Aircraft Hardware: A Short Guide
Every piece of machinery is important on an aircraft, and their largest parts, such as wings and the fuselage, are dependent on the smallest fasteners. Those miniscule pieces like bolts, nuts, rivets, and more, may be much more commercially available, but it is just as important that they are aircraft-grade quality. AN, MS, and NAS standards have been created to ensure that the right components are continually used on aircraft to guarantee safety, even on a vehicle which regularly endures rough treatment. Commercial-grade fasteners are typically made of lower-quality materials such as low-carbon steel which have reduced tensile strength and resistance to corrosion. For this reason, it is crucial that one always chooses aircraft-grade hardware that is verified by its AN, MS, or NAS accreditation. Here is a short guide of relevant information for the various aircraft hardware available, all of which are offered on our website.
Aircraft bolts are often made of alloy steel, stainless or corrosion resistant steel, an aluminum alloy, or titanium for high strength applications. Whereas commercial bolts will have a plain, unmarked head, aircraft bolts will always have a marking of some sort (like an asterisk and the manufacturer's name). You can also identify the material by these markings, and some possible types may be close tolerance bolts (made for a very tight fit) or NAS bolts which have a very high tensile strength.
The standard bolts used in aircraft are AN3 through AN20, and they usually have a hexagon-shaped head and a shank that fits in a hole. The shank should be threaded on one end with the unthreaded portion called the “grip.” Moreover, it is important to note that the stated diameter of a bolt is the width of its grip. Shanks may also be drilled to accept a cotter pin or undrilled, and bolts may also have heads that are drilled for accepting a safety wire or slotted head (clevis bolt) for control cable applications.
Aircraft nuts usually have no identification on them, but are made from the same material as bolts. Because of vibration in aircraft environments, nuts must have some form of locking device to keep them in place. Some commonly used types of locking device are cotter pins used in castle nuts, fiber inserts, lockwashers, and safety wires. On aircraft, you will typically find various castle nuts and self-locking nuts, but plain nuts, wing nuts, and anchor nuts are also used.
The main role of washers in any application are to provide a shim when needed, act as a smooth load-bearing surface, and to adjust the position of castle nuts in relation to a drilled hole in a bolt. As stated before, they are also placed underneath lockwashers to avoid damaging a surface. AN960 are the most common type and are manufactured in both regular thickness and a thinner one which is about half as wide. The dash number following AN960 indicates the size bolt it is used for, and the thinner variety will have an L following it whereas a C indicates that it is made of stainless steel.
AN970 washers have a larger surface area, making them a gentler solution on surfaces they are applied to;therefore, they are a common choice for wood applications. Lockwashers can be in the form of a split ring or have internal, or even external, teeth to grip the surface and nut. Finally, there are nylon and finishing washers which have a countersunk head that sit flush with a surface.
Cotter Pins and Safety Wires
Aircraft cotter pins usually come in two varieties: AN380 which are cadmium plated, and AN381 which are stainless or corrosion resistant. Cotter pins are used for securing bolts, screws, nots, and other fasteners, so there is an extra level of safety involved. Additionally, the dash number of a cotter pin determines the diameter and length of the pin.
Safety wire is widely used on aircraft bolts and nuts, that way if the fastener loosens, it will increase tension on the wire. They are usually made of stainless steel or brass and can be installed using safety wire pliers which twist the wire around a fastener. To avoid wearing the wire, it is important to never over-twist the material(s). It is best to twist the wire about 36 times, cut off any excess wire, and bend its end to avoid it snagging in the future.
Sheet metal is a very useful material in aviation and rivets are especially effective for fastening them to surfaces. Rivets are not only useful for holding aluminum sheets together, but they are also used in securing fittings, nut plates, spars, and ribs. Simply put, they are similar to a bolt without a nut. More specifically, a rivet is a small metal pin which consists of a manufacturer's head, a shank, and a shop head (also called a bucktail). The shop head functions much the same as the head of a bolt, and when a rivet is installed, it expands as it is tightened, making for a very tight fit. In aviation, the most common types of rivets used are solid shanks and blind rivets.
Screws are a very common fastener found on a light airplane and most are made from a lower strength material than bolts. However, structural screws are made of the same material to ensure durability. There are three types of screws usually found on an aircraft: machine screws, structural screws, and self-tapping screws. Machine screws are for general purposes and made from either low carbon steel, aluminum, stainless steel, or brass. By contrast, structural screws are made of steel alloy and are heat treated. They have a grip and length like a bolt and have the same strength. Self-tapping screws are used for attaching removable parts and in areas where the screw can cut its own thread. Each of these types can come in several different materials and are available with different types of heads.
Turnlock fasteners are often used on inspection doors, cowlings, fairings, and other types of removable panels. Their design which can easily lock and unlock with a quarter turn, allows easy opening and access for areas that need maintenance or inspection. The most popular types are Southco, DZUS, and Camlocs which are found on many modern and antique aircraft.
These are not the only hardware used on aircraft, but they are by far the most common and useful in the basic construction of important parts. Regardless of their simplicity, it is crucially important to choose aircraft hardware which stands up to the high standard of quality necessary within aviation. At ASAP Aviation Procurement, customers can find all the aircraft parts and components they need, all with aircraft-grade accreditation. Browse our online catalog at your leisure or send an Instant Request for Quote (RFQ) form today to receive a custom competitive quote in 15 minutes or less!
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