Intended Application

Due to the NAS1291 nut's thin wall and reduced thread height, it should not be used in critical applications. In fact, self-locking nuts such as the NASM21042 series (which succeeds NAS1291 for smaller sizes) are subject to the limitations of use found in MS33588. It states that self-locking nuts shall not be used as follows:

a) At joints in control systems at single attachments, or where loss of the bolt would affect safety of flight unless the threaded parts are held by a positive locking device that requires shearing or rupture of materials before torsional loads would relieve the initial stresses of the assembly.

b) On any externally threaded part that serves as an axis of rotation for another part unless there are no possible torsional loads which can be applied to either the externally or internally threaded part in such a manner as to relieve the initial stresses of the assembly, or unless the threaded parts are held by a positive locking device that requires shearing or rupture of material before torsional loads would relieve the initial stresses of the assembly.

c) With bolts or screws on jet engine aircraft in locations where a loose nut, bolt, or screw could fall or be drawn into the engine air intake duct.

d) With bolts, screws, or studs to attach access panels, doors, or to assemble any part that are routinely disassembled prior to or after each flight for access or servicing.

The entire text of MS33588E can be found below (since replaced by MS33588):

MS33588E