What happened
On September 11, 2009, a Stampe SV 4C biplane, registration F-BDJP, was performing a ferry flight from Cannes Mandelieu to Florence Peretola via Genoa. While flying over the Gulf of La Spezia, the pilot reported a strong smell of burning wood, followed by vibrations and the sudden detachment of the propeller. The loss of the propeller caused an immediate increase in engine RPM.
Following the emergency, the pilot declared a "May Day" and executed an emergency water landing near Punta Castagna. The aircraft struck the water with its right lower wing and the right side of the nose. The impact caused the aircraft to rotate and eventually sink, leaving only the tail visible. The pilot and passenger sustained minor injuries (abrasions) and were able to reach the shore by walking through shallow water. The aircraft was later recovered using lifting balloons and moved to shore.
The investigation
The ANSV investigation focused on the mechanical failure of the propeller assembly. The aircraft had recently undergone an annual inspection in Cannes, which included visual checks of the propeller, bolt torque, and locking mechanisms. Investigators examined the wreckage, specifically the propeller hub and blades.
Analysis of the propeller hub components revealed that the failure of the blades was caused by an loosening of the assembly, which led to the development of orthogonal vibrations and rotational friction. The investigation considered whether the locking tabs on the securing plates had failed due to metal fatigue from repeated bending during maintenance, or if the torque had decreased due to the hygroscopic nature of the wooden propeller blades reacting to changing environmental humidity.
Findings
- The primary cause of the accident was the reduction of the torque applied to the propeller bolts during flight.
- The loss of propeller integrity was driven by the loosening of the assembly, which triggered intense vibrations and friction between the propeller and the metal flange.
- The investigation noted that the wooden propeller's moisture content changes with environmental conditions, which may have impacted the effectiveness of the clamping force.
- The maintenance inspection performed prior to the flight was primarily visual and did not involve a more in-depth verification of the physical integrity of the locking tabs.