AIR TAXI PLT OF C402C CARGO FLT ENCOUNTERS ICING BUT CANNOT GO LOWER DUE TO MVA. DOWNDRAFT CAUSES ALTDEV.

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ON A ROUTINE FLT FROM 1O5 TO RDD I ENCOUNTERED MODERATE RIME ICE AT 11000 FT. THE DEICE BOOTS WERE KEEPING UP WITH THE ACCUMULATION AND I WAS EXPECTING LOWER MINIMUM VECTORING ALTS SHORTLY. THE FORECAST WAS FOR LIGHT TO OCCASIONALLY MODERATE ICING. I RPTED THE MODERATE ICE TO ZOA AND WAS GIVEN AN ALT OF 10000 FT AND A VECTOR FOR THE LOC BACK COURSE APCH TO RWY 16 AT REDDING. IN ABOUT A PERIOD OF 1 MIN I HAD AN ICE ACCUMULATION FROM 1/2 INCH TO SEVERAL INCHES WITH ABOUT 3 INCH RAM'S HORNS ON THE WINGS. MY AIRSPD WENT FROM 140 KTS AT 65% PWR TO BARELY MAINTAINING ALT AT FULL PWR AND 112 KTS. I INFORMED CTR I WAS IN SEVERE ICING AND NEEDED LOWER. THE CTLR TOLD ME I WAS AT THE MVA AND COULD GET LOWER IN 4 MINS. I WAS FACED WITH A CHOICE. DSND AND MAINTAIN 120 KTS; THE RECOMMENDED MINIMUM SPD IN ICING CONDITIONS OR FLY AT 112 KTS AND RISK STALLING AND ACCUMULATING ICE UNDER THE WING AND TAIL BUT BARELY MAINTAIN ALT. UNFORTUNATELY I HAD BEEN IN AND OUT OF LEE WAVES; AND I GOT IN ABOUT A 500 FPM DOWNDRAFT WHICH TOOK ME TO 9500 FT; BELOW THE MVA. THE CTLR INFORMED ME I WAS BELOW THE MINIMUM ALT; BUT THERE WAS NOTHING I COULD DO. EVENTUALLY I GOT TO AN AREA WITH A LOWER MVA AND WAS GIVEN INSTRUCTIONS TO DSND. THE ICE MELTED OFF.