COULD NOT MAINTAIN ASSIGNED ALT BECAUSE OF SEVERE ICING.

Narrative

ACFT LOADED UP WITH ICE AND THE AIRSPD DECREASED UNTIL THE STICK SHAKER WENT OFF. 600 FT WAS LOST TO MAINTAIN AIRSPD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT WAS SAAB 340A WITH THE DOWTY ROTOL COMPOSITE PROP. THERE WAS NO PROB WITH THE ICING EQUIP; HOWEVER; TALKING TO OTHER PLTS; THIS HAS HAPPENED BEFORE. RPTR USUALLY FLIES THE MODEL WITH THE HAMILTON STANDARD PROPS AND THIS HAS NOT HAPPENED BEFORE. DID NOT DECLARE AN EMER. THE AIRPLANE WAS UNDER CTL EXCEPT WITHIN A MATTER OF SECONDS THE SPD WENT FROM AN INDICATED 190-140; THE AUTOPLT KICKED OFF ALERTING THEM TO THE PROB; THEN THE STICK SHAKER; AND THAT LOWERED THE NOSE WITH MAX PWR; THE FO WHO IS ALSO A CAPT NOTIFIED CTR OF THE SEVERE ICING AND REQUESTED A LOWER ALT WHICH WAS IMMEDIATELY GIVEN; BUT BY THAT TIME; THEY WERE 600 FT LOWER AND BREAKING OUT OF THE CLOUDS AT ABOUT 18400 FT; AND WERE CLRED THEN TO 17000 FT. THE ICING EQUIP WAS ON AND THE BOOTS SEEMED TO BE DOING THE JOB OF DE-ICING THE WINGS NORMALLY. THE ARPT SAW ICE BUILD UP ON THE WING AND TURNED ON THE BOOTS AND THE ICE WOULD SHED. THEY WERE BEING RTED BECAUSE OF WX AROUND TSTMS THAT WERE N OF THEM. THE ICING WAS NOT A WING PROB; OR AN ENG PWR PROB; BUT RATHER; SEEMED TO BE A PROP EFFICIENCY LOSS PROB.