ACFT EQUIP PROB MALFUNCTION CREATES A TEMPORARY LOSS OF ACFT CTL FOR PLT OF SMT TWIN.

Narrative

WHILE ON AN IFR FLT APPROX 3 MINS AT CRUISE AT 4000 FT; SMOOTH AIR; IMC ON AUTOPLT; ALT HOLD HDG MODE. CLRED DIRECT TO SOLBERG VOR APPROX 90 MI AHEAD. WITH NO FOREWARNING OR PREVIOUS MALFUNCTION HISTORY; THE TRIM WHEEL FOR PITCH RAPIDLY ROTATED FROM ITS CRUISE SETTING TO FULL NOSE DOWN IN LESS THAN 1 SECOND. THE NEGATIVE G- FORCES CREATED; CAUSED ALL LOOSE EQUIP; MAPS; ETC TO FILL THE CABIN. I AND MY PAX WERE RESTRAINED BY ONLY OUR SEATBELTS. ACFT IS NOT EQUIPPED WITH SHOULDER RESTRAINTS. WE WERE BODY EXTENDED BUT MY L HAND WAS GRIPPING THE YOKE AND I DISCONNECTED THE TRIM AND AUTOPLT. WITH GREAT EFFORT I BROUGHT MY R ARM DOWN AND ELECTED TO CLOSE BOTH THROTTLES AS IAS WAS APPROX 210 KTS. MY PAX IS A HIGHLY EXPERIENCED MULTI-ENG INST RATED PLT AND I ACCEPTED HIS OFFER OF ASSISTANCE AS THE FORCES REQUIRED TO ARREST DSCNT WERE VERY GREAT. ATC NOTED THE ALT LOSS AND I TOLD THEM WE HAD A PROB AND THEY REPLIED NO TFC CONFLICT BUT KEEP THEM ADVISED. I ROLLED THE ELEVATOR TRIM TO NEUTRAL; COORDINATING MY ACTIVITY WITH THE PF WHO HAD TO KEEP BOTH HANDS ON THE YOKE. AS THE SPD DECAYED TO FORMER CRUISING LEVELS; I NOTIFIED ATC OF OUR RETURN TO ASSIGNED. HIS REPLY WAS TO THE EFFECT HE WAS PLEASED AND HIS LAST STATEMENT WAS 'NO PROB.' I PULLED THE TRIM CIRCUIT BREAKER ACCORDING TO THE SUPPLEMENT AND WE NEVER USED THE AUTOPLT AGAIN. THE REMAINDER OF THE FLT WAS ROUTINE. INSPECTION UPON LNDG REVEALED NO SKIN DISTORTION; NO POPPED RIVETS AND CABLE TENSIONS NORMAL. UPON CONTACTING THE MANUFACTURER THEY WERE AT A LOSS TO EXPLAIN HOW A REQUIRED FAULT FOR CERTIFICATION COULD HAVE SUCH RESULTS. THE STC MAX EXCURSION WITH TRIM AMPLIFIER SHORT IS 150 FT WITH AUTOPLT DISCONNECT AND A 3 RPM RATE BY FAULT; EASILY CORRECTED BY PLT FORCE. AT PRESENT; WE ARE INVESTIGATING HOW; YRS BEFORE I BOUGHT THE ACFT; THE INCORRECT TRIM MOTOR WAS INSTALLED. BY SERIAL NUMBER; IT IS THE SAME ONE PUT IN BY AUTOPLT INSTALLER AND HAD NEVER BEEN CHANGED. TIME IN SVC 620 HRS.