Narrative:

While on an IFR flight approximately 3 mins at cruise at 4000 ft, smooth air, IMC on autoplt, altitude hold heading mode. Cleared direct to solberg VOR approximately 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 equipment, maps, etc to fill the cabin. I and my passenger were restrained by only our seatbelts. Aircraft is not equipped with shoulder restraints. We were body extended but my left hand was gripping the yoke and I disconnected the trim and autoplt. With great effort I brought my right arm down and elected to close both throttles as IAS was approximately 210 KTS. My passenger is a highly experienced multi-engine instrument rated pilot and I accepted his offer of assistance as the forces required to arrest descent were very great. ATC noted the altitude loss and I told them we had a problem and they replied no traffic 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 speed 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 problem.' I pulled the trim circuit breaker according to the supplement and we never used the autoplt again. The remainder of the flight was routine. Inspection upon landing 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 maximum excursion with trim amplifier short is 150 ft with autoplt disconnect and a 3 RPM rate by fault, easily corrected by pilot force. At present, we are investigating how, yrs before I bought the aircraft, 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 service 620 hours.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.