PLT OF A MOONEY; M20P; DECLARED AN EMER DUE TO ACFT ICING OF THE AIRFRAME AND PITOT TUBE CAUSING LOSS OF AIRSPD AND ACFT PERFORMANCE.

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IFR FLT PLAN FROM SJC TO SMO CHANGED BY ATC TO VNY DURING FLT. FLT CONDITIONS FOR THE FLT WERE IN AND OUT OF CLOUDS. AT 9000 FT OVER FIM VOR IN IMC; NEW CLRNC TO VNY WAS RECEIVED. AT THE SAME TIME WHAT HAD BEEN ONLY FROST OR TRACE ICE DURING THE COURSE OF THE FLT CHANGED TO RAPID ACCUMULATION OF ICE ON WINGS AND PROP. ALSO LOST THE AIRSPD INDICATOR; WENT TO ZERO; BECAUSE OF BLOCKED PITOT TUBE. DECLARED EMER BECAUSE OF CONCERN OVER STALL CONDITION WITH NO AIRSPD OR VISUAL REF AND BEGAN RAPID DSCNT WITH ATC VECTOR TO S AND LOWER TERRAIN. IN TROUBLESHOOTING THE PROB I REALIZED THE PITOT HEAT HAD NOT BEEN TURNED BACK ON UPON RE-ENTERING CLOUDS. TURING ON PITOT HEAT AND DSNDING BELOW 5000 FT BROUGHT UP AIRSPD INDICATOR AND SHED AIRFRAME ICING. ABLE TO PROCEED FROM SANTA PAULA ON NEW VECTOR FOR LDA APCH TO VNY. PROB WAS A COMBINATION OF PLT ERROR (NO PITOT HEAT) CAUSED BY DISTR OF NEW CLRNC AND SIGNIFICANT CHANGE IN ICING CONDITIONS COMPARED TO EARLIER PORTION OF THE FLT. CURRENCY AND A LOWER COASTAL ROUTING WOULD PREVENT A RECURRENCE OF THE EVENT.