The pilot of the Britten Norman BN-2A-21 Islander was landing at Slate Falls on a Charter Passenger Flight. As the aircraft flared to land it struck the top foot of a four foot snow bank. The snow bank was reportedly located at the threshold lights, and had been left there earlier in the day by a snow clearing crew. The pilot reported that the snow bank was not visible from the air. The pilot, the sole occupant, was not injured. After the occurence, the pilot checked the aircraft, and noted no damage. Several days later, the pilot had difficult fully extending the flaps of the aircraft. Examination revealed structural damage in the area of the engine mount to gear leg attachment point. The aircraft was removed from service for repairs. The pilot reported that the Slate Falls Airport has only been in service for a short time, and that the snow clearing practices have now changed, so that the snow is piled at the end of the clearway, rather than at the runway light area.