Hardware, Materials and Fluid Lines
8% of the exam 2 min
Materials and hardware (8%) with fluid lines (4%): identifying bolts, nuts and rivets, safety wiring, torque, heat treatment and welds, then tubing sizes, flares and hose installation.
Reading bolts, nuts and rivets
In AN3DD5A, 3 is the diameter in sixteenths of an inch, DD is 2024 aluminum, 5 is the length in eighths of an inch and A means an undrilled shank. Head markings: a cross or asterisk for alloy steel, one raised dash for corrosion-resistant steel, two raised dashes for aluminum alloy, and a triangle for a close-tolerance bolt. Fiber-insert elastic stop nuts are not used above 250 °F, and one that turns by hand is replaced; never tap the fiber. The 2117-T rivet is the field rivet and needs no heat treatment; 2017-T and 2024-T icebox rivets are kept cold to delay age hardening, and 5056 is used on magnesium.
Safety wire and torque
Safety wire is new on every application, pulls in the tightening direction, and ends in a pigtail of 1/4 to 1/2 inch (three to six twists) bent back so it cannot snag. Widely spaced bolts wired by the double-twist method are limited to three in a series; closely spaced ones to what a 24-inch wire reaches. Never loosen or over-torque a nut to line up holes. Torque produces the designed preload in the bolt; with an in-line extension the wrench must read less: reading = desired torque × L ÷ (L + E).
Heat treatment, forces and welds
Tempering reduces the brittleness left by hardening and always follows it, never precedes it. Rivets in a lap joint are loaded mainly in shear, the force that tries to slide one layer past another. A good weld is uniform in width with even ripples feathered into the base metal, no burn, and no gas pockets, porosity or inclusions; thorough fusion (penetration) is the most important characteristic, and the edges do not need to be straight. On a micrometer, each numbered barrel line is 0.100 inch, each small division 0.025 inch, and each thimble division 0.001 inch.
Tubing, fittings and hose
Rigid tubing and fitting dash numbers are the outside diameter in sixteenths of an inch: number 6 is 3/8 inch. Aircraft flares are 35° to 37°, never the 45° automotive flare. Double flares are used on soft aluminum tubing 3/8 inch OD and under. AN steel fittings are black and AN aluminum fittings blue. Flexible hose gets 5 to 8 percent slack because it shortens under pressure, is supported at least every 24 inches, and must not be twisted; a lay line that spirals shows a twist.
Where people lose points
✗ "The dash number of an AN bolt is its diameter."
✓ The first number is diameter in sixteenths; the dash number is length in eighths.
✗ "Safety wire can be reused if it is not kinked."
✓ It must be new on every application.
✗ "An extension means the wrench should read more."
✓ It reads less: the extension adds leverage.
✗ "A 45° flaring tool is fine if the nut seats."
✓ Never. Aircraft fittings need 35° to 37°.
✗ "A good weld must have perfectly straight edges."
✓ Penetration matters most; edges need not be straight.
Numbers to memorize
| Two raised dashes (AN bolt) | Aluminum alloy |
| Triangle on bolt head | Close tolerance |
| Elastic stop nut limit | 250 °F |
| Safety wire pigtail | 1/4 to 1/2 inch (3-6 twists) |
| Double-twist series, widely spaced | 3 maximum |
| Torque with extension | Desired × L ÷ (L + E) |
| Field rivet | 2117-T (AD) |
| Aircraft flare angle | 35° to 37° |
| Double flare required | Soft aluminum, 3/8 inch OD and under |
| Flexible hose slack | 5 to 8 percent |
Test yourself
No answers here on purpose — retrieving them is the practice. Drill this domain if any of these stall you.
- Give the meaning of each AN bolt head marking.
- List four general safety wiring rules.
- Explain why icebox rivets are refrigerated.
- Describe a good weld and its most important characteristic.
- State the size of number 8 tubing and when a double flare is required.