What happened
On July 28, 2008, at approximately 19:15 local time, a CASA 1131-E (Buckey BU 131) bearing registration EC-GIT was conducting a private local flight departing from La Axarquia Aerodrome. Approximately 30 minutes into the flight, the aircraft's propeller detached while in cruise.
The pilot performed an emergency landing on a path near the La Viñuela reservoir, northwest of the aerodrome. During the landing sequence, the aircraft slid down a steep slope, resulting in important damage to both wings and the landing gear. The propeller, along with the retaining nut and the front section of the crankshaft, was recovered 10 km away from the landing site. Both occupants of the aircraft were uninjured and were able to exit the aircraft on their own.
The investigation
The investigation focused on the mechanical failure of the propeller assembly. Post-accident inspections revealed that the crankshaft had fractured at the splined forward end that supports the propeller hub.
Laboratory analysis of the broken component determined that the crankshaft was manufactured from EN25 standard steel (Cr, Ni, Mo) and had been hardened and tempered to RC30. The study concluded that the fracture was caused by a fatigue mechanism. The crack originated at one of the radii of the splines at the base of the crankshaft. The analysis identified characteristic fatigue striations (beach marks) indicating that the crack grew under cyclic torsional loads until it reached a critical size, at which point a rapid, brittle fracture occurred. No surface defects or evidence of improper machining were found that would have acted as stress concentutions.
Findings
- The propeller detached in flight due to the fracture of the crankshaft caused by fatigue.
- The fatigue crack initiated at the radii of the splines on the forward part of the crankshaft.
- The crack propagated under torsional loads until reaching a critical size, leading to a brittle failure.
- The initiation of the crack may have been caused by excessive loading on a single spline during a propeller installation procedure where the components were not properly aligned.