PLT OF SMA ENCOUNTERED ICING ON WINGS. UNABLE TO OBTAIN CLRNC TO DSND. ADVISED CENTER HE COULD NOT MAINTAIN ALT. HAD TO DSND FROM 7000 TO 6800 BROKE OUT OF OVCST AND SHED ICE.

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DEPARTED N54 ON IFR FLT PLAN TO OQU. REQUESTED 9000 FT CRUISE. WAS HELD TO 7000 AND 8000 AS FLT PROGRESSED N AS ASSIGNED TO LAKE HENRY; (PA) VOR. HAD PASSED THROUGH AND NEAR SCATTERED PUFFY CLOUDS. FORECAST WAS FOR SCATTERED. WHEN CLRED TO 9000 AS REQUESTED BEGAN TO GET ICING ON WINDSHIELD AND LEADING EDGES OF WINGS (RIME). RPTED ICING TO ATC AND REQUESTED HIGHER OR LOWER; BUT THE CLOUD BEGAN TO GET DARKER. WAS CLRED BACK TO 7000; STILL SOLID IMC. RPTED TO ATC STILL NEEDED LOWER; SOLID IMC; STILL IN ICING CONDITIONS. ATC ANSWERED WITH 'MAINTAIN 7000.' I ANSWERED SOMETHING LIKE 'UNABLE; ICING; MUST DSND; LEAVING 7000.' I BEGAN MY DSCNT; BROKE OUT AT 6800; WAS CLRED VERY SOON THEREAFTER TO 6000 AND THEN TO 5000 FOR REMAINDER OF FLT. I FELT I MUST DSND TO GET OUT OF ICING CONDITIONS EVEN THOUGH ATC TOLD ME TO MAINTAIN 7000. THE CONDITION I ENCOUNTERED WAS NOT FORECAST AND I COULD NOT SEE IT AHEAD SO I DO NOT HAVE ANY RECOMMENDATION ON HOW TO AVOID IT IN THE FUTURE. I REMAINED ON MY ASSIGNED RTE AT ALL TIMES.