.

[Jim Meade]

Sounds like a wiring issue.  Binds when electricity is on, doesn't bind when electricity is off.  No consistent with a control binding or over-center situation, to me.

I don't know your avionics, but my AP can be engaged by buttons on the panel.  Do you have the old Tru-trak?  I thought it was engaged either by the stick or by the panel button.

FWIW, I've practiced runaway trim in a Citation and had an AP engage on takeoff on a C172, on a strip where an abort was not a good option, and I got it handled but it was not a real fun situation until I got to altitude and got the fuse pulled.  Same with runaway trim - I put a hat on the breaker from then on so I could find it by feel to pull.