PLT ENCOUNTERS ICING. ALTDEV ALT EXCURSION.

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OBTAINED WX BRIEFING FOR FLT VIA DUAT ON MORNING OF FLT. HAD ALREADY CHKED DUAT AND WX CHANNEL THE PREVIOUS EVENING AND WX LOOKED GOOD FOLLOWING PASSAGE OF COLD FRONT DURING THE NIGHT. THIS COLD FRONT HAD BROUGHT THE FIRST COOL WX OF THE SEASON. EXPECTED CLR CONDITIONS OVER FILED RTE SO DID NOT PAY TOO MUCH ATTN TO FREEZING LEVEL FORECAST TO BE 5000-6000 FT ALONG NORTHERN PART OF RTE. FIRST LEG OF FLT FROM MQI TO DCA WAS IN BEAUTIFUL CLR CONDITIONS UNTIL RIGHT ABOVE DCA WHERE CEILING WAS 6000 FT BROKEN. DEPARTED DCA FOR RDU. WAS NOT GIVEN FILED RTE WHICH WOULD HAVE TAKEN ME BACK INTO CLR CONDITIONS. WAS RADAR VECTORED TO THE W WHERE CLOUDS HAD DEVELOPED. ENTERED CLOUDS AT ABOUT 5000 FT; FILED ALT WAS 6000 FT. PICKED UP SOME RIME ICE; REQUESTED 8000 FT. CTLR INFORMED ME RIME ICE HAD BEEN RPTED AT 8000 FT. DECIDED I SHOULD CLB WHILE I COULD. ACCUMULATION APPEARED MORE MODERATE AT 8000 FT; SO REQUESTED 10000 FT. DURING CLB TO 10000 FT BEGAN TO SEE TOPS ABOVE; SO REQUESTED VFR ON TOP AT 10500 FT. CTLR TOLD ME TO STANDBY WHILE HE WORKED ON 10500 FT. DUE TO THE AMOUNT OF ACCUMULATION (ABOUT 1/2 INCH OF WING LEADING EDGE AND WHAT APPEARED TO BE MORE ON LEADING EDGE OF STABILATOR) AND THE FACT THAT MY AIRPSD AND RATE OF CLB WAS DECREASING; I ADVISED HIM OF URGENCY. AT THIS POINT HE TOLD ME TO SQUAWK 7700 AND CLB TO 10500 FT. UNFORTUNATELY 10500 FT DID NOT PUT ME COMPLETELY CLR OF CLOUDS. ATC THEN CLRED ME TO 12000 FT. MANAGED A CLB TO 12000 FT AT WHICH POINT I WAS WELL IN THE CLR. RESUMED NORMAL SQUAWK CODE; AS INSTRUCTED BY ATC. APART FROM AIRSPD BEING RATHER SLOW; EVERYTHING WAS FINE AT 12000 FT UNTIL I ENCOUNTERED A DOWN DRAFT; PROBABLY FROM A MOUNTAIN WAVE. DUE TO FREQ CONGESTION AND A PROB WITH ATC UNDERSTANDING MY XMISSIONS; I WAS UNABLE TO INFORM ATC THAT I COULD NOT MAINTAIN 12000 FT UNTIL I WAS ALREADY AT 11600 FT. BECAUSE CLOUD COVER BELOW WAS BREAKING UP; I EVENTUALLY REQUESTED AND RECEIVED CLRNC TO 4000 FT; WHERE I WAS ABLE TO SHED THE ICE. CONTINUED FLT TO RDU WITHOUT FURTHER PROB. I SHOULD HAVE PAID MORE ATTN TO FREEZING LEVEL AND CLOUD COVERAGE NOT ONLY ALONG FILED RTE BUT IN VICINITY. WX OVER MY FILED RTE WAS CLR; BUT ONLY SLIGHTLY FARTHER TO THE W; WHERE I WAS VECTORED; CLOUDS HAD DEVELOPED OVER THE MOUNTAINS OF VIRGINIA. ALTHOUGH I HAVE USED DUAT SINCE THE SVC WAS INTRODUCED; AND CONSIDER MYSELF TO BE EXPERIENCED IN INTERPRETING WX; A CALL TO THE FSS WOULD HAVE BEEN BENEFICIAL IN ALERTING ME TO THESE CONDITIONS. SINCE THIS WAS THE FIRST COOL WX OF THE SEASON; I NEGLECTED TO ANALYZE THE POSSIBILITY OF ICING.