I don't know the answer but I would be very surprised if this nut was not locked in some way. It must have something to stop it coming undone in flight - and that something is obviously working !
There might be a grub screw hidden in a longuitudinal drilling between the nut and the shaft at one end of the nut, rather than a radial drilling on the side.
I am not an aircraft engineer but a photo might be helpful. Are you sure the nut is a discrete item and not part of the cover - in which case the locking device might be located elsewhere.
Secondly, I think this question would be better posted in Tech Log or Engineers & Technicians.
