Date: 202007
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Flight conditions: VMC
Light: Night
Aircraft: PC-12
Operator: Air Carrier
Operating under: Part 135
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class B ZZZ
Aircraft component: Horizontal Stabilizer Jackscrew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We suffered a pitch trim runaway in the downwind leg of the traffic pattern while conducting a night currency flight. I was acting as the pilot monitoring; the flaps were set to 15; and the pilot flying was hand-flying. Upon feeling the changing trim forces; the pilot flying first attempted to comply with the memory item of hitting the trim interrupt switch. He had trouble reaching it by feel underneath the switch cover. After a couple seconds he alerted me to the increasing pitch-down forces and asked me to hit the interrupt. I initially miss-heard him and thought he said 'I hit the trim interrupt' causing a momentary delay. I then saw him fighting the nose-down forces on the yoke and understood what was going on. At this point I hit the trim interrupt. Right before I engaged the switch I heard the automated voice call-out 'trim runaway' which confirmed what I had already determined was the situation. The pitch trim was in a significant nose-down setting which; while it didn't appear to be at the full nose-down stop; was requiring the pilot flying to apply significant back-pressure on the yoke to keep the aircraft level. The aircraft remained under control and no altitude loss was suffered.As we started a descent and turned base I asked the pilot flying if he would like to continue with the landing or leave the pattern to sort out the situation. He indicated that he had sufficient control and we should continue with the landing. The landing gear was lowered and flaps set directly to 40 as we turned final. At this point he asked me to take control of the power so he could use both hands on the yoke. As requested by the pilot flying; I maintained about Vref + 10 knots which; combined with the full-flaps configuration; seemed to work well. I briefly contemplated [advising ATC] at this point; but we were only about 500 feet AGL and I determined that the extra communications required would be more detrimental in a phase of flight where we needed clear communication between myself and the pilot flying since I was controlling the power. We continued to a landing with this set-up; the pilot flying used both hands to flare; the touchdown was normal; and I put the engine into reverse and then idle as our speed reduced. Total time from first indication of the failure to the landing was only a couple minutes at most; it may have been less than one minute.Complicating factors include multiple aircraft types and fatigue. I had just been flying a PC-12 NGX aircraft earlier that day (the event aircraft was a PC-12 NG). I had flown four legs while giving familiarization training to a first officer in the various differences between the aircraft sub-types. The NGX aircraft has a different (and much improved) trim interrupt system including a redesigned button. The pilot flying is also qualified in the PC-24 aircraft which; while I am unfamiliar with; I believe has no trim interrupt. These different elements may have caused a slight delay in our response to the situation. Further issues involve how the PC-12 NG simulator we use for training emulates this failure. As I recall the simulator sounds the 'trim runaway' call-out very quickly. In the event we experienced; there was a several second delay and the trim had moved to almost the full nose-down position before the automated aural call-out occurred.Given the training I have received (and given for that matter) I expected the call-out to occur with this failure much sooner. Since I was the pilot monitoring and could not feel the increasing pitch forces; not hearing an expected call-out contributed to the momentary delay in understanding the situation and what the pilot flying was asking me to do (comply with the memory item). Lastly; fatigue may have been a factor as well. I had given training in a different aircraft earlier in the day; had about a two-hour break; then conducted pilot monitoring duties for two pilots during their night currency flights; who each needed three takeoffs and landings. This event occurred on the last landing of the second pilot's flight. I was certainly fit for the flight but with this failure the difference between a very easy situation and a much more difficult one is a matter of only a couple seconds.




Synopsis: PC-12 flight crew reported a stabilizer trim runaway; causing an air turnback and a precautionary landing.