What happened
On July 19, 2021, an Arcus T motor glider, registration HB-2467, was conducting a private flight near Schänis, Switzerland. After initially flying toward the Engadin, the pilot decided to return to Schänis due to mediocre soaring conditions. While cruising at approximately 1,100 meters, the pilot deployed the auxiliary engine to gain altitude and extend the flight.
Following two unsuccessful attempts to start the engine via windmilling, the pilot increased the aircraft's airspeed for a third attempt. As the engine successfully ignited, a loud bang was heard. The passenger noticed through the rearview mirror that the propeller had detached from the engine. Despite the loss of the propeller, the pilot maintained control of the aircraft and performed a safe landing at Schänis airfield. The detached propeller was later found embedded 35 cm deep in the ground near Benken.
The investigation
SUST examined the wreckage of the HB-2467 and the detached propeller. The investigation revealed that the propeller shaft had suffered a fatigue failure. Physical examination of the shaft showed significant corrosion and pitting, particularly at the transition radius where the diameter changes. The investigation also found that the engine's drive belts were missing and the ignition module plate was bent.
Technically, the propeller shaft was manufactured from ETG88 steel. Investigators also examined a similar shaft from another engine, which showed a 15 mm crack at the same location after only 56 hours of operation. The investigation noted that the engine had 72 hours of total time since new, and the shaft had been installed since the aircraft's manufacture in 2011.
Findings
- The primary cause of the propeller separation was a fatigue failure of the propeller shaft.
- Several contributing factors led to this failure, including insufficient fatigue strength of the shaft design and the absence of specific inspection intervals or life limits for the component.
- The investigation identified a lack of clear identification for parts and drawings from the engine manufacturer, as well as incomplete technical documentation.
- Corrosion and pitting on the shaft surface significantly contributed to the structural degradation of the component.