PLT OF SMT EXPERIENCES FAILURE OF AILERON TRIM MECHANISM RESULTING IN FULL L AILERON TRIM. MANUAL AILERON INPUTS RESULT IN LESS THAN ACCEPTABLE CTL. PLT LANDS AT NEAREST SUITABLE ARPT.

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I TOOK OFF FOR A MEDICAL TRANSPORT OP. I HAD 2 MEDICAL ATTENDANTS AND 1 PATIENT AND HIS WIFE IN THE AIRPLANE. DURING THE CLB AND CRUISE; EVERYTHING LOOKED NORMAL. BUT BEFORE I STARTED DSCNT; I FOUND THE AILERON TRIM INDICATOR SHOWED A LITTLE OFFSET. AFTER I STARTED DSCNT; SUDDENLY I HEARD A VERY LOUD BANG NOISE THEN THE AIRPLANE STARTED TO TURN TO THE R. I TRIED TO CTL THE AIRPLANE TO FLY STRAIGHT AND CHKED ALL ENG INSTS. ALL ENG GAUGES SHOWED FINE. WHEN I CHKED L AILERON; I FOUND AN AILERON TRIM STUCK AT FULL UP POS. AIRPLANE STILL TRYING TO TURN INTO R; FORTUNATELY; I STILL HAD AN AILERON CTL AND TRIED TO FLY AIRPLANE STRAIGHT. EVEN IF IT WAS VERY HVY; I COULD FLY. I CALLED ATC AND EXPLAINED WHAT HAPPENED TO MY AIRPLANE. I TOLD THEM I HAD TO LAND AIRPLANE ASAP. I SAW THE CLOSEST AIRFIELD; WHICH WAS DILLINGHAM AIRFIELD; ABOUT 20 MORE MI AHEAD; AND I FLEW THE AIRPLANE INTO IT. ALL LNDG GEAR AND FLAPS WORKED FINE. THEN I LANDED AT DILLINGHAM AIRFIELD WITHOUT ANY PROB. AFTER I LANDED; I CALLED ATC TO RPT WE LANDED SAFELY AND THANK THEM FOR THEIR HELP. ALSO; I CALLED TO COMPANY OPS TO EXPLAIN ABOUT A SIT AND ARRANGED FOR OTHER TRANSPORTATION FOR THE PATIENT. NEXT DAY; ONE OF OUR MECHS CHKED THE AIRPLANE AND HE FOUND THAT A ROD CONNECTED TO THE AILERON TRIM WAS BROKEN. I NEVER HEARD SUCH A THING BEFORE. HE SAID LIKELY A METAL FATIGUE MUST BE THE CAUSE OF THIS. HOWEVER; FLT CTL IS VERY IMPORTANT. I HOPE THEY WILL CHK AND INSPECT THESE FLT CTL SYS MORE CLOSELY AND CAREFULLY AND THIS THING WILL NEVER HAPPEN AGAIN.