History of Flight
On May 2, 2019, at approximately 1702 central daylight time, a Cirrus SR20 airplane, registration N8160C, was substantially damaged during a personal flight under 14 CFR Part 91 near Mineral Wells, Texas. The pilot and passenger were not injured. While on the downwind leg for runway 31 at Mineral Wells Airport (MWL), the pilot felt a shudder before turning base. Both occupants then observed the right aileron fluttering. The pilot immediately turned to intercept final approach. The right aileron separated from the wing about 0.5 mile from the runway threshold. The pilot maintained roll control and completed an uneventful landing.
Aircraft and Maintenance Information
The right aileron attaches to the wing via two bolts through inboard and outboard hinge supports on the rear spar. Each bolt passes through a bearing in its support and threads into a nut plate riveted to a bracket bolted to the aileron spar. Per the maintenance manual, hinge bolt assemblies should be torqued to 20–25 inch-pounds and safety wired to a tab on the aileron.
The right aileron was installed on August 29, 2007, at 1,059 airframe hours. It was last removed on January 15, 2009, at 1,376 hours, to replace the aileron bearing block. The last annual inspection was completed on November 1, 2018, at 2,094 hours. The mechanic stated he followed the Cirrus SR20 annual inspection checklist, lubricated control surface rod ends and hinges, and did not remove any aileron hinge bolts. Safety wire on the hinge bolts was present during inspection. The checklist did not require aileron removal. The airplane flew 15.2 hours between the annual inspection and the accident.
Wreckage Examination
The separated right aileron was found in one piece along the final approach path, about 0.5 mile from the runway, in a wooded area. The outboard attachment point was embedded with grass and mud; the outboard trailing edge was bent. The inboard attachment bracket remained bolted to the aileron spar, undamaged, with the nut plate and safety wire tab intact. The inboard hinge bolt remained attached through its hinge support and bearing, with washers and spacer present, but was not threaded into the nut plate. No safety wire was on the inboard hinge bolt head.
The inboard hinge bolt was longer than specified (NAS6203-11H vs. NAS6203-9H or -10H) and included an extra small washer. The outboard hinge bolt remained threaded into its nut plate, but the bracket had fractured in overstress; its safety wire was twisted, bent, and fractured. The inboard safety wire exhibited approximately 22 twists per inch, exceeding the specified 6–12. Fractographic analysis showed the wire failed due to torsional overstress.
The inboard hinge bolt had circumferential wear on its grip; threads were intact. The nut plate threads were intact, but the periphery was round (normally deformed oval for locking). The recommended minimum prevailing torque per FAA Advisory Circular 43.13-1B is 8 inch-pounds; the as-received components measured 3 inch-pounds after 720° rotation. The assembly could be rotated by hand through 540° before measurable torque. Thread roots contained black grease.