A C172 FLT INSTRUCTOR WITH A STUDENT GET INTO ICING CONDITIONS WHILE IMC AND EXPERIENCE A COM FAILURE.

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ON AN INSTRUCTIONAL XCOUNTRY FLT WITH PRE-PVT STUDENT; DEPARTED VFR OUT OF SALEM; OR; TO FOLLOW COASTLINE ARPTS FOR GND SPD CHK POINT PRACTICE. UPON ARR NEAR COAST WITH VFR CONDITIONS BECOMING MARGINAL; I TOOK THE OPPORTUNITY TO GIVE THE STUDENT EXPERIENCE IN ACTUAL IMC RATHER THAN TO DIVERT AND LAND VFR -- GET THERE-ITIS MAY HAVE HAD A FACTOR IN THAT DECISION AS THE WK'S XCOUNTRY FLT WAS GETTING LONG. MY DECISION TO GO IFR WAS BASED ON WHAT MY STUDENT TOLD ME ABOUT HIS WX BRIEFING; AS I HAD NOT SPOKEN WITH A PREFLT BRIEFER BUT RATHER GOT MY BRIEFING UNOFFICIALLY FROM AN INTERNET WX SITE. I RECEIVED A POP-UP IFR CLRNC TO CRESCENT CITY ALONG V27 WHERE I PLANNED TO REVIEW WX TO CONTINUE VFR TO SANTA ROSA. ON THE FIRST LEG FROM ONP TO OTH AT 7000 FT MADE PIREP FOR CLOUDS AND NO ICE TEMP INDICATED APPROX 2 DEGS C. ONP-OTH HAS LOWER MEA'S AND AIRMET WAS IN EFFECT FOR ICE ABOVE FREEZING LEVEL. XING OTH MEA'S RAISE ELIMINATING AN ESCAPE FROM ICING CONDITION. MADE DECISION TO CONTINUE BASED ON PERSONAL WX OBSERVATION. 40 NM S OF OTH ON V27 EXPERIENCE MODERATE-SEVERE ICING FROM SUPER-COOLED RAINDROPS. ACCUMULATED ICE CAUSED COM AND NAV FAILURES DUE TO ANTENNA ICE. WE DSNDED THROUGH HOLE IN CLOUDS TO LAND. GOLD BEACH VFR UNDER LOST COM PROC. CALLED FSS ON GND TO GIVE UPDATE OF OCCURRENCE.