Event
On September 9, 2017, about 1545 central daylight time, an Aviat Husky A-1B airplane, registration N299CA, sustained a flight control failure near Prairie du Chien, Wisconsin. The commercial pilot was not injured. The airplane received substantial damage to the left aileron. The flight was operated by Grandview Photo Inc. under 14 CFR Part 91 as an aerial photography mission. Visual meteorological conditions prevailed, and no flight plan was filed. The flight departed Prairie du Chien Municipal Airport (PDC) around 1300, conducted aerial photography over northwest Iowa, and was en route to La Crosse Regional Airport (LSE) when the incident occurred.
Pilot Actions
The pilot reported that after completing the aerial photography, he climbed to 3,500 ft mean sea level and proceeded toward LSE. About five minutes later, he felt light buffeting as the airplane rolled left. Applying right control stick pressure did not stop the roll. He used right rudder to arrest the roll and decided to return to PDC, approximately 10 nautical miles southeast. Using rudder alone, he maneuvered the airplane to PDC and landed without further incident. After parking, he moved the control stick left and right; the outboard end of the left aileron fell to the ground. The pilot and his mechanic disconnected the inboard aileron connection for examination.
Maintenance History
Review of maintenance logbooks showed that on August 8, 2008, the left wing tip was replaced due to hangar rash, with no mention of left aileron damage. No subsequent logbook entries referenced the left wing or aileron. The most recent airframe annual inspection was completed on January 18, 2017, at an airplane total time of 5,348 hours; the airframe had accumulated 5,177 hours since the wing tip replacement.
Examination of the Aileron Hinge Bracket
The aileron hinge bracket was removed and sent to the NTSB Materials Laboratory for detailed examination. The main tube exhibited a complete circumferential fracture just aft of the mounting flange. Scanning electron microscope examination of the fracture surface revealed two fatigue fracture origins leading to two regions of final overstress fracture. The fatigue fractures occurred under low stress over an extended period. No anomalies were found in the origin regions or at the welds, and no evidence of bending was observed at the fracture area.