What happened
On March 28, 2012, a Robin DR250-160, registration F-BNJX, departed Chavenay for a flight including stops at Mortagne-au-Perche and Le Mans. During the flight, a passenger—an experienced instructor—suggested performing engine-failure drills to assist the pilot. While cruising at 3,000 feet near Senonches, the aircraft began experiencing intense vibrations. The pilot attempted to mitigate the shaking by reducing engine RPM to 2,000, but the vibrations persisted and worsened with any increase in power, eventually forcing the engine to idle.
Due to the heavy vibrations and the fear of a structural failure, the occupants decided to perform an emergency landing. The terrain, characterized by dense hedgerows, offered limited options. The passenger took control of the aircraft to execute the maneuver, navigating under power lines to reach a suitable field. After touching down, the passenger intentionally steered the aircraft into a skid to avoid hitting a treeline. While the occupants evacuated safely, the aircraft was destroyed by a post-landing fire.
The investigation
The investigation focused on the source of the vibrations and the mechanical integrity of the propeller assembly. Investigators examined the propeller-hub assembly, which survived the fire. The inspection revealed evidence of friction between the propeller and a spacer (shim) used in the assembly. Crucially, the investigation established that five of the six propeller mounting bolts had failed due to progressive fatigue cracking caused by tensile stress.
It was noted that the wooden propeller used during the accident was a loaner part, which required the use of a spacer because its hub thickness differed from the aircraft's original propeller. This replacement had been installed by a club mechanic only six days prior to the accident. The investigation also considered environmental factors, noting a significant drop in ambient humidity in the region in the days leading up to the installation.