Whenever a nut is subject to rotation, it is required to have a drilled bolt, castellated nut, and cotter key. In the top photo you can see that the proper bolt/nut combination is correctly used on the forward rudder cable link attachment, but not on the aft one. Similarly, in the bottom photo the nut on the control column, which uses a brass insert as a bushing, requires a castellated nut. If a bolt is used through a rod end bearing, the use of a fiber locknut is approved, as the bushing in the rod end bearing rotates instead of the bolt.
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