At approximately 18:45, N9201 departed La Guardia Airport on a VFR flight to Boston. The Lodestar climbed to an altitude of 3,500 feet msl. Approximately 12 miles southeast of Hertford, Connecticut, the right engine became very rough and backfired frequently. The captain was unable to correct this condition and later feathered the right propeller when in the vicinity of Willimantic, Connecticut. A wide right turn was made from a northeasterly to a northwesterly heading toward Bradley Field, Windsor Locks, Connecticut. Communication was established with Bradley Field and the flight was cleared for a straight-in approach to runway 33. The aircraft descended to 2,500 feet msl during the turn and thereafter continued descending on a northwest heading until it struck trees and crashed. PROBABLE CAUSE: “The Board determines that the probable cause of this accident was that after failure of the right engine, accepted single-engine procedure was not followed, which resulted in the aircraft losing altitude and striking the ground.”