Cirrus SR20 N8160C accident near Mineral Wells, Texas

Probable cause

The in-flight separation of the right aileron when the inboard hinge bolt disengaged from its nut plate due to excessively twisted safety wire, which resulted in a torsional overstress fracture that caused the inboard hinge bolt to become unsecured.

NTSB analysis

The pilot was conducting a personal flight in the airport traffic pattern when he felt the airplane “shudder” shortly before the turn onto the base leg for the runway. The pilot and passenger then saw the right aileron “fluttering,” and the pilot immediately turned to intercept the final approach. The right aileron separated from the wing about 0.5 mile from the approach end of the runway. The pilot was able to maintain roll control of the airplane after the right aileron separated, and he completed an uneventful landing on the runway.

Examination of the airplane and the recovered right aileron revealed that the aileron’s inboard hinge bolt was not threaded into its nut plate. The safety wire that normally secured the inboard hinge bolt was found fractured. The safety wire had been excessively twisted when the right aileron was installed; the excessive twisting led to the torsional overstress fracture of the safety wire during the accident flight. Additionally, the nut plate had significantly reduced thread-locking abilities that allowed the unsecured inboard hinge bolt to loosen; the right aileron separated from the wing when the inboard hinge bolt disengaged from the nut plate. Although the inboard hinge bolt assembly included a slightly longer bolt than specified and an additional small washer, these component discrepancies did not contribute to the aileron separation.

The airplane had accumulated about 733 flight hours during the more than 10 years since the right aileron was last removed and reinstalled during maintenance. The most recent maintenance performed on the airplane was an annual inspection completed about 15.2 flight hours before the accident. The mechanic who performed the last annual inspection observed safety wire installed on the aileron hinge bolts.

Text from the NTSB. Open the NTSB docket for CEN19LA133 for the full record.