Metallic Structures can be up to 15 percent of the Airframe test, and its calculation questions are among the most predictable. This guide covers the rules and arithmetic from FAA-H-8083-31B, chapter 4.
Choosing the alloy and the thickness
- If an alloy weaker than the original must be used, use a heavier thickness for equivalent cross-sectional strength. A stronger but thinner material cannot be substituted, because buckling and torsional strength depend largely on thickness.
- Alloy 5052 is used where good workability, corrosion resistance, fatigue strength and weldability are wanted, as in fuel, hydraulic and oil lines.
- Parts with small radii or compound curvatures are formed in the annealed condition, then heat treated to a tempered condition before use on the aircraft.
Worked example: setback
For a 90-degree bend, setback equals the bend radius plus the metal thickness. With a bend radius of 0.250 inch and metal 0.040 inch thick: 0.250 + 0.040 = 0.290 inch. Lay out bend lines at 90 degrees to the grain, if possible, to avoid cracking in the bend.
Worked example: rivet diameter
Multiply the skin thickness by 3 and choose the next larger standard size. For a 0.050-inch skin: 0.050 x 3 = 0.150 inch, and the next larger standard rivet size is 5/32 inch (0.156 inch).
Worked example: replacing a 2017 rivet with a 2117
For general repairs, a 2117 rivet may replace a 2017 rivet of 3/16 inch or less if it is 1/32 inch greater in diameter. Replacing a 5/32-inch 2017 rivet: 5/32 + 1/32 = 3/16 inch.
Worked example: edge distance and row spacing
- The minimum edge distance for universal head rivets is 2 times the rivet diameter, so for a 1/8-inch rivet: 2 x 1/8 = 1/4 inch. Countersunk rivets need 2 1/2 diameters.
- The smallest allowable transverse pitch (distance between rows) is 2 1/2 rivet diameters: 2.5 x 1/8 = 5/16 inch.
Forming and removing rivets
- A properly formed shop head is one and a half times the diameter of the rivet shank, which is why the rivet length is the grip length plus the length needed to form the head.
- To remove a countersunk rivet, drill the head with a bit about 0.003 inch smaller than the shank, at the exact center and to about the depth of the head, to avoid enlarging the hole.
- Annealed DD (2024-T4) rivets are kept in a freezer to retard hardening, because they are too hard to drive as received. Most have been replaced by E-type rivets that can be driven as received.
What a removed or worn rivet tells you
- A joggled shank in a critically loaded rivet that is removed, with misaligned holes, shows partial shear failure. Install the next larger size and find the cause of the shearing stress.
- A dark, smoky trail of aluminum oxide around a rivet head indicates a small amount of relative motion between the rivet and the surface (fretting corrosion). It does not by itself mean the rivet is loose.
Repair method and inspection
- A repair by insertion is used when the damage is too large to patch, or when patch members would interfere with structural alignment, as at a hinge or bulkhead. The damaged portion is replaced by a member identical in material and shape.
- A simple visual inspection cannot confirm hidden cracks in a major structural member; eddy current and ultrasonic inspection are used.
- Brinelling is a shallow, spherical depression in a surface, caused by a part with a small radius pressing on it under high load.
- Use a cetyl alcohol based cutting lubricant for aluminum, titanium and corrosion-resistant steel.
Practice FAA Aviation Mechanic Airframe questions with explanations and timed mock tests.