PLT OF PVT SMA SEL ACFT LOST CTL OF THE ACFT DUE TO ACFT ICING AND DSNDED BELOW ASSIGNED ALT.

Narrative

I RECEIVED A BRIEFING BEFORE THE FLT WITH PIREPS RPTING ICING 6000-9000 FT ON CLBOUT (MLG ACR) FROM STP. I FILED FOR 5000 FT TO AVOID ICE. AFTER TKOF FROM STP I RECEIVED VECTORS THROUGH THE TCA. UPON REACHING 5000 FT I NOTICED ICE FORMING ON THE WING STRUTS. I ASKED FOR LOWER; ATC GAVE ME 4000 FT. HAVING JUST BEEN THROUGH 4000 FT I THOUGHT THIS WOULD SOLVE PROB. IT DID NOT. RIME CONTINUED TO FORM AT A FAIRLY FAST RATE. I AGAIN ASKED FOR LOWER. I WAS CLRED TO 3000 FT. ICE CONTINUED TO FORM. I TRIED TO CLB ONLY TO FIND THE PLANE WAS UNABLE. I ASKED CTR TO ADVISE ON AN ARPT CLOSE WITH BETTER CONDITIONS. I KNEW THAT MAINTAINING AIRSPD WAS BECOMING CRITICAL; AS THE PLANE HAD STALLED ONCE. ATC GAVE ME VECTORS TO MANKATO. I WAS UNABLE TO MAINTAIN THE LAST ASSIGNED ALT (3000 FT). AT ABOUT 2600 FT MSL I NOTICED GND CONTACT. I CANCELLED IFR; AND CONTINUED TO MANKATO AND LANDED WITHOUT EVENT. THE ONLY PREVENTION OF THESE INCIDENTS ARE MORE FREQUENT PIREPS BY MORE PLTS. IF GA PLTS ARE GOING TO FLY IFR WE NEED HELP FROM THE BIG BOYS. THE ATC FACILITIES DID THEIR JOB FLAWLESSLY AND PREVENTED A CATASTROPHE.