PILOT FLYING A PA-46-350 MALIBU MIRAGE ON AUTOPILOT; EXPERIENCES A SUDDEN PITCH DOWN. WHEN THE AUTOPILOT WAS DISCONNECTED; THE AIRCRAFT PITCHED UP VIOLENTLY AND THEN PITCHED DOWN BEFORE PILOT WAS ABLE TO REGAIN CONTROL.

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WE WERE ON A DIRECT IFR FLT FROM ZZZ2 TO ZZZ. THE FLT WAS IN VFR CONDITIONS UNTIL WE ARRIVED ABOUT 15 MILES FROM ZZZ. THE CLOUDS WERE BROKEN AROUND 3000 FT TO 4000 FT (MVFR). WE WERE CLEARED TO DESCEND TO 3000 FT. THE OWNER OF THE ACFT WAS FLYING (HE HOLDS A PRIVATE PLT LICENSE ONLY). WE HAD THE AUTOPLT ON. WE HAD JUST STARTED TO RECEIVE VECTORS FOR THE ILS/DME RWY X APCH INTO ZZZ. THE AUTOPLT SUDDENLY PITCHED THE ACFT NOSE DOWN ABOUT 30 DEGS. WE WENT THROUGH OUR ASSIGNED ALT OF 3000 FT DOWN TO ABOUT 2400 FT. (AIRSPD WAS AROUND 165 KTS.) BEFORE I COULD GET THE AUTOPLT DISENGAGED; THE PLANE SUDDENLY PITCHED UP VIOLENTLY AND THEN PITCHED DOWN AGAIN WITH ENOUGH FORCE THAT EVERYTHING WAS FLYING AROUND THE COCKPIT AND CABIN. I HIT THE TOP OF THE COCKPIT HARD; IT STUNNED ME FOR A FEW SECONDS. DURING THIS TIME ZZZ APCH WAS CALLING AND VERY UPSET THAT WE HAD NOT ANSWERED THEM. MY FIRST PRIORITY WAS TO GAIN CONTROL OF THE ACFT. I GOT THE AUTOPLT DISENGAGED AND REGAINED CONTROL OF THE ACFT. I COULD NOT GET THE LOCALIZER TO ENGAGE. I CALLED ZZZ APCH AND LET THEM KNOW WE HAD AN AUTOPLT ISSUE AND COULD NOT INTERCEPT THE LOCALIZER; AT THIS TIME WE WERE IN VFR CONDITIONS AND HAD ZZZ IN SIGHT. WE WERE ABOUT 2 TO 3 MILES NORTH OF ZZZ LEFT OF THE CENTER LINE OF RWY X. APCH CLEARED US FOR THE VISUAL APCH TO RWY X. WE CHANGED TO CTAF FREQ AND FLEW OVER THE ARPT AND LET THEM KNOW WE HAD A PROBLEM AND THEN REENTERED THE RIGHT DOWNWIND FOR RWY X AT ZZZ. THERE WAS A C-172 IN THE PATTERN DURING THIS TIME. WE LANDED WITHOUT FURTHER INCIDENT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE PA-46-350 MALIBU MIRAGE THEY WERE FLYING HAS A KING-150 FLIGHT CONTROL SYSTEM AUTOPILOT WITH FLT DIRECTOR. REPORTER STATED THEY HAD SET THEIR VERTICAL AT 3000 FT; AND WHEN THAT ALTITUDE WAS REACHED; THE ACFT STARTED THE UNCOMMANDED PITCH DOWN; THEN A VIOLENT PITCH UP AND DOWN AGAIN BEFORE HE COULD REGAIN CONTROL. TWO AUTOPILOT DISENGAGE OPTIONS ARE AVAILABLE; ONE ON THE CONTROL PANEL AND THE OTHER IS A RED BUTTON ON THE CONTROL WHEEL. REPORTER STATED HE WAS QUITE SURPRISED AT THE AMOUNT OF FORCE REQUIRED TO TRY AND PHYSICALLY OVERRIDE THE AUTOPILOT WHEN THE FIRST UNCOMMANDED PITCH DOWN HAPPENED. WHEN HE DISENGAGED THE AUTOPILOT USING THE CONTROL WHEEL RED BUTTON; THE ACFT PITCHED UP AGAIN. REPORTER STATED HE FELT THE AUTOPILOT STILL WANTED TO RE-ENGAGE. SINCE THEN; OTHER PILOTS HAVE INFORMED HIM THEY HAVE HAD SIMILAR EXPERIENCES IN CESSNA-414'S WHERE THE ACFT PITCHES UP WHEN THEY DISENGAGED THEIR KING-150 AUTOPILOT. REPORTER STATED HE WAS GRATEFUL THESE UNCOMMANDED FLIGHT CONTROL PITCHES DID NOT HAPPEN WHILE ON ILS AT 500 FT.