On September 9, 2017, about 1545 central daylight time, an Aviat Husky A-1B airplane, N299CA, experienced a flight control failure near Prairie du Chien, Wisconsin. The commercial rated pilot was not injured and the airplane sustained substantial damage to the left aileron. The airplane was registered to and operated by Grandview Photo Inc., under the provisions of 14 Code of Federal Regulations Part 91 as an aerial photography flight. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The flight departed Prairie du Chien Municipal Airport (PDC), Prairie du Chien, Wisconsin, about 1300, flew over northwest Iowa for the aerial photography portion of the flight, and was destined for La Crosse Regional Airport (LSE), La Crosse, Wisconsin.
The pilot reported that he concluded the aerial photography portion of the flight, climbed the airplane to 3,500 ft mean sea level and proceeded toward LSE. About 5 minutes later he felt "light buffeting" as the airplane rolled left. He attempted to counteract the left roll by inputting right control stick pressure, but the airplane continued to roll left. He input right rudder to stop the left roll and decided to return to PDC, which was about 10 nautical miles southeast of his location. He used the rudder to maneuver the airplane to PDC and landed uneventfully. After parking the airplane, he moved the control stick left and right several times and the outboard end of the left aileron fell to the ground. The pilot and his mechanic disconnected the inboard aileron connection and removed it for examination.
A review of the airplane maintenance logbooks revealed that on August 8, 2008, the left wing tip was replaced due to "hangar rash" and there was no specific mention of damage to the left aileron. There were no other logbook entries that mentioned the left wing or left aileron. The most recent airframe annual inspection was completed on January 18, 2017, at and airplane total time of 5,348 hours; the airframe had accumulated 5,177 hours since the wing tip was replaced.
The aileron hinge bracket was removed and sent to the NTSB Materials Laboratory for a detailed examination. The examination revealed that the main tube exhibited a complete circumferential fracture just aft of the mounting flange (figure 1).
Figure 1 – Aileron Hinge Bracket The fracture surface on the flange side was examined with a scanning electron microscope, which revealed two fatigue fracture origins that led to two regions of final overstress fracture. The fatigue fractures occurred under low stress over an extended period of time. No anomalies were noted in the origin regions. The fractures did not occur at the welds and no anomalies were noted with the welds. The examination did not find any specific evidence of bending at the fracture area.