4.2 Materials, Hardware, and Process Control

Aircraft Materials and the Temper System

Materials, hardware, and processes form the mechanic's everyday quality system. The ACS spans material applications, heat treatment, metalworking, loads, bolts, nuts, screws, pins, washers, turnlock fasteners, cables and fittings, safety wire, precision measurement, soldering, torquing, compatibility, markings, welds, and repair suitability. A small hardware decision can affect an entire structure.

Aluminum alloys are identified by a four-digit number where the first digit names the principal alloying element: 2xxx = copper, 6xxx = magnesium and silicon, 7xxx = zinc. The two anchor aerospace alloys are 2024 and 7075. 2024-T3 (copper, introduced by Alcoa in 1931) is used for fuselage and wing skins and general structure. 7075-T6 (zinc) has the highest mechanical strength of common aircraft aluminum — roughly 74,000–78,000 psi ultimate tensile — but lower fracture toughness, so it is used for highly loaded parts such as spars and stringers. The letter-and-number suffix is the temper:

A suffix such as -O (annealed) marks the most formable condition; clad ("Alclad") aluminum bonds a thin pure-aluminum layer over the alloy for corrosion protection. Steel alloys are designated by the four-digit SAE/AISI system (e.g., 4130 chrome-moly for engine mounts and tube structure). Titanium offers a high strength-to-weight ratio and excellent heat and corrosion resistance for firewalls, exhaust shrouds, and high-temperature structure.

Bolts, Markings, Grip, and Fit

Hardware is identified by head markings so material and strength are not guessed:

Grip length is measured from the underside of the head to the start of the threads and should equal or slightly exceed the thickness of the material being clamped, so no threads bear inside the joint (washers fine-tune the stack-up). In an AN part number such as AN3-10A, the first dash number is diameter (3 = 3/16 inch) and the second is grip length (-10), while a trailing letter such as A indicates no drilled hole. Fit matters: a close-tolerance bolt is driven into a reamed hole with a light tap.

Torque, Safetying, and Process Control

Torque is not the same as clamping force, but it is the practical means of producing the intended preload. Thread cleanliness, lubrication, washers, run-down friction, and wrench calibration change the relationship; under-torque allows joint movement and over-torque stretches the fastener, crushes material, or shortens fatigue life.

Safetying is a process, not decoration. The double-twist safety-wire method wires two or more units so any loosening tendency increases wire tension in the tightening direction. Cotter pins must fit the hole with minimal side play — one prong bent over the bolt end (not exceeding the bolt diameter) and the other down toward the nut; AN381 corrosion-resistant pins serve where nonmagnetic or corrosion-resistant material is required. Turnbuckles and turnlock fasteners have their own specified locking methods.

A Worked Torque-Extension Example

When a crowfoot or extension is added in line with the torque-wrench handle, it changes the effective lever length, so the wrench indicates a different value than is actually applied at the fastener. The standard relationship is T_w = T_a × L / (L + E), where T_w is the wrench setting to dial in, T_a is the actual torque wanted at the bolt, L is the wrench lever length, and E is the added extension length. 7 inch-pounds** dialed on the wrench. If the mechanic instead dialed 50, the joint would be over-torqued to about 60 inch-pounds.

This calculation is a classic General-test item, and the extension must be in the same plane as the handle for the formula to hold.

Cables, Measurement, and Process Control

Control cables are identified by diameter and construction (for example 7×19 extra-flexible for control runs, 7×7 for less-flexed applications); their fittings are swaged or woven and turnbuckles are safetied by clip or wire to a defined standard. Inspect cables for broken wires, corrosion, wear at pulleys/fairleads, and proper tension.

Precision measurement requires clean, zeroed, in-calibration tools used within range. A micrometer reads the barrel and thimble (and a Vernier micrometer adds ten-thousandths); a Vernier caliper reads the main scale plus the aligned Vernier graduation. Both are easily misread by a full graduation, and temperature, dirt, burrs, and excess feel pressure shift the reading, so the tool must be confirmed suitable and accurate before the number is trusted.

Soldering needs clean surfaces, the correct solder and flux, and heat control to avoid cold or starved joints; welding is inspected for sound contour and fusion while rejecting cracks, porosity, undercut, and incomplete penetration per the applicable data, and heat treatment of a part may be required after welding to restore strength. Finally, a Suspected Unapproved Part (SUP) — one with questionable markings, paperwork, condition, or source — must not be installed merely because it fits the hole. Maintenance quality depends on traceability and conformity to the approved type design, not on whether a part happens to fit.