Most airframes are aluminum alloy. The first digit of the alloy number says its main added element: 2024 has copper (strong, used for skins and structure), 7075 zinc (stronger still), 6061 magnesium and silicon (easy to weld and form), and 1100 is almost pure aluminum. Alclad sheet has a thin layer of pure aluminum on each side to resist corrosion. The temper follows: -T heat treated (2024-T3), -O annealed (soft), -H strain hardened. For welded steel tube structures the usual steel is 4130 chrome-molybdenum.
Reading part numbers
AN bolts: the number after AN is the diameter in sixteenths of an inch, and the dash number is the grip length, first digit whole inches and second digit eighths. An AN4-7 is 4/16 = 1/4 inch across and 7/8 inch long; an AN5-12 is 5/16 inch and 1 2/8 = 1 1/4 inches. A cross or asterisk on the head means standard steel; a triangle means close tolerance.
Rivets: MS20470 (or AN470) is the universal head, MS20426 the 100° countersunk head. Then the alloy letters and two numbers: diameter in thirty-seconds and length in sixteenths. MS20470AD4-6 is a 2117 aluminum rivet, 4/32 = 1/8 inch across and 6/16 = 3/8 inch long.
AD (2117): dimple on the head, driven as received, the most common.
D (2017) and DD (2024): raised dot and two raised dashes; harder, so heat treated and kept in a freezer ('icebox rivets') until driven.
B (5056): raised cross, for magnesium.
A (1100): plain head, soft, for non-structural parts.
Nuts, safety and torque
Self-locking nuts (fiber or all-metal) hold by friction; they must not be used where the bolt or nut turns in service. Otherwise use a castle nut with a new cotter pin. At least one full thread should show past the nut. Safety wire is twisted and installed so that its pull tends to tighten the part, never loosen it.
Torque values come from the maintenance manual. If an extension makes the wrench longer, the wrench must read less than the torque wanted: TW = TA × L ÷ (L + E), where L is the wrench's length and E the extension's.
Wanted 300 in-lb, wrench 10 in, extension 5 in
Set the wrench to 300 × 10 ÷ 15 = 200 in-lb
Practice
What is the diameter of an AN5 bolt, in inches? Write it as a fraction.