FLC OF AN LTT TURBOPROP EXPERIENCED UNFORECAST ACFT ICING DURING DSCNT AND APCH RESULTING IN THE FLT CTLS BECOMING LESS RESPONSIVE; BUT STILL SUFFICIENT TO CTL THE ACFT.

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DURING A SCHEDULED FLT FROM MSP TO RFD; RFD HAD EARLIER IN THE DAY FREEZING RAIN ON AND OFF ALL DAY CHANGING OCCASIONALLY TO SNOW OR ICE PELLETS. WHEN WE GOT OUR DEP WX CURRENT RFD WX CALLED FOR 700 BROKEN; 3 MI; LIGHT ICE PELLETS; FOG. AFTER DEP WE CALLED FLT WATCH AND CHKED CURRENT PIREPS AND WX. RFD WAS 700 BROKEN; 2 MI; LIGHT ICE PELLETS; FOG. ALL THE PIREPS ALONG OUR RTE OF FLT WERE FOR EITHER NO ICE OR LIGHT TO MODERATE RIME AND MODERATE TO SEVERE TURB ABOVE FL180. FLT WAS CONDUCTED AT 13000 FT; IMC ENTIRE FLT. NO ICE WAS ENCOUNTERED ENRTE AND THERE WERE ALSO NO PIREPS IN THE RFD AREA; CLOSEST WAS MSN. FIRST DSCNT WENT TO 9000 FT; OUTSIDE AIR TEMP WAS APPROX -4 DEGS C WITH LIGHT MIXED ICE AND SNOW. NEXT DSCNT AT APPROX 25-30 NM OUT TO 4000 FT. DURING DSCNT RADAR SHOWED THIN LINE OF WX (LIGHT RAIN) ABOUT 1 MI WIDE A COUPLE MI AHEAD. PASSED THROUGH THIS AREA DSNDING THROUGH 6000 FT. OUTSIDE AIR TEMP +3 DEGS C; NO ICE; LIGHT RAIN. UPON REACHING 4000 FT STARTED TO ENCOUNTER FREEZING RAIN. OUTSIDE AIR TEMP 0 TO -1 DEG C. CTR OVERHEAD WINDOW GLAZED OVER WITHIN 1 MIN. WE WERE APPROX 20 NM OUT BEING VECTORED FOR ILS RWY 7. REQUESTED DSCNT TO 2600 FT (MINIMUM ALT FOR ILS). TEMP ON ATIS WAS 28 DEGS F. WING BOOTS WERE COVERED ALL THE WAY BACK TO THE WING AND SIDE DV WINDOWS HAD SOME ICE BUILDUP; SPINNER WAS GLAZED OVER APPROX 1/2 WAY TO PROP. TEMP AT 2600 FT WAS APPROX -2 DEGS C AND CONTINUED TO GET INTERMITTENT FREEZING RAIN OR DRIZZLE. NO UNCOMMANDED CHANGES IN PITCH OR ROLL OCCURRED; HOWEVER; CTLS WERE SLOPPY. DURING DSCNT ON APCH FREEZING RAIN CHANGED TO ICE PELLETS. FROM RECENT ARTICLES ON THIS EVENT AND ON SCDD; WE FOLLOWED THEIR RECOMMENDATIONS ON CHANGING ALT AND COURSE; HOWEVER; WE WERE UNABLE TO COMPLETELY LEAVE THIS REGION OF FREEZING RAIN. ONLY OTHER THOUGHT WAS TO CLB BACK TO LEVEL WHERE IT WAS STILL RAIN AND ABOUT +2 DEGS AND HOPE IT MELTED OFF; HOWEVER; SHORTEST DURATION SEEMED TO BE TO CONTINUE WITH APCH AND LAND. EXTENDED FLT IN THESE CONDITIONS WOULD HAVE DEFINITELY PRODUCED ADVERSE FLT CHARACTERISTICS OR CTL PROBS.