On July 20, 2020, at 1600 central daylight time, an Aero Commander S2-R, N1662S, was substantially damaged when it was involved in an accident near Harlan, Iowa. The pilot was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot stated that, during a spray pass, he felt the airplane yaw abruptly left and remain in an uncontrollable left yaw for about 10 to 15 minutes, at times pitching uncontrollably downward. The top 2/3 of the rudder had separated and the remainder was attached. When the top portion of the rudder completely separated, the pilot regained control of the airplane and landed without further incident. The separated portion of the rudder was not recovered. The top rudder hinge had fractured through and had brown-colored features consistent with corrosion. The remaining portion of the rudder and stabilizer was retained for metallurgical examination. Postaccident metallurgical examination revealed the upper rudder hinge was fractured through the two hinge barrels attached to the vertical stabilizer aft spar. The interior surfaces of the barrels were mostly covered with a tacky black grease. Thick grease deposits were also present on adjacent surfaces above the upper barrel and below the lower barrel. Portions of the upper hinge barrel interior fracture surfaces had flat fracture features in the radial plane with curving boundaries, features consistent with fatigue. Ratchet marks were also observed consistent with fatigue emanating from multiple origins. The fatigue regions covered approximately 90% of the fractured cross-section. The vertical stabilizer was cleaned, inspected, and repaired March 8, 2011, in accordance with designated engineering approved data. Airworthiness Directive 79-10-10, To Prevent Separation of the Single Upper Hinge from The Rudder Assembly, was complied with June 3, 1997, per Federal Aviation Administration Form 337 by installing an additional rudder hinge.