The pilot picked the aircraft up to a hover and made a hovering turn in preparation for departure. The nose of the aircraft began to rise. The pilot stated later that he could not control the aircraft. The aircraft began to descend and the tail rotor struck a security fence around the heliport. The main rotor then struck a wind sock pole, a tree, light pole, and concrete bench during the accident sequence. The accomplishment of the manufacturer's Service Bulletin (SB) #67-05 had been partially completed. The SB provided for protection against ice in the hydraulic servo by installation of protective covers. The protective covers had been installed to protect the other two main rotor servos but not the servo which controlled fore and aft cyclic control. The Omaha area experienced freezing precipitation the night before the accident, the temperature had not risen above freezing during the day, and freezing precip was forecast. This was the first flight of the day for this aircraft.