Accident Overview
On July 20, 2020, at approximately 1600 central daylight time, an Aero Commander S2-R, registration N1662S, sustained substantial damage during an accident near Harlan, Iowa. The aircraft was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported no injuries.
Sequence of Events
According to the pilot, while conducting a spray pass, the airplane yawed abruptly to the left and remained in an uncontrollable left yaw for about 10 to 15 minutes. During this period, the aircraft also pitched uncontrollably downward at times. The top two-thirds of the rudder had separated, while the remainder remained attached. When the top portion of the rudder completely detached, the pilot regained control and landed without further incident.
Postaccident Examination
The separated rudder portion was not recovered. The top rudder hinge was found fractured, with brown-colored features consistent with corrosion on the fracture surfaces. The remaining rudder and stabilizer were retained for metallurgical testing.
Metallurgical examination revealed that the upper rudder hinge had 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, with thick grease deposits on adjacent surfaces above the upper barrel and below the lower barrel. Portions of the upper hinge barrel interior fracture surfaces exhibited flat fracture features in the radial plane with curving boundaries, consistent with fatigue. Ratchet marks were also observed, indicating fatigue emanating from multiple origins. The fatigue regions covered approximately 90% of the fractured cross-section.
Maintenance History
The vertical stabilizer was cleaned, inspected, and repaired on March 8, 2011, in accordance with designated engineering approved data. Airworthiness Directive 79-10-10, which addresses prevention of separation of the single upper hinge from the rudder assembly, was complied with on June 3, 1997, per Federal Aviation Administration Form 337, by installing an additional rudder hinge.