Flight Crew Air Transport Pilot (ATP)
Fatigue; Distraction; Workload
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance; General Maintenance Action
Environment - Non Weather Related; Aircraft; Human Factors
We departed out of BOS off of runway 9. Climbing out of 6;400 feet to FL260; the autopilot was engaged. The command bars were set in pitch mode ~5 degrees with altitude armed and not yet captured. Heading mode was the active lateral guidance. When the autopilot was engaged; the aircraft immediately pitched un-commanded to 9-10 degrees and was immediately followed by a nose down descending pitch of ~6 degrees. I am unsure what the exact pitch down was at the time; however; the aircraft was descending +1500 feet/min. Recognizing the autopilot was not responding to commands; I disconnected the autopilot. The autopilot disconnected; and the controls were extremely heavy with a nose down tendency. The autopilot had trimmed the aircraft nearly full nose down. By the time the aircraft was back under positive control; we had lost 300 feet (6;400 feet to 6;100 feet indicated) in the climb. Primary flight control movement was free and correct. The aircraft was hand flown until we topped the clouds. Reaching FL240; we attempted to reengage the autopilot. The flight director was commanding nav mode and altitude capture. The same results occurred. The last attempt was after we leveled off at FL260. In level flight with nav and altitude commanded; the autopilot still displayed the same results: a rapid pitch to 8-10 degrees nose up followed by a rapid descent to about 6 degrees nose down with the pitch trim motor trimming full nose down. The flight director commanded level flight during this occurrence. No Caution and Warning System (CAWS) messages or aural warnings were received. [Captain] called Maintenance and Company to inform them of the situation. VFR conditions prevailed in [destination]; thus; it was a good place to land. To combat fatigue from hand-flying; [Captain] and I alternated Pilot Flying/Pilot Monitoring duties. Several minutes before the descent; we collectively decided to run the QRH procedure for a failed autopilot since the conditions were near identical to what we had received. Further; we did not want the autopilot servo to engage un-commanded. The procedure called for the autopilot circuit breaker to be pulled. ATC gave us a crossing restriction on the arrival into [destination]: cross ZZZZZ Intersection at 11;000. During the descent; we ran the QRH procedure. The Autopilot circuit breaker was pulled; and the entire autopilot control panel became inoperative: the autopilot; altitude alerts; yaw damper; and flight director went out. Since the QRH did not specify we would lose these items; it came as a bit of a shock. Our workload increased drastically. As I was setting up the approach for the RNAV into ZZZ; [Captain] asked what altitude we were supposed to be at; and we then realized that we had blown our assigned altitude of 11;000 by 500 feet (10;500 feet). During this; we had been given a controller change to Approach. Checking on with the controller; we stated that we were 10;500 climbing 11;000 and stated that we had descended through our assigned altitude and were correcting for the mistake. The controller was very calm; and relaxed and did not mention a notice of pilot deviation. Had the QRH specified that the altitude alerts would be inoperative; we would not have had the altitude deviation. After landing; the autopilot was written up per the MEL and maintenance request. In the initial cruise; we also suffered a de-ice boot fail. We ran the QRH for a de-ice boot failure. The CAWS appeared at 35 seconds into the cycle. We maintained flight in non-icing conditions and landed in ZZZ. The de-ice boots were written up per the MEL and maintenance request. Limit autopilot failures to VFR conditions. Increase awareness that the QRH will not state the loss of altitude alerts; flight director; yaw damper; etc with this procedure. In my opinion; we would not have had the altitude deviation had the QRH stated it would become inoperative.
A PC-12 First Officer reported that they overshot an altitude crossing clearance during descent. Autopilot failure and increased workload were mentioned as key contributors.