Which safety device is acceptable for a castle nut installed on secondary structure?

A cotter pin is a safety device commonly used to secure fasteners in place, particularly when there is a potential for loosening from vibrations or movement. When it comes to a castle nut, which is designed with slots or holes for securing, the cotter pin fits well into its intended function. The cotter pin passes through the hole in the bolt shank and engages with the slots in the castle nut, effectively preventing it from rotating loose.

In contrast, while locking wire and thread locker can be effective for many applications, they are not specifically designed to fit the unique characteristics of a castle nut. Similarly, while washers are essential for load distribution and preventing wear, they do not serve as a locking mechanism for securing castle nuts. Thus, the cotter pin is the appropriate device to ensure that a castle nut remains firmly in place on a secondary structure.