PC-12
Flight Crew Private; Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 45; Flight Crew Total 1450; Flight Crew Type 45
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Aircraft
[We were] at FL230 when the plane suddenly pitched aggressively down and autopilot disengaged.  I immediately grabbed [the] yoke and pitched up and tried to access the situation.  During this time the plane climbed about 1000 feet to FL240.  A safety pilot was with me and we discussed issue as we worked to get plane down to assigned altitude. After reviewing the situation we configured plane for level flight with manual control and decided to engage the autopilot again.  Same thing happened.  After a long time we went through all of the information on board and decided that perhaps we had a simple problem of incorrect nose pitch on autopilot and decided to try autopilot one more time with manual control of pitch up.  This did not work as it resulted in an even worse altitude excursion and ATC informed us that the traffic space was tight and we needed to hold that altitude and forget diagnosing autopilot.  We asked for descent to FL190 and flew the [remainder of the] trip without event.In hindsight I simply should have only tried the diagnosis one time or asked ATC for a block altitude to try a couple of things with autopilot. Weather at [destination] around [arrival] time was forecast to be around 800 feet with snow and 2-4 miles visibility; although it had been improving during flight according to onboard weather data. Fortunately I felt comfortable and was proficient at hand flying rest of trip and RNAV approach although conditions had improved enough that a visual approach could have been done. Decided to do approach for safety factor after stress of incident; long day and night conditions plus additional practice flying hand approach.
PC-12 pilot reported an altitude excursion resulted when the autopilot suddenly pitched down.