On August 30, 1993, at 1230 central daylight time, a Fairchild SA-227-AC, N446MA, registered to Mesaba Aviation, Inc. of Minneapolis, Minnesota, and operated by a crew of one ATP rated pilot and a commercial instrument rated pilot, experienced a restriction of aileron control while maneuvering to land at Grand Rapids/Itasca County Airport, Grand Rapids, Minnesota, on runway 34 (5,755' x 100' dry/asphalt). The flight landed without incident and a broken aileron cable was found on examination. The flight was a CFR 14 Part 91 repositioning flight that had departed Thief River Falls, Minnesota, exact time unknown, on an IFR flight plan. No passengers were on board the flight. Visual meteorological conditions prevailed at the time of the incident. The airplane sustained no structural damage and the two crew members reported no injuries. The pilot stated that during a right turn, the yoke snapped free to the right. Some aileron control to the left was possible, but limited. There was no aileron control to the right. Minneapolis Center was contacted and advised of the situation. The crew elected to land with one half flap setting. During the final approach, the crew experienced what appeared to be aileron lockup, but with force, freed the controls. The airplane landed on runway 34. On examination the aileron cable was found to be broken. The aileron cable was replaced and the airplane was returned to service. Due to this incident, Mesaba conducted a complete system survey on their fleet of 21 Metro III aircraft, to inspect aileron cables. No additional discrepancies were found. A metallurgical examination of the broken aileron cable was conducted on September 27, 1993, by AADFW, Inc., in Euless, Texas, at the request of the Federal Aviation Administration to determine the tensile strength, chemical composition, and failure mode of the broken cable. A copy of the report is attached as an addendum to this report. The conclusion stated in the report was that the pulley over which the cable was routed was phenolic and did not appear to have any wear to the extent that would be expected if a steel cable in contact with the pulley had worn as much as the cable in this investigation. The reported indicated that it was more probable that the cable came off the pulley and had worn against a small diameter steel pin. The report further stated that service of the cable over a pin would provide higher stresses in the cable because of bending over an unacceptably small diameter pin. Such service would also provide a pivot pin that is most likely harder than the cable against which the cable would rub and wear. Service such as described above would be expected to result in fatigue failures of the cable wires that would appear exactly like what was observed in the laboratory.