PLT OF SMT ENCOUNTERED MORE ICING THAN ACFT COULD MAINTAIN ALT WITH; STARTED DSCNT AND FINALLY OBTAINED REVISED CLRNC FROM CENTER.

Narrative

ACFT WAS CRUISING AT 16000 FT ENCOUNTERING LIGHT; TO TRACE ICING. ALL DEICE AND ANTI-ICE SYS WERE OPERATING NORMALLY. ACFT THEN ENTERED AREA OF MODERATE ICING. I REQUESTED A LOWER ALT. CENTER ADVISED UNABLE DUE TO TFC. SHORTLY AFTER THIS ACFT ENTERED AREA OF SEVERE ICING. DUE TO FREQ CONGESTION; I WAS UNABLE TO ADVISE CENTER OF THE PROBLEM. ACFT PICKED UP A LARGE LOAD OF ICE; DEICING EQUIP WAS UNABLE TO CTL THE ICING. ACFT WAS SOON UNABLE TO MAINTAIN ALT AND STARTED TO DSND. ACFT HAD DSNDED ABOUT 500 FT WHEN CENTER WAS ADVISED AS TO THE DSCNT. THEY ADVISED DSCNT UNABLE DUE TO TFC. I REPLIED WE WERE COMING DOWN. THE CENTER THEN ASKED IF WE WANTED TO DECLARE AN EMER. I REPLIED NEGATIVE; AS ACFT WAS IN CTL AND HAD EXITED SEVERE ICING AT THIS TIME. CENTER THEN ASSIGNED US A HDG AND CLRNC TO 10000 FT. DSNDING OUT OF 12000 FT ALL ICE WAS SHED AND CENTER ADVISED. ANY TFC CONFLICT IS UNKNOWN TO ME. NO OTHER ACFT WAS ADVISED TO TFC CONFLICT OR ASKED TO DEVIATE; THAT I HEARD. I HAD BEEN UNABLE TO ADVISE CENTER OF PROBLEM; A LOT OF ACFT WERE TRYING TO CHANGE ALTS; BUT NO RPTS AS TO ICING WERE GIVEN. FORECASTS HAD ONLY CALLED FOR A CHANCE OF LIGHT ICING. IN RETROSPECT; I SHOULD HAVE INSISTED ON DSNDING SOONER AND HAVE ANTICIPATED THIS TYPE OF SITUATION SOONER. CONTRIBUTING FACTORS I FEEL WERE THE FREQ CONGESTION AND ATC PROCS PUTTING ALL THE TFC OVER 1 FIX; LEAVING LITTLE ROOM FOR POSSIBLE DEVS.