SMA IFR ENCOUNTERS ICING CONDITIONS ENRTE TO RBL. IS CLRED TO CLB TO FL190 WHERE FORECAST WAS TO BE ON TOP. AT FL190 PICKING UP LIGHT ICING AND REQUEST DSCNT TO 5000 FT. CLRED TO DSND TO 16000 FT AND ADVISE INTENTION. REQUEST VECTOR TO EUG. CHANGED TO EUG APCH CTL AND DSNDED TO 9000 FT BTWN LAYERS AND ELECTED TO CONTINUE FLT WITH NO FURTHER PROBS. DURING RAPID DSCNT HAD DEVIATED FROM V23 4 DEGS.

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DEPARTED THUN FIELD; PIERCE COUNTY; WA (1S0) AT APPROX XA00Z UNDER IFR. CEILING 3000-4000 FT OVCST. OCCASIONAL LIGHT RIME OR MIXED ICE RPTED/FORECAST FROM FREEZING LEVEL (7000-9000 FT MSL) TO FL200. PIREPS RPTED TOPS AT 16000 FT TO FL180 ALONG RTE OF FLT. CLBED TO FL190 AND INTERCEPTED J-1 WITHOUT INCIDENT. STILL IMC AT FL190. CLRED TO PROCEED AS FILED TO RED BLUFF; CA (V23) AFTER BATTLE GROUND VOR. SEVERAL MINS S OF BATTLE GROUND VOR BEGAN TO PICK UP TRACE RIME ICE AND REQUESTED CLB TO FL210. STILL IN IMC AT FL210 (+5 DEGS F) BEGAN TO ACCUMULATE MIXED ICE ON WINDSHIELD; LEADING EDGE OF WINGS; TAIL; AND PROP (INFERRED). REQUESTED IMMEDIATE DSCNT TO 5000 FT MSL. INITIALLY CLRED TO DSND TO 16000 FT AND EVENTUALLY CLRED TO 5000 FT BY ZSE. DSCNT MADE AT 2000 FPM. ASKED BY CTR TO STATE THE NATURE OF OUR PROB; REPLIED 'WE'VE ACCUMULATED SOME ICE.' ASKED BY CTR TO REPEAT NATURE OF PROB; STATED 'WE'RE PICKING UP A LITTLE ICE.' CTR ASKED THAT WE STATE OUR INTENTIONS; AND WE REPLIED 'CONTINUE HDG 175 DEGS TO EUGENE; OREGON.' INADVERTENTLY DEVIATED TO E OF V23 (AT LEAST 4 DEGS) DURING RAPID DSCNT WHILE ATTN WAS FOCUSED ON ASCERTAINING WHETHER ACFT PERFORMANCE WAS DETERIORATING. CTR HANDED US OFF TO CASCADE APCH DURING DSCNT. LEVELED OFF AT 9000 FT MSL (BTWN CLOUD LAYERS). CLRED TO MAINTAIN 9000 FT AND GIVEN CLRNC AND RADAR VECTOR DIRECT TO ROSEBURG VOR BY CASCADE APCH. PROCEEDED ON COURSE WITHOUT FURTHER INCIDENT. PLT AND 1 PAX ON BOARD.