Incident Summary
Shortly after takeoff, a counterweight and clamp separated from one blade of the right propeller. The detached counterweight and clamp struck the adjacent blade, causing that blade to separate approximately 12 inches from the hub. The resulting imbalance induced severe vibration from the right engine. The propeller governor struck the engine cowling and broke into pieces. The flight crew shut down the engine using the condition lever. However, the propeller did not feather when commanded, and it continued to windmill, causing vibration and drag until landing.
Metallurgical Findings
Metallurgical examination of the failed components indicated that a blade counterweight clamp screw had failed, leading to the clamp's separation. The fracture surface of the screw initiated at the screw threads and exhibited features typical of intergranular cracking. Cadmium, used as a plating on the screw after manufacturer, was present on the fracture surface.