FLC OF ACR LTT ACFT DEVIATED FROM ASSIGNED ALT WHEN LOSING ACFT CTL DUE TO ENTERING SEVERE ICING CONDITIONS.

Narrative

PREFLT WX CHK: KEPT EYE ON WX ALL DAY (SNOW; FOG; ETC). TALKED WITH PLT WHO HAD JUST COME FROM BKW AND BLF. PULLED UP 'AL' WX ON COMPUTER (SA; FT; NOTAMS; PLT RPTS). WX RPTED BASICALLY TYPICAL WINTER 1/4-3/4 BUILDUP ON OUR AIRPLANE; NON-STICKING TO THE GND. SEVERE ICING WARNING POSTED. FLT (PRE-DEP): GREEN LIGHTS FOR ICING BOOTS DID NOT ILLUMINATE ON GATE TEST. ENGS WERE SHUT DOWN. MAINT CALLED. BOOTS AND LIGHT CAME ON DURING MAINT RUNUP. FLT: DEPART CLT FOR BKW -- UNEVENTFUL. APPROX 6 MI S OF BKW; CRW APCH GAVE FLT A DSCNT TO 6000 FT. THIS PUT THE ACFT IN THE TOPS OF THE CLOUDS. CAPT (PF) SLOWED THE ACFT UP FOR THE APCH. R TRIM WAS NEEDED TWICE. TROUBLE HOLDING ALT. TOO LOW AIRSPD FOR PWR. CAPT WAS FLYING AND MONITORING ICING. FO WAS VERY INEXPERIENCED WITH ICING. FO MADE ANNOUNCEMENT TO PAX REGARDING PROPS SLINGING ICE ONTO FUSELAGE. CAPT WITH ICE OBSERVER LIGHTS ON. FO WAS INSTRUCTED TO MONITOR WINDSHIELD WITH FLASHLIGHT. THE ICE HAD A STRANGE SNOWFLAKE LOOK. ENG ANTI-ICE/IGNITION; PROP SLOW CYCLE WERE ON CONTINUOUSLY. AIRFRAME BOOTS WERE CYCLED BY CAPT. LIGHTS ILLUMINATED. CAPT OBSERVED BOOTS POPPING ICE. SNOW LOOK WAS TOWARD REAR OF BOOT ALSO. CAPT WAS IN PROCESS OF TURNING FO TO PF TO ALLOW CAPT TO MONITOR ICING DURING THE APCH WHEN ACFT STARTED TO BUFFET AND LOSE ALT. FULL PWR APPLIED AND ATC ADVISED OF IMMEDIATE CLB. ACFT CONTINUED BUFFET LOSS OF AIRSPD; APPROX 50 FPM CLB AND HANGING. CAPT TOLD FO TO ASK MECH IN CRW FOR MORE PWR FROM ENGS. TOO SLOW -- CAPT MADE CHOICE TO CUT COMPUTER OFF. ACFT WALLOWED OUT OF CLOUDS WITHOUT PWR ADVANCE WITHOUT COMPUTER. COMPUTER CUT ON. FLT RETURNED TO CRW.