RWY EXCURSION RESULTED FROM TEMPORARY LOSS OF ACFT CTL IN SMT IN A NIGHT OP.

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DEST WAS AUW. WX RPTED AS 600 OVCST; 2 MI IN LIGHT FREEZING DRIZZLE AND FOG. ACTUALLY 400 AND 2. ATTEMPTED AND MISSED VOR APCH. DIVERTED TO NEARBY CWA. WAS TOLD TO EXPECT ILS RWY 8. APCH PROCEEDED NORMALLY. BY THIS TIME HAD SPENT APPROX 20 MINS ACCUMULATING AND SHEDDING LIGHT TO MODERATE RIME ICE. WINDSHIELD WAS ICED OVER EXCEPT FOR THE HOT PLATE SECTION DIRECTLY IN FRONT OF THE L PLT POS. WIND WAS A L QUARTERING TAILWIND OF 5-10 KTS. RWY WAS ICE COVERED BUT ATIS INDICATED THE CTR 80 FT WAS SANDED. WITH THE L CRAB ANGLE ESTABLISHED EARLY IN THE APCH. COULD NOT SEE THE R SIDE OF THE RWY UPON BRAKING OUT. HAD DIFFICULTY RECOGNIZING AND CORRECT FOR WIND DRIFT DURING TRANSITION TO VISUAL BECAUSE OF TUNNEL VISION CAUSED BY 1) THE SIZE OF THE HEATED SECTION OF WINDSHIELD; 2) DISTORTION IN THE HEATED WINDSHIELD; AND 3) THE FACT THAT IT WAS DARK. TOUCHED DOWN R OF THE CTRLINE AND IMMEDIATELY SLID R AND OFF PAVEMENT. ACFT STRUCK A LNDG LIGHT. WAS ABLE TO RETURN TO THE RWY AND DECELERATE NORMALLY. DAMAGE TO 1 PROP BLADE; R OUTBOARD GEAR DOOR; AND R FLAP. THE BACK COURSE APCH WAS AN OPTION BUT QUESTIONABLE DUE TO THE CEILING AND BACK COURSE MINIMUMS.