After takeoff, the pilot heard a 'loud bang' followed by severe vibration. The pilot returned and landed at the departure airport without further incident. Examination revealed the left engine's number two propeller counterweight had separated, and exited through the propeller spinner. The counterweight then impacted the number one propeller blade, of the left engine, which separated about 12 inches from the hub. Examination of the counterweight clamp leading edge bolt hole revealed that the internal threads in the hole were stripped by motion of the leading edge bolt out of the hole. Additionally, the hole contained a double set of thread roots, indicative of cross threading of a bolt or mistapping of the threads. The propeller counterweight bolts had been replaced during an overhaul performed by the propeller manufacturer on April 12, 1995. The propeller counterweight bolts were also replaced by a certified propeller repair station, during maintenance on April 8, 1996, and at the last overhaul, which was performed on January 28, 1997. According to the propeller overhaul manual, after removal of a counterweight clamp bolt, an undersized tap should be used to remove the loctite retaining compound which was to be applied to the counterweight clamp threads during installation. The manual also stated '...Visually inspect threads for damage. Cross threading, damaged or missing threads is cause for retirement of the clamp.' A representative from the propeller repair station stated that their mechanics use a compressed air plastic bead spray to clean the clamp threads and they do not use an undersized tap. According to the propeller manufacturer, the use of a compressed air plastic bead spray to clean the propeller counterweight clamp threads was not an approved procedure.

The National Transportation Safety Board determines the probable cause(s) of this incident as follows:

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