Text in force at 23-63: from 23-48, effective 11 Mar 1996.
[(a) Each removable fastener must incorporate two retaining devices if the loss of such fastener would preclude continued safe flight and landing.
(b) Fasteners and their locking devices must not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation unless a non-friction locking device is used in addition to the self-locking device.]
61 FR 5148 · Source