SMA HAS ROUGH ENG; ALTERS COURSE; UNABLE TO REACH ARPT. LANDS THREE FOURTHS MI FROM ARPT.

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WE DEPARTED STEVENSVILLE; MT; (32S) ON FRIDAY AFTERNOON AND CLBED UP TO 11000 FT IN VFR CONDITIONS. WE WERE UNABLE TO PICK UP OUR CLRNC FOR ABOUT 20 MINS AND CIRCLED ABOVE 32S. MSO FINALLY CLRED US TO MSO VOR AND ON TO HLN. IT TOOK US ABOUT ANOTHER 10 MINS TO REACH THE VOR; AFTER RPTING OVERHEAD WE INTERCEPTED THE 096 DEG RADIAL (V-20 OUTBOUND. AT THIS TIME WE WERE BTWN CLOUD LAYERS AND THE TEMP WAS ABOUT 10 DEG F. AFTER WE WERE ESTABLISHED FOR ABOUT 5 MINS MSO HANDED US OFF TO SALT LAKE CTR. WE CONTINUED ON OUR FLT FOR ABOUT 5-10 MINS BEFORE HAVING PROBLEMS. AT FIRST IT FELT LIKE WE WERE ENCOUNTERING SOME TURB; BUT AFTER ONLY A COUPLE OF MINS THE TURB ENDED AND A SHAKING CONTINUED AND GOT WORSE QUICKLY. ABOUT THIS TIME THE MANIFOLD PRESSURE STARTED DROPPING RAPIDLY ON THE LH ENG. THE R SEAT PLT IMMEDIATELY TOOK THE FLT CTLS WHILE I WENT THROUGH THE EMER PROCS. ALL ENG CTLS WENT FORWARD; MAGS CHKED ON. BOOST PUMPS TURNED ON; CARB HEAT PULLED ON; FUEL CHKED ON; XFEED TURNED ON AND I CHKED THE ENG INSTS. THE R SEAT PLT STARTED A 180 DEG R TURN BACK TO MSO AND TOLD SLC ABOUT OUR SITUATION. THEY CLRED US BACK TO MSO AND TO CHANGE TO MSO TWR FREQ. I THEN CONTACTED MSO TWR. THEY CLRED US FOR THE VOR-C APCH AND AN ALT THAT WE WERE UNABLE TO HOLD. AFTER EXPLAINING THE ALT PROBLEM THEY CLRED US DIRECT TO THE VOR AND TO RPT BREAKING THROUGH THE CLOUDS. I KEPT CHKING AND DOUBLECHKING THE PROCS AND INSTS. AS BEFORE THE INSTS ALL LOOKED NORMAL EXCEPT FOR THE MANIFOLD PRESSURE WHICH HAD DROPPED OFF RAPIDLY. AT FIRST WE THOUGHT IT WAS STRUCTURAL FAILURE DUE TO THE SEVERE SHAKING; BUT SINCE THE ENG CONTINUED TO WINDMILL WE BEGAN TO SUSPECT INDUCTION ICING. WE TRIED EVERY COMBINATION OF THROTTLE; MIXTURE; AND CARB HEAT AND NOTHING WORKED. I THEN TOOK THE CTLS WHILE THE SECOND PLT WENT THROUGH THE EMER CHKLIST. HE WAS ALSO UNABLE TO GET A RESTART. I THEN DECIDED TO FEATHER THE LH ENG. WHILE WORKING THE THROTTLE CTLS TO CONFIRM THE CORRECT ENG THE ENG RESTARTED AND RAN FOR ABOUT 20 SECONDS AND STARTED TO QUIT AGAIN. I DISCOVERED THAT BY WORKING THE THROTTLE EVERY 10 SECONDS THE ENG CONTINUED TO DEVELOP THRUST. SINCE IT APPEARED THAT WE WERE GETTING BETTER THAN ZERO THRUST WE DECIDED TO LEAVE THE ENG RUN. WE BROKE THROUGH THE CLOUDS ABOUT 4 MI E OF THE ARPT ABOUT LINED UP WITH RWY 25. WE CONTINUED ON LONG FINAL WITH GEAR UP; FLAPS UP AND L ENG RUNNING. WHEN IT WAS APPARENT WE WERE UNABLE TO MAKE THE RWY WE DECIDED ON AN ALF-ALPHA FIELD IMMEDIATELY TO OUR R. WE HAD THE FIELD MADE SO I STARTED SHUTTING THE ACFT DOWN. I SHUT OFF MAGS; PUMPS AND PULLED THROTTLES AND MIXTURES BACK. MEANWHILE THE R SEAT PLT HAD STARTED EMER EXTENSION OF THE GEAR. SINCE THE GND WAS COMING UP QUICKLY I TRIED TO MAKE AS SOFT A LNDG AS POSSIBLE WITH THE NOSE GEAR OFF. WE TOUCHED DOWN GOING UP A SLIGHT HILL. THE NOSE ENDED UP COLLAPSING AFTER HITTING A GOPHER MOUND. THE PROPS HIT AND WE SLID ABOUT 150 YDS. AFTER SHUTTING OFF PWR AND GRABBING A FIRE EXTINGUISHER WE EXITED THE ACFT WITH NO INJURIES.