The aircraft was established in normal cruise at 7,000 ft when the pilot heard a loud bang from the right engine. The pilot believed that the right engine had failed, but as he commenced the propeller feathering procedure he realised that the propeller was missing. On closer inspection it was clear that the entire right propeller, including the hub and spinner, had detached. The aircraft returned to Darwin where a safe landing was accomplished. A metallurgical examination of the crankshaft revealed its failure was consistent with the development of an abnormally high force during flight. The engine bearers had been broken and the engine was supported only by control linkages, cables and the engine cowling. This damage was consistent with the separation of one propeller blade, or portion of a blade. The resulting imbalance causing an overload failure of the engine bearers and crankshaft. The propeller was not recovered.