Part-66 Module 6: materials, corrosion and aircraft hardware

Module 6 is the largest of the early modules for category B1. It mixes metallurgy with very practical hardware knowledge, so many questions are about codes and part numbers you need to recognise on sight.

Metals

• SAE steels: the first digit is the alloy type (1 carbon, 4 molybdenum), the last two the carbon content. 4130 is chrome-moly with about 0.30% carbon.
• Heat treatment: harden by heating and quenching; temper to reduce brittleness; anneal by slow furnace cooling; normalise by cooling in still air. Case hardening (carburising, nitriding) gives a hard surface and tough core.
• Aluminium alloys: 2xxx copper (2024), 6xxx magnesium-silicon (6061), 7xxx zinc (7075). Tempers: -O annealed, -T4 naturally aged, -T6 artificially aged. Alclad has a pure aluminium skin for corrosion protection.
• Magnesium is the lightest structural metal; fight magnesium fires with dry sand or a Class D extinguisher, never water. Titanium keeps its strength at high temperature and gives brilliant white sparks.
• Hardness tests: Brinell (ball, impression diameter), Rockwell (depth), Vickers (diamond pyramid).

Composites and non-metallics

Fibres carry the load and the matrix holds them. Carbon is black and very stiff but must be isolated from aluminium; aramid is yellow and tough; glass suits radomes. The tap test finds delamination: a dull sound means a void. Know pot life, shelf life, vacuum bagging and scarf repairs.

Corrosion

The anode corrodes. Magnesium is the most anodic common metal; aluminium corrodes when coupled with steel. Learn the types: pitting, intergranular, exfoliation (layers lifting in extrusions), filiform (worm-like under paint), fretting (smoking rivets), stress corrosion cracking and oxygen concentration cells (corrosion where oxygen is lowest). Never use steel wool on aluminium.

Fasteners

• AN bolts: diameter in sixteenths (AN6 = 3/8 in); dash number length in inches and eighths (-12 = 1 1/4 in); A after the dash = undrilled shank; a triangle marks close tolerance. Grip length equals material thickness.
• Nuts: nylon-insert self-locking nuts to about 121 °C, not on rotating bolts; castellated nuts with cotter pins; tighten further, never back off, to align a slot.
• Lockwire pulls in the tightening direction and is never reused.
• Rivets: AD (2117) dimple, D (2017) raised dot, DD (2024) two dashes and refrigerated, B (5056) raised cross for magnesium. MS20470 universal head, MS20426 100° countersunk. Diameter in 32nds, length in 16ths. Length = grip + 1.5 D; edge distance at least 2 D; pitch at least 3 D.

Lines, bearings, transmissions and cables

AN flares are 37°. Lay lines show hose twist. Tapered roller bearings carry radial and thrust loads; brinelling is denting, spalling is fatigue flaking. Gear speed is inverse to tooth count. Use 7 × 19 cable for primary controls, no more than three threads outside a turnbuckle, and fairleads for changes of direction up to about 3°.