PC-12
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem
Weather; Aircraft
On the initial climb; Pilot Flying (PF) engaged autopilot at approximately 6000 ft with 10;000 ft selected on the altitude selector. After 1 -2 seconds the aircraft pitched over and entered an uncomanded descent of approximately 1000 - 1500 FPM. After a loss of approximately 300 ft; the PF disconnected the autopilot; leveled the aircraft and re-entered the climb. During the pitch over and descent the command bars did not follow the pitch over and descent; but rather showed a nose low indication. There were no Cockpit Aural Warning System (CAWS) or aural warnings associated with the event. Once stable in the climb again; we attempted to re-engage the autopilot with the same result. The autopilot was again disconnected; and the decision was made to hand fly the remainder of the climb. We reached VMC conditions at approximately FL200; and leveled off at FL260. With residual rime ice on the wings; we opted to leave the boots on to help remove it quicker. It was at this time that the de-ice boots amber CAWS illuminated. We ran the procedure; but the boots failed a second time and were left in the off position per the QRH. As we're were in VMC conditions; the remaining ice eventually sublimated. In cruise we reviewed the QRH and determined that the Autopilot Trim procedure was the only thing that closely resembled what we had experienced; although the conditions for the procedure include the Autopilot Trim CAWS and aural warning; which we did not have. We decided to try the autopilot one more time to see if it reacted the same way from a cruise condition with the altitude captured. This time engaging the autopilot caused the aircraft to initially pitch up approximately 8 degrees; followed by a pitch down and a descent. The PF disengaged the autopilot and leveled back at FL260. Altitude deviation was less than +/-100 ft. We opted to hand fly the remainder of the flight; and after contacting operations and maintenance; we decided to continue to our planned destination. After further discussion between the crew; we also decided to safe the system per the Autopilot Trim checklist in the QRH prior to reaching 10;000 ft. After completing our initial descent below the flight levels we were assigned a further descent of 11;000 ft. I assumed the the controls; while the FO began with the QRH checklist; the last item of which calls for the Autopilot circuit breaker to be pulled. Upon pulling the circuit breaker; at approximately 12;500 ft we lost our flight director; yaw damper and altitude controller; which also disabled the aural altitude warnings. We continued with a stable descent but realized we had missed our assigned altitude by approximately 700 ft; stopping the descent at 10;300 ft. We climbed back up; to 11;000 ft and reported the deviation to ATC; also noting that we were in the process of working an avionics issue. The controller acknowledged the deviation; but had no further response. The remainder of the flight was flown without incident; with a visual approach and normal landing.
PC-12 Captain reported that they connected the autopilot during climb and the aircraft pitched over and went into an uncommanded descent.