What happened
On May 5, 2011, a Westland Bell 47G-3B-1 (Soloy) was performing a vineyard spraying mission, transitioning from Hoheneck to Benningen am Neckar. At approximately 09:45 local time, while flying at an altitude of about 600 feet AGL, the pilot experienced a loud impact. Simultaneously, the turbine RPM increased while the main rotor RPM decreased.
In response to the engine failure, the pilot initiated an autorotation toward an open field located to the left of the flight path. During the descent, the tail rotor made contact with an obstacle, and the helicopter performed a hard landing. Following the impact, the pilot shut down the engine, disconnected the battery, and successfully evacuated the aircraft despite sustaining serious injuries.
The investigation
The BFU examined the wreckage and the engine components. The investigation focused on the failure of the engine output shaft. Metallurgical analysis of the engine output shaft was conducted by the Institute of Materials Science at the Technical University of Braunschweig. The examination of the aircraft's maintenance records showed that the main drive shaft had undergone visual inspections and lubrication as required, though the most recent crack inspection for the shaft was conducted in 2006.
Findings
- The investigation established that the engine output shaft broke at the connection to the main drive shaft.
- Metallurgical analysis revealed that the failure was caused by a large number of individual, propagating fatigue cracks.
- The fatigue cracks were driven by bending stresses, likely resulting from a misalignment between the shaft and the flange.
- The structural design of the flange, characterized by significant stiffness changes and associated stress concentrations, contributed to the development of these fatigue cracks.