Runaway trim! Scary stuff!

[OR75]

On ‎10‎/‎9‎/‎2016 at 2:33 PM, Bennett said:

Not necessarily true. When I took a lightning strike that fused much of my panel, and about all of the instruments that were powered, the pitch servo ran to full down trim and jammed in position. I finally was able to regain control by killing breakers , and forcing the manual trim wheel. In the case of the "rebuilt" pitch servo (different Mooney) the servo ran full down, and it too jammed. This time it was easier to manually force the trim wheel after punching out on the on/off switch, AP disconnect, and attempting to disengage the autopilot via the yoke electrical trim switch. When the servos jam in their fully extended position, you have a serious problem. A runaway servo is unusual, but it does happen. I spent a fair amount of time discussing this with a Bendix/King engineer as to their certification tests for the KAP 150. Their filing states that their test pilot was able to recover from a full down servo excursion in 400'. The ATC. "Snitch" will go off in a 300' deviation in altitude when IFR and there is another aircraft in the vicinity. It took me 13 months before a FAA senior engineer agreed with me and Bendix/King, that a 400' exclusion because of a runaway pitch servo was not actionable, and dropped the potential violation they attempted.

I won't disagree with you (disconnecting a runaway trim by any mean is a success)

But I am saying that a properly functioning servo should disconnect with manual force. It is designed for that.

 

[PeytonM]

As an update, my avionics tech DID find a broken wire in my trim switch on the yolk, but could not determine whether that was the cause, or whether it came un-soldered when he lifted the switch out to examine it.  (I need to follow up with Century; they did not offer any reason why several transistors on the trim board failed.)  I'm over a month out since getting the repair completed on and I'm doing manual trimming during critical phases of flight  e.g. takeoff and landing.

[Fly By]

Picked up my plane after a new autopilot had been installed (non MSC shop).  During the takeoff roll at rotation I discovered something was horribly wrong.  The plane was extremely nose heavy and began to porpoise.  It bounced back on the runway and required a very large amount of elevator pull to keep it from impacting the runway and possible prop strike.  Adrenaline is a good thing!

Post incident inspection revealed that the trim had been improperly reassembled resulting in two full turns of the trim wheel nose down when the trim indicator was indicating normal takeoff range!  The trim was readjusted, checked, and has been fine since.

[Bob_Belville]

5 hours ago, Fly By said:

Picked up my plane after a new autopilot had been installed (non MSC shop).  During the takeoff roll at rotation I discovered something was horribly wrong.  The plane was extremely nose heavy and began to porpoise.  It bounced back on the runway and required a very large amount of elevator pull to keep it from impacting the runway and possible prop strike.  Adrenaline is a good thing!

Post incident inspection revealed that the trim had been improperly reassembled resulting in two full turns of the trim wheel nose down when the trim indicator was indicating normal takeoff range!  The trim was readjusted, checked, and has been fine since.

Lee, the previous owner of my E was retired Air Force, living in DC, working for NASA. A few days after he delivered it to me when I got to fly it I sent him a note that neither the trim nor flap indicators were visible (completely off scale or disconnected). He wondered why anyone would need to look down there. He went by the yoke position and looked out the window to verify flaps. There were several other little systems he did not need enough to keep maintained.

[flyboy0681]

1 minute ago, Bob_Belville said:

Lee, the previous owner of my E was retired Air Force, living in DC, working for NASA. A few days after he delivered it to me when I got to fly it I sent him a note that neither the trim nor flap indicators were visible (completely off scale or disconnected). He wondered why anyone would need to look down there. He went by the yoke position and looked out the window to verify flaps. There were several other little systems he did not need enough to keep maintained.

You have got to be kidding. Position of the yoke?

[kevinw]

There's a good article about runaway trim in the current issue of AOPA Pilot (Jan 2017 issue, page 87) titled "The Other Autopilot Failure". In the article one reason this happens is the pilot disconnects the AP while putting forward or back pressure on the yoke which may prevent the pitch servo from disconnecting. To make matters worse, the more you fight it the more it wants to go in the other direction. Other than pulling the breaker or shutting off the master, the way to disconnect it is to pitch in the direction the servo is going and it will release. The lesson is always disconnect the AP with the controls neutral. Worth reading for those of you who subscribe. 

[Andy95W]

5 hours ago, flyboy0681 said:

You have got to be kidding. Position of the yoke?

Yes.  On the legacy airplanes with the spring bungees connected to the elevator, the yoke moves as you move the trim.  That is actually how I reset my trim after landing- by yoke position.

[flyboy0681]

9 minutes ago, N1395W said:

Yes.  On the legacy airplanes with the spring bungees connected to the elevator, the yoke moves as you move the trim.  That is actually how I reset my trim after landing- by yoke position.

I watch it move all the time but I couldn't tell you where it's positioned when the trim is neutral. However, I do know exactly where it should be positioned when doing my walk around and examining the tail section.

[Fly By]

On modern transport category aircraft there are painted stripes to indicate the limits and zero trim positions.  This might be a nice enhancement on our Mooneys that would provide a visual confirmation during walk around that the trim is set for takeoff since our stabilizers move relative to the fuselage just like the big boys!