Part-66 Module 6 cheat sheet

SAE 4130 chromium-molybdenum low-alloy steel is the standard tube material for fuselage frames, engine mounts and landing gear; it is normally supplied normalised or normalised and stress-relieved.

Hardening steel requires heating above the upper critical temperature to form austenite, holding, then quenching in water, oil or air; tempering must follow immediately to restore toughness.

Austenitic stainless steels (301, 302, 304, 321, 347) are non-magnetic, cannot be hardened by heat treatment, and are strengthened only by cold working.

Grade 321 (titanium-stabilised) and 347 (niobium-stabilised) resist sensitisation and are preferred for exhaust systems and welded parts exposed to 425–870 °C.

Martensitic and ferritic stainless steels (410, 416, 440C) are magnetic and hardenable; 440C is used for bearings and high-hardness wear parts.

17-4PH is a precipitation-hardening stainless steel: solution treated, machined, then aged (e.g. H900) to reach high strength with minimal distortion.

Main heat-treatment defects are decarburisation (loss of surface carbon) and quench cracking; hardness testing and magnetic-particle inspection reveal them.

Stainless steel must not be worked with carbon-steel tooling or wire brushes - embedded iron particles cause rust staining and galvanic attack on the surface.

2024 (Al-Cu) offers high strength and fatigue resistance but poor corrosion resistance; airframe skins use Alclad 2024, which has a thin pure-aluminium cladding.

7075 (Al-Zn-Mg-Cu) is the highest-strength common aircraft aluminium; it is notch-sensitive and prone to exfoliation and stress corrosion, so it is always protected.

6061 (Al-Mg-Si) is weldable, corrosion resistant and heat treatable, used for tubing, fittings and moderately stressed structures.

Temper designations: O = annealed, H = strain hardened, T = heat treated; T3 = solution treated, cold worked and naturally aged; T6 = solution treated and artificially aged.

Magnesium alloys are the lightest structural metals but ignite readily; magnesium chips and dust are a fire hazard requiring dry powder or D-class extinguishing agents.

Ti-6Al-4V is an alpha-beta titanium alloy with a high strength-to-weight ratio and excellent corrosion resistance, used for engine parts, landing gear and fasteners.

Anodising grows a hard, corrosion-resistant oxide film on aluminium and provides a key for paint; it does not increase fatigue strength and thick coatings can reduce it.

Nickel-base alloys such as Inconel (heat resistant) and Monel (copper-nickel, seawater resistant) are used in exhaust systems and marine environments.

Carbon-fibre/epoxy has the highest specific stiffness and strength but is brittle, electrically conductive and galvanically incompatible with aluminium, so isolation plies or sealant are used.

Aramid (Kevlar) has excellent tensile and impact properties but poor compressive strength and absorbs moisture; it fuzzes when machined and is hard to cut.

Glass fibre (E-glass, S-glass) is cheaper, electrically insulating and corrosion resistant; S-glass has higher strength where strength-to-weight matters.

Prepreg has a limited shelf life and out-life at room temperature; it is stored frozen and must be thawed in sealed bags to prevent condensation before lay-up.

Cure cycles control ramp rate, dwell temperature, pressure and vacuum; voids and porosity result from insufficient pressure or trapped volatiles.

Honeycomb cores give high stiffness-to-weight but can trap water; ingress is found by tap testing, X-ray or thermography and drained through weep holes.

Damage types include delamination, disbond, matrix cracking, fibre breakage, porosity and foreign objects; tap testing finds disbonds and ultrasonic C-scan finds internal defects.

Repairs use tapered scarf or stepped joints to restore the load path, and the repair ply orientation and stacking sequence must match the original layup.

A corrosion cell needs an anode, a cathode, an electrolyte and a metallic path; breaking any one of these (e.g. sealing faying surfaces) stops the attack.

Galvanic series from anodic to cathodic: magnesium, zinc, aluminium, cadmium, steel, lead, tin, brass, copper, passive stainless steel, titanium, graphite.

A small anode coupled to a large cathode is the worst case - a steel fastener in a large aluminium skin corrodes rapidly.

Pitting is localised breakdown of the passive film by chlorides; crevice corrosion occurs in stagnant trapped gaps such as under gaskets and clamps.

Intergranular and exfoliation corrosion follow grain boundaries; exfoliation lifts surface layers and is common in 7075 extruded and forged sections.

Stress corrosion cracking requires combined tensile stress and a specific corrosive environment; stress relief and protective finishes prevent it.

Fretting corrosion occurs at clamped joints under vibration, producing dark oxide debris (cocoa staining) that acts as a fatigue crack initiator.

Protection methods: cladding, anodising, chromate conversion coating, primer plus topcoat, sacrificial anodes, corrosion-inhibiting compound and faying-surface sealant.

AN bolt dash numbers give diameter in 1/16 in steps (AN3 = 3/16 in), with letter suffixes for drilled heads and thread length; head markings identify the manufacturer and type.

Grip length must match the clamped thickness so that no thread lies in the shear plane and the nut engages the specified number of full threads.

For riveted joints, edge distance should be at least 2D and pitch about 3D–4D; closer spacing risks tear-out and wider spacing reduces joint efficiency.

Solid rivet alloys: 2117-T4 (AD), 2017-T4 (D, refrigerated), 2024-T4 (DD), 7050 (E), 5056 (B); rivet strength must suit the material being joined.

Blind rivets (Cherrymax, Cherrylock, Avex) are used where only one side is accessible; hole size, grip range and stem break must match the rivet specification.

Hi-Lok and Hi-Shear fasteners use a collar that shears off at a preset torque, giving a controlled preload without using a torque wrench.

Torque figures apply to dry or lubricated threads and friction strongly affects the resulting preload; the nut is normally turned, not the bolt.

Safety wire must be routed so any tendency of the fastener to loosen pulls the wire tight, with 6–8 twists per inch and a 1/4 to 1/2 in pigtail.

Rigid tubing materials: 6061-T6 aluminium for low-pressure systems, 304/321 corrosion-resistant steel for high pressure and heat, and titanium in some high-pressure circuits.

A 37° flared fitting requires a smooth, concentric, crack-free flare; over-tightening work-hardens the flare and leads to cracking in service.

Flareless (ferrule) fittings rely on a swaged ferrule biting the tube; they allow only a limited number of reassemblies and require the correct insertion depth.

Minimum bend radius is typically 2–3 times the tube diameter measured to the centreline; tighter bends cause ovality and wall thinning.

Tube support clamps are spaced per the maintenance manual, must not crush the tube, and use a rubber or plastic sleeve to prevent chafing and galvanic contact.

Flexible hose must be installed without twist, within its minimum bend radius, and supported at intervals; hose assemblies also carry a shelf life and installation date.

Hose assemblies are proof-pressure tested and marked with operating pressure, fluid type, date and part number; fire sleeves are fitted in designated fire zones.

Dissimilar metal tube and fitting combinations need a barrier (sealant, sleeve or insulating washer) to prevent galvanic corrosion at the joint.

Hooke's law applies within the elastic range; for a helical spring the rate is k = Gd⁴/(8D³N), where G is shear modulus, d wire diameter, D mean coil diameter and N the number of active coils.

Increasing wire diameter raises the rate by the fourth power; increasing coil diameter or the number of active coils lowers the rate.

Compression springs have open coils, tension springs have closed coils with hooks, torsion springs apply torque, and Belleville washers are disc springs giving high load in a short space.

Shot peening induces compressive residual stress at the spring surface and improves fatigue life; it must be done to the approved specification.

Scragging (presetting) removes residual set so the free length remains stable; a spring that has taken a permanent set in service is replaced, not re-used.

Common spring materials: music wire for small springs, chrome-vanadium and chrome-silicon for high-fatigue duty, 17-7PH stainless for corrosion and heat resistance.

End condition changes the number of active coils: plain, plain and ground, and squared and ground, the last being usual for compression springs.

Springs must not be stressed beyond their tested load and must be installed with the correct end orientation and without side loading or coil binding.

Ball bearings carry radial loads plus moderate thrust; cylindrical rollers carry higher radial loads; tapered rollers carry combined radial and thrust loads in one direction.

Needle roller bearings fit a small radial envelope; thrust bearings carry axial load only and are usually paired with a radial bearing.

Bearing internal clearance allows thermal expansion; preloading opposed pairs removes clearance, increases stiffness and reduces vibration.

Failure modes: brinelling from static overload or impact, spalling from subsurface fatigue, fretting from micro-movement, and scoring from lubrication breakdown.

Correct fits are essential - an inner race spinning on a shaft wears the shaft; use the specified interference and heat for installation rather than impact force.

Gear types: spur for parallel shafts, helical for smoother quieter drive with thrust load, bevel for intersecting shafts, worm for high ratios, planetary for high torque density.

Gear ratio = teeth on driven gear ÷ teeth on driving gear; torque ratio is the inverse of the speed ratio.

Backlash is measured with a dial indicator at the pitch circle; belts and chains need correct tension and pulley or sprocket alignment.

Cable construction: 7x7 for general runs, 7x19 where maximum flexibility over pulleys is needed, 1x19 for stiff straight high-strength runs; construction and diameter are marked on the cable.

Swaged terminals (e.g. Nicopress) require the correct die, a go/no-go gauge check after swaging and a proof load test before installation.

Turnbuckles have one right-hand and one left-hand thread; a minimum number of threads must remain engaged at each end and the barrel must be safety-wired.

Pulleys must match the cable size and be fitted with guards to prevent the cable jumping out; fairleads guide cables through structure without chafing.

Cable tension is set with a tensiometer to the rigging figures and corrected for temperature, since tension changes with structure temperature.

Inspection looks for broken wires (limits per strand and per length), corrosion, kinks, flattening and pulley wear; a kinked or corroded cable is replaced, never repaired.

Cables must be lubricated with the specified cable lubricant and must not rub on structure, since chafing causes rapid wire wear and failure.

Constructional stretch settles after the first loadings, so rigging tensions are rechecked after initial flights or after cable replacement.

In the AWG system a smaller number means a larger conductor; wire size is selected so that voltage drop at maximum current remains within the specified limit.

Insulation types include Tefzel (ECTFE), PTFE and polyimide (Kapton); polyimide is moisture sensitive and needs special handling and protection.

Crimped terminals are preferred to soldered joints in vibrating aircraft structures; use the correct tool and die, then inspect and pull-test the termination.

Connector families: keyed circular MS/EN connectors, D-sub miniature connectors for avionics and coaxial connectors for RF; pin numbering must match the wiring diagram.

Cable shielding (braid or foil) is grounded at the end(s) specified to control EMI; the shield must never be used as a current-carrying conductor.

Power and signal wiring is routed separately with the specified separation, supported by correct clamps and protected from chafing, heat and fluid.

Bonding resistance between structure and components is checked in milliohms with a bonding meter; a poor bond causes arcing, static build-up and interference.

Insulation resistance is measured with a megohmmeter at the specified test voltage and continuity with a low-resistance ohmmeter, never on a live circuit.