On March 14, 2019, about 1730 Pacific daylight time, a Great Lakes 2T-1A-2 airplane, N101GL, sustained substantial damage when it was involved in an accident near San Diego, California. The private pilot and his passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. In a report submitted to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported that, about 5 minutes after takeoff, the left upper aileron came loose from the inboard hinge and remained attached by the one remaining hinge. The pilot returned to the departure airport and landed uneventfully. Examination of the airplane revealed that a wing rib aft of the spar at the outboard aileron cutout was substantially damaged. The failed aileron hinge strap (two components) was secured by the NTSB IIC and sent to the NTSB Materials Laboratory for examination. The pilot mentioned that while there were no inspection criteria from the manufacturer relative to the hinge straps, all hinge straps were inspected as part of each annual inspection. Examination of the aileron hinge strap fracture surfaces by the NTSB Materials Laboratory technician revealed crack arrest marks and thumbnail-shaped patterns on the fracture surfaces emanating from both the top and bottom surfaces of the hinge strap. The thumbnail-shaped patterns are ratchet marks and their presence on both the top and bottom surfaces is consistent with asymmetric reverse-bending fatigue crack propagation. The fatigue cracking propagated from the top and bottom surfaces through almost the entire cross-section of the hinge strap.