DEPARTING WITH A SURFACE TEMP OF 40 DEGS; A C185 FINDS ICE ACCUMULATION ON THE WINGS AT A 4000 FT CRUISE ALT. CLRNC TO A LOWER ALT IS OBTAINED BUT AN EXCURSION OF THE ASSIGNED ALT OCCURS DUE TO INABILITY TO MAINTAIN SPD DUE TO ICE.

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AFTER FILING AN IFR FLT PLAN AND PICKING UP THE CLRNC WITH BRADLEY APCH; I CLBED TO 4000 FT OVER ELLINGTON ARPT AND PROCEEDED DIRECT TO GARDNER VOR; THEN PLANNING TO LAND AT NASHUA; NH. THE WX AT THE TIME WAS 1600-1800 FT OVCST WITH THE TEMP AROUND 40 DEGS F. DURING THE CLB; THE C185 AMPHIBIAN BEGAN PICKING UP TRACES OF MIXED ICE ALONG THE LEADING EDGES OF THE WINGS. SHORTLY AFTER REACHING 4000 FT; I DECIDED TO DSND TO 3000 FT TO TRY AND REACH WARMER TEMPS AND TURN BACK TO WESTOVER AFB WHERE I KNEW THE CEILINGS WERE BETTER THAN 1500 FT. BRADLEY CLRED ME TO 3000 FT AND WAS VECTORING ME TO WESTOVER. THROUGHOUT THE DSCNT AND UPON REACHING 3000 FT; ICE BEGAN ACCUMULATING MUCH FASTER THAN EXPECTED. SHORTLY THEREAFTER; FULL PWR WOULDN'T ALLOW ME TO HOLD ALT AT 3000 FT. WHILE TRYING TO MAINTAIN CTL OF THE ACFT AND FIND THE APCH PLATES FOR WESTOVER; I HAD DSNDED TO 2600 FT. AFTER CONFIRMING MY ALT; BRADLEY CLRED ME TO 1500 FT AND GAVE ME A VECTOR DIRECT TO THE ARPT. AT AROUND 1800 FT; I BROKE OUT OF THE OVCST LAYER AND LANDED AT WESTOVER. THIS INCIDENT HAS TAUGHT ME TO EXERCISE MORE CAUTION WHEN ENCOUNTERING ICING CONDITIONS AND TO COMMUNICATE MY SIT BETTER WITH ATC SO THEY ARE ABLE TO DO THEIR BEST TO HELP ME OUT.