What type of fasteners commonly requires the use of cotter pins?

The correct answer is based on the function and design of castellated nuts. Castellated nuts have a series of slots cut into one end that allows for additional locking mechanisms to be employed. When these nuts are used, it is essential to secure them in place with a cotter pin. The cotter pin is inserted through a hole in the bolt or threaded shaft, which passes through the slots in the castellated nut. This configuration prevents the nut from loosening under vibration or rotation, ensuring the integrity of the assembly.

This method of securing fasteners is particularly important in applications where safety is paramount, such as in aircraft structures where components must maintain their integrity during operation. The conformance of the cotter pin to the castellated nut design provides a reliable solution for preventing accidental loosening.

Other options do not typically require cotter pins for their securement. While locknuts may provide a friction fit that helps prevent loosening, they do not necessitate the use of cotter pins. Likewise, cap screws and flat washers serve different purposes in fastening and do not employ cotter pins in their typical applications.