What happened
On July 26, 2016, a pilot was conducting agricultural spraying operations over the vineyards on the eastern bank of the Moselle river, near Mesenich and Senheim. After completing several morning flights and a midday break, the pilot prepared for the final scheduled flight of the day.
Approximately one minute after takeoff, while on approach to the spraying area, the aircraft experienced a sudden bang, followed by a massive loss of engine power, warning lights, and the activation of the engine-out warning horn. Due to the low altitude, the pilot immediately initiated an autorotation, maneuvering the aircraft parallel to the hillside.
The MD Helicopters HU 369E struck the approximately 40-degree slope with minimal forward speed. The impact caused the helicopter to slide roughly 20 meters down the incline, eventually coming to rest upright on a road. The pilot sustained one injury (light) and was able to exit the aircraft independently. The helicopter was heavily damaged, with the tail boom broken and the landing gear fractured.
The investigation
The BFU examined the engine, a Rolls-Royce M250-C20R/2, which was disassembled for inspection by the manufacturer in the United States under the supervision of the NTSB. The investigation focused on the cause of the sudden power loss and the physical damage to the engine components.
Findings
- The investigation established that the engine failure was caused by a high-cycle fatigue failure of a blade in the fourth turbine stage.
- This failure caused the power turbine to burst, resulting in the sudden loss of engine performance.
- The damage pattern was consistent with previous known issues involving the Rolls-Royce M250-C20 engine series equipped with the "enhanced" CEB-1365 power turbine.
- The pilot had been operating within the legal requirements, and the aircraft had been maintained according to all regulations.
- While the manufacturer had identified a specific N2 RPM range (71% to 88%) that should be avoided to prevent such fatigue, the BFU noted that the aircraft manufacturer had not yet implemented a visual marker on the N2 gauge to highlight this critical range.