A PA34 ENCOUNTERS ICE AND FREEZING RAIN BEYOND THE CAPABILITIES OF THE INSTALLED DEICING SYS TO REMOVE IT. ALT WAS LOST DOWN TO THE MINIMUM VECTORING ALT FOR THE AREA.

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I WAS ON IFR FLT PLAN FROM MMU TO 27K. WX BRIEFING CALLED FOR RIME/MIXED ICING TO 8000 FT ALONG RTE. ACFT WAS APPROVED FOR FIKI; AND ALL EQUIP WAS WORKING. WHILE IN IMC AT 8000 FT; WAS PICKING UP VERY LIGHT RIME ICE WITHOUT ANY PROBLEM REMOVING WITH DEICE EQUIP. SUDDENLY ENCOUNTERED HVY FREEZING RAIN (OUTSIDE AIR TEMP -10 DEGS). IN SPITE OF PITOT HEAT; AIRSPD WENT TO ZERO. FREEZING RAIN COATED ACFT AND OVERWHELMED BOOTS AND HEATED WINDSHIELD. PROP DEICE WAS INEFFECTIVE. WX BRIEF WRONG -- NO MENTION OF POTENTIAL FOR FREEZING RAIN. NO SIGMETS ISSUED WHILST ENRTE FOR FREEZING RAIN. REQUESTED IMMEDIATE DSCNT. WAS CLRED TO 6000 FT; HOWEVER; COULD NOT STOP DSCNT AT 6000 FT DUE TO CONTAMINATED LIFTING SURFACES AND LACK OF AVAILABLE PWR BECAUSE OF ICED PROPS. CONTINUED DSCNT TO 4100 FT WHERE FREEZING LEVEL WAS LOCATED. FREEZING RAIN BECAME HVY LIQUID PRECIPITATION; WHICH CLRED PITOT TUBE ICE; RESTORING AIRSPD INDICATION. RAIN PLUS HEATED PROPS AND BOOTS CLRED ICE FROM ACFT AND CONTINUED FLT AT MINIMUM VECTORING ALT OF 4100 FT. LATER; CTR DEVIATED ME NW TO AVOID HIGHER MVA. TURNED ON COURSE AND COMPLETED BALANCE OF FLT AT 4000 FT; CLR OF CLOUDS. DUE TO BEING IMC; I COULD NOT ANTICIPATE THE ENCOUNTER WITH FREEZING RAIN. NO ONBOARD RADAR ALSO WAS A FACTOR. PRINCIPLE REASON FOR THIS NEAR ACCIDENT WAS INCORRECT WX FORECAST. WHEN I EXPERIENCED TOTAL AIRSPD LOSS; IN IMC; I SHOULD HAVE DECLARED AN EMER -- ESPECIALLY AFTER INABILITY TO HOLD ALT. HVY HEADWINDS MADE FOR A LONG AND BUMPY TRIP AND FATIGUE WAS THEREFORE A FACTOR. NERVOUS PAX WAS ALSO A SMALL FACTOR. IMMEDIATE ASSISTANCE FROM ZOB WAS A POSITIVE FACTOR.