Part-66 Module 6 key terms

A heat treatment that heats steel above its critical temperature, holds it there, then cools it very slowly, usually in the furnace. It softens the metal, relieves internal stresses and refines the grain structure.

A process that adds carbon to the surface of low-carbon steel by heating it in a carbon-rich atmosphere, then quenching and tempering. The result is a hard, wear-resistant skin over a tough, shock-resistant core.

A chromium-nickel stainless steel, usually about 18% chromium and 8% nickel. It is non-magnetic, cannot be hardened by heat treatment, work-hardens rapidly and has excellent corrosion resistance. Types 304 and 316 are common.

A stainless steel with more carbon and little or no nickel, magnetic and hardenable by quenching and tempering. It gives high strength and wear resistance but poorer corrosion resistance, and is used for cutlery and some aircraft parts.

An aluminium-copper-magnesium alloy with high strength and good fatigue resistance, widely used for aircraft skins and structures. Its corrosion resistance is poor, so it is usually clad with pure aluminium (Alclad) or painted.

An aluminium-zinc-magnesium-copper alloy, one of the strongest wrought aluminium alloys, used for highly stressed parts such as wing spars and fittings. It has low corrosion resistance and is normally protected by cladding, anodising or paint.

A light, very strong metal with outstanding corrosion resistance and good performance at elevated temperature. It is used for engine parts and highly loaded structures, but it is expensive, difficult to machine and its fine chips are a fire hazard.

The lightest common structural metal, used for castings such as gearbox housings and wheels. It corrodes easily, needs protective treatment, and its turnings and dust can ignite and burn fiercely.

The plastic material, usually epoxy or polyester resin, that surrounds and bonds the reinforcing fibres in a composite. It carries load between fibres, protects them from moisture and damage, and holds the part's shape.

Composite material supplied as fibre cloth or tape already impregnated with a controlled amount of uncured resin. It must be stored cold, has a limited out-life, and is laid up and cured in an autoclave.

A lightweight core of thin metal or composite sheet formed into hexagonal cells, bonded between two skins to make a sandwich panel. It gives very high stiffness and strength for very little weight, but trapped water can cause damage.

Separation between the layers of a laminated composite, caused by impact, poor bonding, trapped moisture or manufacturing defects. It reduces strength and stiffness and is often found by tapping or ultrasonic inspection.

Corrosion that occurs when two different metals are in electrical contact in the presence of an electrolyte. The less noble metal becomes the anode and corrodes faster; the further apart the metals in the galvanic series, the worse it is.

A piece of less noble metal, such as zinc or magnesium, deliberately attached to a structure so that it corrodes instead of the protected metal. It is used in tanks, pipelines and immersed or buried parts.

Cracking of a metal caused by the combined effect of a sustained tensile stress and a corrosive environment, for example chlorides on stressed stainless steel. The cracks are often fine and hard to find, and failure can be sudden.

Damage at the contact surfaces of two parts that rub slightly under load, such as riveted joints or bearing seats. It produces fine dark oxide debris, loosens the joint and can start fatigue cracks.

The thickness of material that a bolt or rivet clamps together, measured under the head and not including washers. Choosing the correct grip length ensures the unthreaded shank, not the threads, lies in the shear plane.

A bolt whose shank is machined to a tight tolerance so it fits closely in a reamed hole. It carries shear loads and prevents movement between the joined parts, giving a stronger, more fatigue-resistant joint.

A stainless steel wire threaded through drilled heads or nuts and twisted in the tightening direction so vibration cannot loosen the fastener. It is used on critical fasteners where self-locking features alone are not enough.

A nut with a built-in locking feature, such as a nylon insert or a deliberately deformed thread, that resists loosening under vibration. Its locking action must be checked, and it may only be reused a limited number of times.

The conical end formed on a rigid tube so that, when the B-nut is tightened, the flare is clamped against a matching cone on the fitting. This seals the joint without a separate sleeve and is standard on aircraft flared fluid lines.

A tube fitting in which a separate ferrule (sleeve) is crushed onto the plain end of the tube by the nut, sealing and gripping it. No flaring is needed, but assembly and inspection need care.

The smallest curve a tube or cable may be formed into without kinking, wrinkling or losing strength. Too sharp a bend restricts flow, stresses the material and can crack or flatten the bore.

A metal sleeve, swaged or screwed onto the end of a flexible hose, that clamps the hose onto the insert fitting. It must match the hose and fitting type and be correctly crimped to prevent leaks or blow-off.

The load needed to compress or extend a spring by a given distance, usually stated in newtons per millimetre. A higher rate means a stiffer spring; rate depends on wire diameter, coil diameter and number of coils.

A helical spring designed to resist a pushing force, shortening under load and returning to its free length when released. It is used in valves, controls and landing gear, and must not be compressed to solid height in service.

A spring that works by twisting, storing energy as its ends are rotated relative to each other. It is used for hinges, control surfaces and return mechanisms, and must be fitted so that it is loaded in the correct direction.

A process that bombards a metal surface with small round particles to induce compressive stress in the surface layer. On springs and other highly stressed parts it greatly improves resistance to fatigue cracking.

A bearing with conical rollers running between conical races, able to carry both radial and axial loads. It is used for wheels, gearboxes and shafts and is normally fitted as a matched pair with a set preload.

The deliberate axial force applied during assembly to a bearing, bolt or joint so the parts stay tight in service. Correct preload removes internal clearance, reduces vibration and stops rolling elements from skidding.

The small amount of free movement between the teeth of two meshed gears, or between a shaft and its bearing, when they are not loaded. Too little causes overheating and wear; too much causes noise and lost motion.

A bearing with no rolling elements, just a sleeve or bush of bearing material running against a shaft and separated by a film of oil or grease. It is simple, quiet and cheap but needs reliable lubrication.

The process of compressing a metal sleeve onto a cable end to form a permanent eye or terminal. The correct die, insertion depth and proof load are essential, because a badly swaged terminal can pull off.

An adjustable connector with left-hand and right-hand threads at its two ends, used to tighten and rig control cables. After adjustment it must be locked, and a minimum number of threads must stay engaged.

A flexible cable running inside an outer conduit, used where the load path must curve, as in cockpit controls for vents or brakes. The conduit carries the reaction force, so the inner cable can work in both push and pull.

A guide, usually a grooved block or bush, that leads a cable or rod along the correct path and stops it rubbing on structure. Fairleads also keep a cable from jumping off its pulley.

A standard system for stating wire size in which the number decreases as the wire gets thicker. Each gauge has a defined cross-sectional area and current rating, and a replacement wire must be the same size or larger.

An electrical terminal attached to a wire by mechanically compressing it with a tool rather than by soldering. A correct crimp needs the right tool, the right terminal for the wire size, and a pull check.

The electrical connection of metal parts of an aircraft to each other and to the structure, giving a low-resistance path. It prevents static build-up, provides a current return path and reduces radio interference and lightning damage.

A measurement of how well the insulation of a wire, cable or connector resists leakage current, taken with a megohmmeter at a specified test voltage. A low reading indicates moisture, contamination or damaged insulation.