ATX PLT FLIES INTO FORECAST ICING CONDITIONS IN ACFT NOT EQUIPPED TO DEAL WITH ICING.

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DURING A WX BRIEFING; I WAS TOLD BY A BRIEFER THAT THERE WAS AN AIRMET FOR POSSIBLE LIGHT TO MODERATE ICING FROM 500 TO 900 FT UP TO 20000 FT MSL. HOWEVER; THE BRIEFER POINTED OUT THAT THERE WERE NO PLT RPTS OF ICING ALONG THAT RTE; SO I FILED FOR AND RECEIVED A CLRNC FOR 8000 FT MSL FROM SGF TO MKC. DURING THE FLT AT 8000 FT I NOTICED THE OUTSIDE AIR TEMP FROM MY TEMP PROBE WAS NEAR FREEZING SO I REQUESTED A LOWER ALT AND RECEIVED CLRNC TO 6000 FT. AFTER APPROX 15 MINS I STARTED TO PICK UP A TRACE TO LIGHT ACCUMULATION OF RIME ICE. I REQUESTED A LOWER ALT AGAIN AND WAS CLRED TO 4000 MSL. I POPPED OUT OF THE BASES AT 4500 AND ALL OF THE ACCUMULATION WAS GONE. I CONTINUED THE FLT TO MY DEST WITHOUT INCIDENT. THE PROB AROSE FROM THE MISUNDERSTANDING OF FLT INTO KNOWN ICING CONDITIONS. MY ACFT IS NOT EQUIPPED WITH DEICE/ANTI-ICE AND THE ACFT MANUAL STATES NOT APPROVED FOR KNOWN ICING. HOWEVER; KNOWN ICING IS NOT DEFINED. I FOUND OUT THROUGH A FELLOW PLT THAT FLT INTO KNOWN OR FORECAST ICING IS NOT APPROVED; UNLESS DEICE EQUIP IS ON BOARD AND SPECIAL CERTIFICATION OF EQUIP IS REQUIRED FOR KNOWN ICING. I THINK MANY PLTS ARE CONFUSED ABOUT WHAT KNOWN ICING IS; PERHAPS MAKING THAT INFO MORE READY AVAILABLE THROUGH ADVISORY CIRCULARS OR BULLETINS; ESPECIALLY SINCE THE ICING SEASON IS APCHING.