A C-208 CARGO PLT SUFFERS A TXWY EXCURSION DURING A NIGHT OP WHEN TAXIING WITH A FROSTED WINDSHIELD AT ICT; KS.

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I RPTED FOR DUTY; CHKED IN WITH DISPATCH; AND CALLED FOR A WX BRIEFING WITH FSS. I THEN PROCEEDED OUTSIDE AND PERFORMED A PREFLT ACFT CHK. WHILE PERFORMING THE PREFLT; I OBSERVED A THIN LAYER OF FROST (LESS THAN 1/32 INCH THICK) COVERING THE WINDSCREEN AND PORTIONS OF THE ACFT (APPROX 25% OF THE ACFT WAS COVERED WITH FROST). WHILE WEARING A GLOVE; I DRAGGED A FINGER ACROSS THE WINDSCREEN AND EASILY CLRED AN AREA FREE OF FROST; WHILE NOTING ITS THICKNESS. I ELECTED NOT TO DEICE THE ACFT WITH FLUID AND CHOSE TO DEFROST THE WINDSCREEN WITH ENVIRONMENTAL HEAT; AND ELECTRICALLY DEFROST THE HOT PLATE. 5 MINS PRIOR TO LOAD COMPLETION; I CALLED FOR CLRNC AND TURNED ON THE WINDSHIELD ANTI-ICE FOR APPROX 60 SECONDS; CLRING THE HOT PLATE FROM FROST. AFTER LOADING WAS COMPLETE; I SECURED THE CARGO DOOR AND CHOCKS AND WAS READY FOR ENG START. I CALLED GND FOR TAXI AND WAS CLRED FOR TAXI TO RWY 19R. I WAS MARSHALED OUT OF MY SPOT AND BEGAN TAXIING SLOWLY TO RWY 19R WITH ALL AVAILABLE HEAT ON THE WINDSCREEN. THE RTE I CHOSE TO RWY 19R WAS APPROX 800-1000 FT TOTAL TAXI DISTANCE. HOWEVER; WHILE TAXIING; CARGO RAMP LIGHTING AND MY ACFT TAXI LIGHTS WERE CAUSING A GLARE EFFECT ON THE FROSTY WINDSCREEN. THE ACFT LEFT THE HARD SURFACED CARGO AREA. I IMMEDIATELY APPLIED BRAKES AND STOPPED; COMING TO REST IN A GRASSY AREA BTWN THE CARGO RAMP (W OF) AND TXWY D (E OF). I IMMEDIATELY INFORMED GND CTL THAT I WAS IN THE GRASS W OF THE CARGO RAMP. THEY REPLIED 'OK' AND AGAIN ASKED FOR MY POS. I REPLIED WITH MY POS AND ASKED THEM TO HOLD ON SO I COULD ASSESS THE SIT. I THEN OPENED THE PLT'S DOOR AND TURNED ON ALL FORWARD LIGHTING TO SEE THE AREA BETTER. THE GRASS WAS CUT SHORT AND THE GRADE OF THE AREA WAS SUCH THAT I HAD NO CONCERN OF A PROP STRIKE. I INFORMED GND THAT I NEEDED TO MAKE A 180 DEG TURN TO RETURN TO THE CARGO RAMP AREA; AND THEY APPROVED MY REQUEST. HOWEVER; PRIOR TO MOVING THE ACFT; I NOTICED A DRAINAGE GRATE APPROX 10 FT TO THE L OF MY ACFT (S OF MY POS) AND FEARED COLLAPSING THE GRATE IF COMING IN CONTACT WITH IT. I THEN INFORMED GND THAT THE AREA AHEAD OF ME WAS CLR AND ASKED TO TAXI STRAIGHT AHEAD TO TXWY D. THEY APPROVED MY REQUEST; AND I TAXIED SLOWLY AHEAD THROUGH APPROX 50 FT OF GRASS AND RETURNED TO HARD SURFACE ON TXWY D. I CONTINUED TO RWY 19R AND CALLED TWR SHORT OF RWY 19R AND REQUESTED A SHORT DELAY. I THEN PERFORMED AN OVERSPD GOVERNOR CHK AND RECHKED THE PAR UNIT FOR ANY EXCEEDENCES IN ENG PARAMETERS. I NOTED NO EXCEEDENCES; ABNORMALITIES; OR VIBRATIONS THAT WOULD INDICATE ANY DAMAGE TO THE ACFT. I THEN CALLED TWR AND RECEIVED TKOF CLRNC. I COMPLETED THE FLT AS SCHEDULED AND POSTFLT EXAM REVEALED NO DAMAGE TO THE PROP; ENG INLETS; MAIN LNDG GEAR; OR NOSEWHEEL. IT IS IN MY OPINION THAT THIS POTENTIAL INCIDENT COULD HAVE BEEN PREVENTED HAD I TAKEN THE TIME TO CLR THE WINDSCREEN WITH A SOFT CLOTH; SOFT RUBBER OR PLASTIC SCRAPER; OR WITH DEICE FLUID. IN THE FUTURE; THE ABOVE PROCS WILL BE PRACTICED BY THIS PLT. IN THE EVENT THE ABOVE PROCS CANNOT BE PERFORMED; I WILL USE ALL AVAILABLE HEAT TO CLR THE WINDSCREEN. HOWEVER; I WILL ALSO ALLOW THE ADEQUATE TIME NEEDED FOR THE HEAT TO COMPLETELY DEFROST THE ENTIRE WINDSCREEN BEFORE MOVING THE ACFT IN A MOVEMENT OR NON MOVEMENT AREA.