Narrative:

On a routine flight 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 altitudes shortly. The forecast was for light to occasionally moderate icing. I reported the moderate ice to ZOA and was given an altitude of 10000 ft and a vector for the localizer back course approach to runway 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 airspeed went from 140 KTS at 65% power to barely maintaining altitude at full power and 112 KTS. I informed center I was in severe icing and needed lower. The controller told me I was at the MVA and could get lower in 4 mins. I was faced with a choice. Descend and maintain 120 KTS, the recommended minimum speed in icing conditions or fly at 112 KTS and risk stalling and accumulating ice under the wing and tail but barely maintain altitude. 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 controller informed me I was below the minimum altitude, but there was nothing I could do. Eventually I got to an area with a lower MVA and was given instructions to descend. The ice melted off.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.