PLT OF AN EMS HELI DSNDED BELOW ASSIGNED ALT DUE TO ENCOUNTERING ACFT ICING DURING AN IFR FLT.

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I DEPARTED ABC ARPT IFR TO ACD ON AN EMS FLT WITH CLRNC TO CLB TO 5000 FT. AT ABOUT 3000 FT AND 5 MI NW OF ABC; I NOTICED A LIGHT RIME ICE ON THE WINDSCREEN. I NOTIFIED CTR THAT I WAS PICKING UP ICE AND WOULD LIKE TO STAY AT 3000 FT; PROCEED DIRECT TO ACD NDB (IAF) AND EXECUTE THE LOC RWY 5 AT ACD. ACD NDB IS 5 MI SW OF ACD WITH AN IAF XING ALT OF 3700 FT. IN HINDSIGHT; THIS WAS NOT A SMART REQUEST. I WAS VERY FAMILIAR WITH THE AREA I WAS IN AND DID NOT WANT TO CLB ANY HIGHER. I WAS NOT THINKING OF THE 3700 FT IAF ALT AT ACD NDB. MY FAMILIARITY WITH THE AREA GAVE ME A FALSE SENSE OF SECURITY THAT I COULD 'CUT CORNERS.' ATC DENIED MY REQUEST FOR OBVIOUS REASONS TO STAY AT 3000 FT. ATC CLRED ME DIRECT ACD; BUT SAID I NEEDED TO BE A MINIMUM OF 4500 FT. I ACKNOWLEDGED AND CONTINUED MY CLB; NOW REMEMBERING THAT THE IAF REQUIRED 3700 FT. NOW ENRTE TO ACD NDB (ABOUT 8 MI AWAY) AND CLBING; I NOTICED MY RATE OF CLB AND AIRSPD DETERIORATING. I DETERMINED THE INCREASED ALT WAS CAUSING INCREASED ICE ACCUMULATION. AT ABOUT 4200 FT I REQUESTED A DSCNT TO 3700 FT WHILE ENRTE TO ACD DUE TO THE INCREASED ICE ACCUMULATION. XZ CTR INFORMED ME THAT I NEEDED TO BE 'ESTABLISHED' TO DSND AND DENIED MY REQUEST. I SHOULD HAVE DECLARED AN EMER AT THAT POINT. MY ACFT WAS NOT EQUIPPED TO FLY IN ICING CONDITIONS. ATC COULD HAVE BETTER SERVED ME; AND I COULD HAVE DEVIATED FROM MY ASSIGNED ALT HAD I DECLARED AN EMER. I WAS RELUCTANT TO DECLARE AN EMER FOR FEAR IT WOULD DRAW ATTN TO ME AND THE SIT I HAD PUT MYSELF IN. AT 4500 FT AND CRUISE PWR SETTING I WAS ONLY ABLE TO MAINTAIN 10-20 KTS ABOVE MY MINIMUM IFR SPD. I MADE THE DECISION TO DSND AND INCREASE AIRSPD. I WAS STILL ABOUT 2 MI FROM THE IAF. I TOLD XZ CTR THAT I WAS DSNDING TO 3700 FT. THEY ASKED IF I WAS 'ESTABLISHED.' I TOLD THEM THAT I WAS; EVEN THOUGH I WAS NOT. I JUST WANTED TO GET A COMFORTABLE AIRSPD BACK ON THE ACFT AND GET BACK DOWN TO A LOWER ALT. I WAS CLRED FOR THE LOC RWY 5 AND COMPLETED IT TO AN UNEVENTFUL LNDG AT ACD.