C182 PLT ON AN IFR FLT PLAN ENCOUNTERS ICE NEAR SEATTLE; REQUESTS HIGHER ALT BUT IS UNABLE TO CLB BECAUSE HE FORGOT THE AUTOPLT WAS ON.

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WE WERE ON AN IFR FLT ON OUR ASSIGNED ALT OF 12000 FT IN VFR CONDITIONS WHICH THEN CHANGED AS WE ENTERED THE TOPS OF CLOUDS. WE HAD CHKED WITH FLT WATCH AND NOT RECEIVED ANY PIREPS OF ICING IN THE VICINITY. WE THEN NOTED THE ACCUMULATION OF ICE ON THE LEADING EDGE OF THE WINGS. WE IMMEDIATELY REQUESTED CLRNC TO A HIGHER ALT AS WE COULD TELL THAT WE WERE JUST AT THE TOPS. AFTER BEING CLRED TO 13000 FT WE ATTEMPTED TO CLB. HOWEVER; WE BEGAN TO NOTICE A DECREASE IN AIRSPD AND WERE UNABLE TO CLB. I BELIEVE THAT THE AUTOPLT WAS ON AND THE ALT HOLD WAS ENGAGED INADVERTENTLY. THIS LIKELY CONTRIBUTED TO THE PERCEIVED INCREASED DIFFICULTY IN CLBING. AT THIS TIME OVER-CTL AND NOT APPRECIATING THE FACT THAT THE AUTOPLT WAS ENGAGED CAUSED US TO BELIEVE THAT WE WERE HAVING CTL SURFACE DIFFICULTIES. I DECLARED AN EMER. WE WERE LOSING ALT UP TO 1000 FPM AND I REQUESTED A VECTOR OVER LOWER TERRAIN. I HAD SOME DIFFICULTY HEARING SOME OF THE XMISSIONS FROM ATC. ONCE WE REALIZED THE AUTOPLT WAS ENGAGED AND WE WERE MANUALLY OVERRIDING IT; WE DISENGAGED THE AUTOPLT AND WERE THEN ABLE TO EASILY MAINTAIN OUR HEADING AND ALT. WE WERE VECTORED TO PROGRESSIVELY LOWER ALTS AND PROCEEDED UNEVENTFULLY. ATC WAS VERY HELPFUL. I IMAGINE SOME SPATIAL DISORIENTATION CONTRIBUTED TO THE PROB.