Standard Aircraft Nuts

All nuts used in aircraft construction must have some sort of locking device to prevent them from loosening and falling off. Many nuts are held on a bolt by passing a cotter pin through a hole in the bolt shank and through slots or castellations in the nut. Others have some form of locking insert that grips a bolt's threads or relies on the tension of a spring-type lock-washer to hold the nut tight enough against the threads to keep it from vibrating loose. There are 2 basic types of nuts, self-locking and non self-locking.As the name implies, a self-locking nut locks onto a bolt on its own while a non self-locking nut relies on either a cotter pin, check nut or lock washer to hold it in place.



Aircraft nuts usually have no identification on them but they are made from the same material as bolts. Due to the vibration of aircraft, nuts must have some form of a locking device to keep them in place. The most common ways of locking are cotter pins used in castle nuts, fiber inserts, lockwashers, and safety wire. The aircraft nuts you will most likely encounter are castle nuts, self-locking nuts, and plain nuts. Wing nuts and anchor nuts are also used.

Castle nuts
AN310 and AN320 castle nuts are the most commonly used. See Figure 4. Castle nuts are fabricated from steel and are cadmium plated. Corrosion resistant castle nuts are also manufactured ( AN310C & AN320C—remember when you encounter a "C" it will designate stainless). Castle nuts are used with drilled shank bolts, clevis bolts, and eye bolts. The slots in the nut accommodate a cotter pin for safetying purposes. The thinner AN320 castellated shear nut has half the tensile strength of the AN310 and is used with clevis bolts which are subject to shear stress only. The dash number following the AN310 or AN320 indicates the size bolt that the nut fits. In other words, an AN310-4 would fit a ¼ inch bolt. See figure 1 for example of nut.

(Figure 1 )

Plain Aircraft nuts
Plain nuts require a locking device such as a check nut or lockwasher. They are not widely used in most aircraft. AN315 is the designation used for a plain hex nut. These nuts are also manufactured with a right hand thread and a left hand thread. The check nut used to hold a plain nut in place is an AN316. If a lockwasher is used a plain washer must be under the lockwasher to prevent damage to the surface.





Basics of Aircraft Nut Installation

• When using a castle nut, the cotter pin hole may not line up with the slots on the nut. The Mechanics General Handbook states "except in cases of highly stressed engine parts, the nut may be over tightened to permit lining up the next slot with the cotter pin hole." Common sense should prevail. Do not over tighten to an extreme, instead, remove the nut and use a different washer and then try to line the holes again.
• A fiber nut may be reused if you are unable to tighten by hand.
• At least 1 thread should be projecting past the fiber on a fiber nut installation.
• No self-locking nuts on moving part installations.
• Do not use AN364 or AN365 fiber nuts in areas of high temperature—above 250 degrees F.
• Shear nuts are to be used only in shear loads ( not tension ).
• Plain nuts require a locking device such as a lockwasher or a check nut.
• When using a lockwasher, place a plain washer between the surface of the airplane part and the lockwasher.
• Shear nuts and standard nuts have different torque values.
• Use wing nuts only where hand tightness is adequate.