See the giant hole in the side of the hex? The end of the prop shaft has a similar hole. There is a steel pin that gets hammered in to those holes when it is assembled to prevent it coming unscrewed, then a steel cable threaded through a hole in the pin to keep it from coming out of the nut. Helicopters are giant vibrators, and they have to keep them from coming unscrewed in the event they come loose.
All of that is somewhat unnecessary though, as they generally have left-handed threads so that the rotor's torque forces it to tighten, and the material and torque specs of every fastener should be enough to keep them from coming loose. (Threads depend on the direction the rotor spins.)
Source - 35 years in manufacturing, including aerospace. Threads are one of my specialties.
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