COMMUTER ACFT HAS DEICE BOOT RUPTURE ON CLBOUT. EMER DECLARED.

Narrative

ON CLBOUT AFTER LEAVING CLOUD LAYER AT 14000 FT MSL IN LIGHT MIXED ICING; CYCLED DEICE BOOTS AND ACFT STARTED TO SHAKE AND SHUDDER. INDICATED AIRSPD WAS 200 KTS; ALT WAS APPROX 15000 FT MSL. DISCONNECTED AUTOPLT AND ACFT ROLLED APPROX 15 DEGS L. I WAS ABLE TO RIGHT ACFT AND TRIM FOR WINGS LEVEL FLT. HOWEVER; ACFT CONTINUED TO SHAKE. WE DECLARED AN EMER AND PROCEEDED TO BE VECTORED TO PORTLAND; OR. WE WERE CLRED TO VARIOUS LOWER ALTS AND I INCREASED AIRSPD TO 225 KTS. THE VIBRATIONS BECAME WORSE AS WELL AS CTLABILITY. I THEN REDUCED AIRSPD TO APPROX 175 KTS. THE VIBRATION DECREASED. ON THE DSCNT WE AGAIN ENTERED ICING CONDITIONS AND LEFT ICING CONDITIONS APPROX 5000 FT MSL; HOWEVER; VIBRATIONS CONTINUED. WE DID AN ILS APCH TO PDX AND LANDED AT FLAPS 12 DEGS AND APCH WAS FLOWN AT 150 KTS. NORMAL LNDG AND ROLLOUT. AFTER ENG SHUTDOWN; IT WAS DISCOVERED THAT THE L DEICE BOOT; OUTBOARD OF THE ENG NACELLE HAD RUPTURED. THERE APPEARED TO BE APPROX A 16 INCH TEAR IN THE BOOT AND IT WAS 'PEELED' BACK OVER THE UPPER AND LOWER WING SURFACES. HENCE THE L ROLL AND VIBRATION. THE FO AND MYSELF WERE UNSURE OF THE CAUSE OF THE PROB. WE SELECTED A REDUCED FLAP SETTING AND HIGHER APCH SPD TO MINIMIZE THE POSSIBILITY OF STALLING THE ACFT.