What was found
ON DESCENT TO RWY 27, ON ABOUT AN 8 MILE FINAL I REDUCED POWER TO SLOW DOWN. WHEN I WENT TO ADD POWER ONCE AGAIN THE LH SIDE PRODUCED NO ADDITIONAL THRUST. UPON LOOKING AT THE ENGINE INDICATIONS I NOTICED N1 WAS AT 35%, AUTO IGNITION WAS ACTIVATED, AND AUTO FEATHER HAD BEGUN TO FEATHER THE PROP. SINCE I WAS ON APPROACH AND STABLE I ELECTED TO COMPLETE THE ENGINE FAILURE MEMORY ITEMS TO SECURE THE ENGINE. LANDING WAS UNEVENTFUL. HELENA AIRPORT IRE/RESCUE MET ME ON THE RAMP BUT NO ADDITIONAL ASSISTANCE WAS REQUIRED.