What happened
On February 25, 2014, a Robin DR40-135CDI Ecoflyer, registration F-HBIT, departed Annecy for a local flight accompanied by two passengers. While cruising near Mont Charvin, the pilot heard a sharp mechanical bang, followed by the activation of FADEC A and B warning lights. The engine parameters immediately showed an abnormal propeller speed exceeding 2,800 RPM, while engine power fluctuated between 0% and 5%.
Following engine failure procedures, the pilot attempted to glide toward Albertville aerodrome. However, as the aircraft descended into the valley, increasing headwinds made the aerodrome unreachable. To avoid overhead power lines on a nearby railway, the pilot executed a forced landing in a bramble field approximately 5.5 km from the runway. The impact resulted in three persons injured and the destruction of the aircraft.
The investigation
The BEA examined the wreckage and the engine components, including the FADEC unit and the propeller assembly. While the airframe sustained significant damage from the ground impact, the engine showed no signs of external leaks or fuel contamination. Data retrieved from the FADEC revealed that at approximately 13:41 local time, the engine speed abruptly increased from a stable 3,560 RPM to roughly 4,700 RPM, far exceeding the normal operating limit.
Investigators focused on the torque and vibration damper, which connects the engine to the propeller. Microfractographic analysis of the damper's splined hub revealed that it had failed due to progressive fatigue cracking. Furthermore, investigators discovered that one of the Belleville washer stacks within the damper cage was improperly positioned, with seven washers misaligned relative to the guide.
Findings
- The loss of propeller drive was caused by the failure of the splined hub that transmits rotation from the engine to the propeller.
- This failure was driven by fatigue cracking, which resulted from an imbalance of forces within the damper cage.
- The imbalance was caused by the incorrect positioning of a washer stack during assembly.
- The investigation noted that this specific damper model was difficult to assemble correctly and has since been replaced by a newer design that resolves these mounting issues.