Narrative:

On final approach; tower told us there was moderate turbulence reported and cleared us to land on runway xy. We were on the ILS and proceeding with an autopilot coupled approach. As we continued our approach we experienced a wave of moderate-severe turbulence that disengaged the autopilot. The turbulence started at approximately 1200 feet MSL. We reengaged the autopilot and successfully recaptured the localizer and glide-slope. At approximately 600 feet MSL; we encountered another wave of moderate-severe turbulence that was more intense than the first one. The autopilot disengaged again and I noticed the CDI was deflecting rapidly. I also noticed generator 2 voltage dropping from 22V. I called for missed approach as the CDI was rapidly approaching half scale deflection. The pilot flying pressed the go-around button and began to climb as I called tower and advised we were on the missed. Tower gave us a heading; told us to climb to 3000 feet and asked our intentions. At that moment; I advised we would like to try the approach again. During the process of cleaning up the airplane; we heard the 'sink rate' call out and I noticed we were descending. I instructed PF to climb. He told me he couldn't and needed help. I noticed he was pulling back on the yoke. As I grabbed the yoke I realized it was extremely heavy. I looked down and saw the stab trim was all the way down. I tried to re-trim and about that time we heard rapid beeping. I quickly hit trim interrupt-norm and pushed the test button on the autopilot. I then tried again to re-trim and was successful. ATC called and advised they showed us in a descent and asked if everything was ok. I advised we had a control issue and we were trying to climb. We were both pulling back pressure on the yoke and wings were level but was still slowly descending until we were able to get the stab trim back. I then advised ATC we were electing to go to [a nearby airport]. Tower handed us off to approach and we were given our route. I elected [said airport] due to previous knowledge that it was VFR. I did not know if the turbulence caused damage or if the autopilot/trim was the problem or any other anomalies so I did not want to go back into IMC conditions; especially did not want to conduct any low instrument approach procedures. We continued our flight without any more control problems so I did not [advise of any emergency conditions]. We did not get any central aural warning system warnings during this event. Closer to [diversion airport]; approach advised us that no one was landing [there] visually and told us to expect GPS 24. At that time; we were in VMC conditions and had [a third airport] in sight. I cancelled IFR with approach. They advised us cancellation was received; maintain VFR and gave us tower frequency. We proceeded to [the third airport] and landed successfully.

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Original NASA ASRS Text

Title: PC12 flight crew reported a missed approach during an ILS due to turbulence; autopilot disengagement and full nose down trim. Crew reported a successful divert to a VMC airport.

Narrative: On final approach; tower told us there was moderate turbulence reported and cleared us to land on RWY XY. We were on the ILS and proceeding with an autopilot coupled approach. As we continued our approach we experienced a wave of moderate-severe turbulence that disengaged the autopilot. The turbulence started at approximately 1200 feet MSL. We reengaged the autopilot and successfully recaptured the localizer and glide-slope. At approximately 600 feet MSL; we encountered another wave of moderate-severe turbulence that was more intense than the first one. The autopilot disengaged again and I noticed the CDI was deflecting rapidly. I also noticed Generator 2 voltage dropping from 22V. I called for missed approach as the CDI was rapidly approaching half scale deflection. The Pilot Flying pressed the go-around button and began to climb as I called tower and advised we were on the missed. Tower gave us a heading; told us to climb to 3000 feet and asked our intentions. At that moment; I advised we would like to try the approach again. During the process of cleaning up the airplane; we heard the 'sink rate' call out and I noticed we were descending. I instructed PF to climb. He told me he couldn't and needed help. I noticed he was pulling back on the yoke. As I grabbed the yoke I realized it was extremely heavy. I looked down and saw the stab trim was all the way down. I tried to re-trim and about that time we heard rapid beeping. I quickly hit trim interrupt-norm and pushed the test button on the autopilot. I then tried again to re-trim and was successful. ATC called and advised they showed us in a descent and asked if everything was ok. I advised we had a control issue and we were trying to climb. We were both pulling back pressure on the yoke and wings were level but was still slowly descending until we were able to get the stab trim back. I then advised ATC we were electing to go to [a nearby airport]. Tower handed us off to Approach and we were given our route. I elected [said airport] due to previous knowledge that it was VFR. I did not know if the turbulence caused damage or if the autopilot/trim was the problem or any other anomalies so I did not want to go back into IMC conditions; especially did not want to conduct any low Instrument Approach Procedures. We continued our flight without any more control problems so I did not [advise of any emergency conditions]. We did not get any Central Aural Warning System warnings during this event. Closer to [diversion airport]; approach advised us that no one was landing [there] visually and told us to expect GPS 24. At that time; we were in VMC conditions and had [a third airport] in sight. I cancelled IFR with approach. They advised us cancellation was received; maintain VFR and gave us tower frequency. We proceeded to [the third airport] and landed successfully.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.