What happened
On September 1, 2018, a Dewoitine D2-6, registered as HB-RAG, departed from the grass runway 32 at Thun (LSZW) as part of a formation flight alongside an identical aircraft. Shortly after the aircraft transitioned into level flight at approximately 600 feet, the pilot detected unusual vibrations coming from the engine. Reacting to the instability, the pilot immediately reduced engine power to roughly 1200 RPM, performed a short circuit, and successfully landed the aircraft back on the grass runway without further incident. There were no injuries to the pilot or any other persons.
The investigation
Following the flight, maintenance personnel inspected the engine and discovered a deep, corroded crack on the crankshaft. This defect was located beneath the propeller cone and only became visible once the propeller had been removed. Further disassembly revealed that the crankshaft had actually fractured at the site of the corrosion.
Records indicated that the engine had undergone an extensive overhaul during the 2016/17 winter season following a previous accident involving a different aircraft. During that overhaul, the crankshaft had been subjected to a magnetic particle inspection, which had failed to detect any irregularities. At the time of the vibration event, the engine had only accumulated approximately 20 flight hours since that inspection.
Findings
- The primary cause of the engine vibration was a corroded and fractured crankshaft.
- The pilot's decision to immediately reduce power and land prevented a potential engine failure at a low altitude, which could have resulted in an emergency landing.
- The existing crack was located under the propeller cone, making it difficult to detect during standard visual inspections.
- A significant delay in reporting occurred, as the SUST was not notified of the serious incident until two months after the event, following a report through the EU occurrence portal.