What happened
On May 12, 2016, during a pre-flight inspection at Rybnik Gotartowice Airport (EPRG), a professional pilot performing routine checks on a 3X47 Navigator UL 002 noticed unusual movement of the engine relative to the engine mount. Upon removing the engine cowlings to investigate, the pilot discovered that one of the bolts securing the engine to the mount had sheared off. Specifically, the lower right mounting bolt (from the pilot's perspective) was broken. Following the discovery, the aircraft was immediately withdrawn from service, and both the manufacturer and the PKBWL were notified.
The investigation
The investigation focused on the physical examination of the failed M10 x 50 bolt. Investigators determined that the failure occurred at the transition point between the threaded and cylindrical sections of the bolt. The fracture surface indicated that the crack propagated rapidly from an initial point of origin. The analysis of the fracture morphology showed a dark area where the crack initiated, adjacent to the thread circumference, followed by a lighter-colored area representing the final, rapid fracture.
Findings
- The failure of the M10 x 50 bolt was caused by stress concentration at a notch located at the transition between the bolt's threads and its shank.
- The investigation concluded that the initial crack likely formed because the engine mount bolt assembly had loosened during operation.
- This loosening subjected the bolt to bending forces, which facilitated the crack's development.
- The crack propagation occurred rapidly, which may have prevented the defect from being detected during previous maintenance inspections.