Further to OPR's comments -
Certification Specifications contain the following statement -
Fasteners
(a) Each removable bolt, screw, nut, pin or other fastener whose loss could jeopardise the safe operation of the rotorcraft must incorporate two separate locking devices.
The fastener and its locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(b) 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.
At the outset this seems to be prudent and simple but there are other subtleties. Installation of locking devices implies that someone has made the effort to install things correctly and
when doing a "duplicate inspection" the second inspector can at least see that the locking is in place.
As quoted by OPR the locking devices do NOT have anything to do with the integrity of the fastener. If the fastener has become loose and it is relying upon the locking device
something is wrong with the connection and the safety system works.
There are numerous issues that safety locking can overcome and incorrect torque is only one of them.
Incorrect hardware
Worn hardware
incorrect assembly of the fastener etc etc.
Nuts and bolts can be more complex than a lot of people appreciate.
I know of one particular large manufacturer who does not comply with the above statement but that is another story.......................