AN SF340 FLC ENRTE FROM PVD TO LGA DECLARED AN EMER AFTER OBSERVING ERRATIC PROP RPM.

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THIS STATEMENT CONCERNS A PART 121 SCHEDULED ACR FLT OPERATED IN AN SF340B ACFT WHICH DEPARTED PVD AND ARRIVED LGA. I WAS THE CAPT AND THE PF. THE TKOF; CLB AND INITIAL CRUISE SEGMENT OF THE FLT WERE NORMAL AND UNEVENTFUL. DURING CRUISE WHILE OVER MAD VOR WE NOTICED A DECREASE OF PROP RPM AND AN INCREASE OF TORQUE ON THE L ENG. WE PERFORMED THE ERRATIC ENG OP CHKLIST FOLLOWED BY THE PROP RPM UNDERSPD CHKLIST. AT THIS TIME WE NOTIFIED ATC THAT WHILE OUR L ENG WAS STILL PRODUCING PWR; WE WERE EXPERIENCING ERRATIC L PROP RPM; ALL OTHER ENG INDICATIONS WERE NORMAL AND WE REQUESTED TO BE CLRED DIRECT TO LGA. ATC IMMEDIATELY CLRED US DIRECT LGA AND WE PROCEEDED AS CLRED AT 8000 FT MSL. I XFERRED CTL OF THE ACFT AND COM #1 RADIO COMS TO THE FO AND CONTACTED COMPANY MAINT ON COM #2. MAINT SUGGESTED THAT WE LOCK OUT THE TORQUE MOTOR; WHICH WE HAD ALREADY DONE; AND CHK FOR POPPED CIRCUIT BREAKERS. NO CIRCUIT BREAKERS WERE POPPED; MAINT HAD NO FURTHER SUGGESTIONS. I THEN BRIEFED THE FLT ATTENDANT OVER THE INTERCOM FOLLOWED BY THE PAX OVER THE PA. WHEN I RETURNED TO COCKPIT COMS; THE FO WAS DSNDING TO 4000 FT AND SPEAKING TO LGA APCH CTL. LGA APCH TOLD US TO EXPECT THE EXPRESSWAY VISUAL RWY 31 APCH. WE INFORMED LGA APCH THAT WE WANTED TO ENTER A R BASE FOR RWY 31 AND DECLARED AN EMER TO FACILITATE PRIORITY HANDLING. I TOOK CTL OF THE ACFT FROM THE FO AND WE COMPLETED THE DSCNT; APCH; AND BEFORE LNDG CHKLISTS. THE APCH AND LNDG WERE MADE WITHOUT FURTHER INCIDENT.