On January 22, 1998, about 2230 eastern standard time, a Robinson R-44 Helicopter, N972SA, experienced a partial cyclic control stick failure while hovering to land at the Kendall-Tamiami Executive Airport, Miami, Florida. The helicopter was operated by the pilot under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed, and no flight plan was filed for the local, personal flight. The private pilot was not injured, and the helicopter sustained no damage. The flight departed Fort Lauderdale, Florida at 2200. According to the pilot, as he was attempting to land, the helicopter began to drift right. He applied left cyclic control, but "just past the center of cyclic movement, the cyclic felt as if it hit a stop". The pilot rotated the cyclic 360 degrees to ensure the cyclic was not set at full right trim. There was no effect. The pilot made a run-on landing, using right turns, without further incident. Further examination of the cyclic revealed that the lateral trim actuator shaft was not properly aligned. The shaft exhibited wear concurrent with its position in a guide bearing. After sufficient quantity of the shaft was worn, the shaft became lodged in the guide bearing. Once lodged, the lower end of the shaft and a guide spring became loose and interfered with the surrounding structure, resulting in the partial loss of lateral control. After this incident, the inspection criteria for the lateral cyclic trim system was reviewed. The criteria did not include a measurement of the shaft wear. There was a service bulletin, SB-19, that dealt with excessive shaft wear on Robinson R-44 helicopters which directed mechanics to replace the lateral cyclic trim assembly shaft, Part Number (P/N) C585-1, if the diameter was decreased 0.001 inch per 100 hours or a diameter of less than 0.238 inch. This Service Bulletin, however, was for R-44 helicopters Serial Numbers (S/N) 0002 through 0321, and it did not apply to the incident helicopter. Subsequent to this incident, a new Service Bulletin, SB-26, was released for R-44 helicopters, S/N 0002 through 0420 and 0425, 0426, and 0427. SB-26 stated that the lateral cyclic trim assembly shaft, P/N C585-1, should be measured in several places. If the shaft diameter varies more than 0.004 inches in any 0.50 inch of length, the assembly should be replaced before further flight. After the release of SB-26, a priority Airworthiness Directive (AD), AD 98-04-12, was issued to require compliance with the terms of SB-26.