Aeronautical engineers have determined that the strongest aircraft designs use lightweight materials such as aluminium joined with rivets. But choosing the right size and type of rivet to join aircraft pieces is similar to a tailor choosing the right thread and stitch size to join pieces of fabric: Only the right size rivet for the job will do.
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To maintain safe and airworthy designs, the FAA, or Federal Aviation Administration, publishes Advisory Circulars that list the rivets that can be used to build aircraft and assigns them a mil-spec part number. FCC AC 43.13-1B is the governing specification for aircraft hardware. Tables in the circular list a universal, solid-shank, protruding-head rivet that can be used both inside and outside the aircraft, for example, as MS20470. When a surface must be free of bumps for better aerodynamics, engineers select a countersunk head rivet with part number MS20426 in AC 43.13-1B.
The most commonly used material for rivets is an aluminium alloy. The FAA has identified six grades and assigned them the numbers 2117-T, 2017-T3, 2024-T, 1100 and 5052; the number 5056 identifies magnesium alloys. The FAA further assigns material codes and specifies markings on the rivet head to help buyers identify the grade of material. The 2117T grade has material code "AD" and the rivet has a dimpled head marking, for example.
The rule of thumb for selecting rivet diameters is to choose a diameter three times the thickness of the material to be riveted. If the material is one-eighth inch thick, for example, a three-eighths diameter rivet is selected. Rivet diameters are specified in thirty-seconds of an inch in FAA tables and supplier catalogues.
A rivet's length is set by the actual length of material that will be riveted. A protruding-head rivet, for example, is measured only from the base of the head, whereas the entire length of a countersunk rivet is measured. Rivet lengths are specified in sixteenths of an inch in FAA tables and supplier catalogues.
Aircraft parts catalogues list rivets according to the FAA designations, followed by standardised numbers that indicate the length and diameter of the rivet, expressed in sixteenths and thirty-seconds of an inch, respectively. An example will clarify: For rivet part number MS20426AD-3-4, MS20426 is the mil-spec number assigned to the countersunk-head rivet; AD indicates the 2117-T aluminium alloy; 3 equals the diameter measured as three-thirty-seconds of an inch; 4 equals the length as four-sixteenths or one-eighth of an inch. You could further identify the rivet by its dimpled head marking. You may also see a ".5" at the end of a length indicating that rivet can be purchased in a half size.
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