AN IAI-1124 CREW HAD A RUNAWAY TRIM AUTOPLT FAILURE.

Narrative

WE WERE APPROX 30-40 MINS INTO OUR 1 HR FLT. OUR FLT WAS COMPLETELY NORMAL UNTIL OUR AUTOPLT/TRIM STARTED THE ACFT DOWN AT APPROX 20-25 DEGS NOSE DOWN. BY THE TIME WE BOTH REACHED UP TO DISENGAGE THE TRIM/AUTOPLT WE WERE DSNDING THROUGH FL300 AT PROBABLY 4000 FPM. WITH BOTH OF US PULLING BACK TO STOP OUR DSCNT; WE REGAINED CTL OF THE ACFT AND HAD TIME TO TALK TO ZOB. THEY SAID THEY HAD OBSERVED OUR DSCNT AND WANTED TO KNOW WHAT WAS OUR STATUS. WE EXPLAINED AND WERE TOLD THAT THERE WAS NO CONFLICT OR PROB. WE WERE FURTHER ASKED IF WE NEEDED ANY SPECIAL HANDLING AND WE DECLINED. THE ACTUAL PROB IS STILL BEING TROUBLE-SHOT. BUT ON INITIAL CONVERSATION WITH AVIONICS MANUFACTURER; IT MUST HAVE BEEN A CATASTROPHIC TRIM/AUTOPLT FAILURE SOMEWHERE ALONG THE LINE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR TALKED WITH HIS MAINT CHIEF WHO SAID '...FOUND SOMETHING WRONG;...NOT SURE THAT IT CAUSED THE PROB.' ALL TRIM IN THIS ACFT IS ELECTRIC WITH SMALL INDICATORS THAT SHOW POS OF THE TRIM SURFACES. THE ELEVATOR TRIM INDICATOR IS NORMALLY NEAR THE END OF ITS TRAVEL IN TRIMMED FLT SO THAT IT IS DIFFICULT TO SEE A SEVERE OUT OF TRIM NOSE DOWN SIT. THE ELEVATOR TRIM SOUNDS AN AURAL ALARM WHEN IT IS MOVING. THERE WAS NO ALARM IN THIS INCIDENT. SUPPLEMENTAL INFO FROM ACN 271538: THE ACFT STARTED A SLIGHT PITCH UP ARC; ABOUT 2 DEGS ABOVE LEVEL FLT ATTITUDE. THE ACFT THEN PITCHED NOSE DOWN ABOUT 20-25 DEGS NOSE DOWN. THE AUTOPLT AND PRIMARY TRIM WERE DISCONNECTED AND A RECOVERY TO LEVEL FLT WAS MADE. THE FLT WAS CONTINUED TO CLEVELAND'S CUYANOGA COUNTY ARPT ON SECONDARY TRIM AND THE AUTOPLT OFF. A NORMAL APCH AND LNDG WAS MADE.