Module 6: Materials and Hardware

1. Module Overview

Module 6 of the EASA Part-66 syllabus provides the foundational knowledge of the materials and hardware used in aircraft construction and maintenance. This module is essential for certifying staff, as it covers the properties, behaviours, and correct handling of metals, non-metals, fasteners, and fluid systems. The content is structured to build from basic material science (Level 1) through to detailed inspection and repair criteria (Level 3). The module is divided into key areas: material properties, corrosion, aircraft hardware, non-destructive testing, and fluid lines.

This study material synthesises the core knowledge required for the B1.2 (piston-engine aeroplane) licence category, focusing on the practical application of these principles in a maintenance environment.

2. Key Concepts Explained in Detail

2.1 Material Properties and Identification

Understanding the fundamental properties of materials is critical for selecting the correct material for a repair and for identifying potential failure modes.

Ductility: The ability of a material to deform plastically under tensile stress without fracturing. This is a crucial property for materials that are formed, such as rivets, and for structures that must absorb energy without sudden failure.
Malleability: The ability of a material to deform plastically under compressive stress (e.g., hammering or rolling).
Elasticity: The ability of a material to return to its original shape after a load is removed. The elastic limit is the point beyond which permanent deformation occurs.
Toughness: The ability of a material to absorb energy and plastically deform before fracturing. It is a measure of a material's resistance to fracture under impact.
Hardness: The resistance of a material to indentation, scratching, or abrasion. It is often related to wear resistance and tensile strength.
Strength: The ability of a material to withstand an applied load without failure. This can be further defined as:
Tensile Strength: The maximum stress a material can withstand while being stretched or pulled before necking or fracturing.
Compressive Strength: The capacity of a material to withstand loads tending to reduce size.
Shear Strength: The maximum stress a material can withstand before failure in a direction parallel to the applied load.

Ferrous vs. Non-Ferrous Metals:

Ferrous Metals: Contain iron as a primary constituent. Examples include carbon steels, stainless steels, and cast iron. They are generally magnetic and prone to corrosion (rusting).
Non-Ferrous Metals: Do not contain significant amounts of iron. Examples include aluminium, titanium, magnesium, copper, and their alloys. They are generally more corrosion-resistant than ferrous metals.

Key Aircraft Metals:

Aluminium Alloys: The most common airframe material. Pure aluminium is soft and weak; it is alloyed with elements like copper, magnesium, manganese, and zinc to increase strength. The heat-treatable alloys (e.g., 2024, 7075) are used for structural applications.
2024-T3: High strength-to-weight ratio, good fatigue resistance. Used for wing skins, fuselage structures.
7075-T6: Higher strength than 2024 but lower fracture toughness. Used for highly stressed fittings.
2117-T4: Primarily used for rivets due to its good workability in the T4 condition.
Titanium Alloys: Offer an excellent strength-to-weight ratio and superior corrosion resistance, especially at elevated temperatures. They are used in critical structural components, engine parts, and fasteners. They are expensive and difficult to machine.
Magnesium Alloys: The lightest structural metal. Highly susceptible to corrosion, especially galvanic corrosion. Used in non-structural applications like gearbox housings.
Steels: Used for highly stressed parts, engine components, and landing gear.
Low-Carbon Steel: Used for general-purpose hardware.
High-Strength Low-Alloy (HSLA) Steel: Used for structural components.
Nickel-Chromium Alloys (e.g., Inconel): Retain strength and resist oxidation at high temperatures, making them ideal for exhaust manifolds and turbine components.
Bronze: An alloy of copper and tin. It has excellent bearing properties, low friction, and good wear resistance, making it suitable for gears and bushings that run against steel shafts.

Heat Treatment of Aluminium Alloys:

Heat-treatable aluminium alloys are strengthened through a solution heat treatment and ageing process.

Solution Heat Treatment: The alloy is heated to a specific temperature to dissolve alloying elements into a solid solution, then rapidly quenched (e.g., in water) to trap them in a supersaturated state.
Natural Ageing (T4): The alloy is left at room temperature to precipitate hardening over several days. 2024-T4 is achieved this way.
Artificial Ageing (T6): The alloy is heated to a moderate temperature to accelerate the precipitation process, producing a higher strength condition (e.g., 7075-T6).
Rivets: 2117-T4 rivets are driven in the T4 condition while they are still soft. They naturally age-harden over time, so they must be driven within a specific period after quenching to ensure they form a proper shop head.

Fastener Markings:

ISO Metric Bolts: Marked with a property class, e.g., "8.8". The first digit (8) indicates the ultimate tensile strength in hundreds of MPa (800 MPa). The second digit (0.8) is the ratio of yield strength to tensile strength (0.8), giving a yield strength of 640 MPa.

2.2 Corrosion

Corrosion is the deterioration of a material, usually a metal, due to a chemical or electrochemical reaction with its environment. It is a primary concern in aircraft maintenance.

Types of Corrosion:

Surface / Uniform Corrosion: Occurs evenly over a large exposed area. On aluminium, it appears as a white, powdery deposit. On steel, it is the familiar reddish-brown rust. The primary action is to mechanically remove the corrosion and assess the depth of attack to determine if the component is within allowable limits.
Pitting Corrosion: Highly localised attack that creates small pits or holes on the surface. It is common on aluminium propellers and skins. The depth of pits must be measured and compared to allowable limits in the maintenance manual. Deep pits may require blending out or replacement.
Crevice Corrosion: Occurs in shielded areas (crevices) where moisture and oxygen are trapped, such as under loose gaskets, hose clamps, or lap joints. A differential oxygen concentration cell is created, leading to localised attack. Stainless steel is particularly susceptible in oxygen-depleted, chloride-rich environments.
Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially. This is a major concern when fastening dissimilar metals (e.g., steel fasteners in aluminium or magnesium structures). Prevention involves isolating the metals with non-conductive washers or coatings.
Intergranular Corrosion: Attack along the grain boundaries of a metal. It can occur in aluminium alloys if they are improperly heat-treated or in stainless steels if they are sensitised (heated to a specific temperature range). It is difficult to detect visually and can severely weaken a structure.
Stress Corrosion Cracking (SCC): Occurs when a metal is under tensile stress in a corrosive environment. The combination of stress and corrosion leads to cracking.
Filiform Corrosion: Occurs under a painted or coated surface, appearing as a network of fine "worms" or threads. It is a form of crevice corrosion.

Corrosion Prevention and Control:

Protective Coatings: The primary purpose of paint and other coatings is to act as a barrier against moisture and contaminants, preventing corrosion.
Anodising: An electrochemical process that produces a thick, hard, corrosion-resistant oxide layer on aluminium alloys. It also provides an excellent base for paint. It does not significantly increase tensile strength.
Cadmium Plating: A sacrificial coating applied to steel fasteners and components. It provides corrosion protection and reduces friction. Critical Note: Cadmium melts at approximately 321°C. In high-temperature areas (e.g., exhaust stacks), it can melt and diffuse into the steel, causing liquid metal embrittlement (LME) . Cadmium-plated fasteners must not be used in these locations.
Corrosion-Inhibiting Compounds (CICs): Applied to internal structures (e.g., wing interiors) to penetrate crevices and faying surfaces. They are often applied by high-pressure spray or fogging to ensure complete coverage. Safety: These compounds contain solvents; adequate respiratory protection and ventilation are mandatory during application.
Material Selection and Isolation: Using compatible materials and isolating dissimilar metals with non-conductive washers or coatings to prevent galvanic corrosion.

Corrosion Removal:

The general procedure for corrosion removal is:

Mechanically remove the corrosion products using appropriate abrasive materials (e.g., aluminium wool, abrasive pads).
Assess the depth of the remaining damage.
If the damage is within allowable limits (per SRM/AMM), blend out the area smoothly and re-apply the protective finish.
If the damage exceeds allowable limits, the component must be repaired or replaced per approved data.

2.3 Aircraft Hardware: Fasteners

Aircraft fasteners are critical components that must be correctly selected, installed, and safety-wired.

Thread Forms:

Unified National (UN): The standard thread form for most aircraft fasteners. It includes Unified National Fine (UNF) and Unified National Coarse (UNC) series.
Metric (ISO): Increasingly common on newer aircraft.
Acme, Buttress, Square: Used for special applications like jackscrews or power transmission.

Bolts, Screws, and Studs:

Bolts: Used in structural applications. They are identified by the material, head marking, and grip length.
Close-Tolerance Bolts (e.g., Class 5 fit): Have a shank that is nearly an exact fit in the hole. They are used in applications requiring precise alignment and where shear loads are significant.
Standard Bolts (e.g., Class 3 fit): Have a looser fit (more clearance) and are used for general applications.
Shear Pins: Designed to carry shear loads only. They are often used in applications where a predictable failure is required.
Screws: Similar to bolts but are generally used for non-structural applications or where the threads engage directly into a component (e.g., a casting).
Studs: Threaded at both ends. One end is screwed into a component (e.g., an engine crankcase), and a nut is used on the other end.

Nuts:

Self-Locking Nuts: Use a non-metallic insert (e.g., nylon) or an elliptical distortion of the threads to create friction, preventing loosening without additional safetying.
Prevailing Torque: The torque required to turn the nut on a thread without a load. This must be within specified limits to ensure the locking feature is effective. A nut that has been dropped may be damaged and must be replaced.
Installation: Washers are not installed under self-locking nuts unless specifically called out in the AMM/IPC, as this can change the grip length and preload.
Castellated Nuts: Have slots cut into the top to accept a cotter pin for safetying. They are used with bolts that have a hole drilled through the shank.

Washers:

Plain Washers: Distribute the load under a nut or bolt head and prevent damage to the surface.
Lock Washers: Designed to prevent the nut from rotating and loosening under vibration by creating a spring action or biting into the surfaces. They do not increase bolt strength or seal.

Rivets:

Solid Rivets: The most common type of permanent fastener in airframe construction. They are installed by using a rivet gun on the manufactured head and a bucking bar on the tail to form the shop head by upsetting the shank.
Rivet Length: For a universal head rivet, the length of the rivet protruding above the material should be 1.5 times the rivet diameter to form a proper shop head.
Installation Technique: Use lower air pressure and controlled hammering to form a uniform shop head without deforming the surrounding material.
Drilling Out: Use a high-speed steel drill bit with a depth stop to avoid enlarging the hole.
Blind Rivets: Used where access to the back side is not possible.
Cherrymax (or similar): Have a mechanical lock between the stem and the head, providing high strength and fatigue resistance, making them suitable for structural repairs.
Hi-Lok Fasteners: A two-piece fastener consisting of a pin and a threaded collar. The collar is torqued with a wrench while the pin is held with a hex socket, providing consistent preload. They are installed from one side.

Safetying:

Cotter Pins (Split Pins): A safetying device inserted through a hole in the bolt and nut to prevent the nut from turning and loosening due to vibration. They are single-use items and must be replaced and bent correctly to secure the nut.
Lock Wire: Wired through holes in fasteners to prevent loosening.
Turnbuckles: Used to adjust cable tension. After adjustment, the threaded terminals must be visible through the inspection hole to ensure sufficient engagement, and the barrel is safety-wired.

Torque Application:

A torque wrench applies a precise amount of torque to a fastener to achieve the correct clamping force (tension) without over-stressing the fastener or the assembly. If a bolt rotates without increasing torque, the threads are likely stripped or damaged. The bolt must be removed and the internal threads inspected; if stripped, a thread insert (e.g., Heli-Coil) may be required per the AMM.

2.4 Aircraft Hardware: Fluid Lines, Hoses, and Seals

Rigid Fluid Lines (Tubing):

Materials: Aluminium alloy, stainless steel, and titanium.
Bending: The minimum bend radius for aluminium tubing is typically 3 times the tube diameter. For a 1/4-inch OD tube, the minimum bend radius is 3/4 inch. Exceeding the minimum bend radius places excessive stress on the tube and can cause kinking.
Fittings: Flared fittings (e.g., AN fittings) use a B-nut and a flared tube end. A leaking B-nut indicates a damaged or deformed flare or washer. The correct repair is to replace the sealing washer (or re-flare the tube if permitted) and re-torque the B-nut to the specified value. Over-tightening can damage the flare. Thread sealant is not used on flare fittings.

Flexible Hoses:

Construction: Typically consist of an inner tube, a reinforcement layer (e.g., textile or wire braid), and an outer cover.
Inspection: Crazing, cracking, or chafing of the outer cover indicates deterioration. A chafed outer braid with broken wires compromises the pressure containment and the hose must be replaced. Taping is not an approved repair.
Service Life: Hoses with a fire-resistant outer cover that show signs of crazing or cracking must be replaced immediately.
Installation: Hose length must be per the manufacturer's instructions, considering routing, bend radius, and movement. Excessive slack can cause chafing, and tension can cause failure.
Minimum Bend Radius: The primary reason for respecting the minimum bend radius is to protect the integrity of the hose-to-fitting interface. Exceeding it places excessive stress on the end fittings, which can lead to fitting fatigue, cracking, and eventual leakage or failure.
Swaged Fittings: These are permanently attached by the manufacturer. Re-swaging a new fitting on a used hose is not permitted. If a swaged fitting leaks, the hose assembly must be replaced.

Seals:

O-Rings: Used for static seals or limited dynamic applications.
U-Cup Seals: Designed for dynamic sealing applications, such as a reciprocating piston rod, providing low friction and effective sealing.
Gaskets: Used for static joints between flat surfaces.

2.5 Non-Destructive Testing (NDT)

NDT methods are used to inspect components without causing damage.

Dye Penetrant Inspection (DPI): Used to detect surface-breaking cracks in non-porous materials (ferrous and non-ferrous).
Procedure: Clean the surface -> Apply penetrant -> Allow dwell time -> Remove excess penetrant (wipe with a clean cloth or use water for water-washable penetrant) -> Allow the surface to dry completely -> Apply developer to draw out penetrant from defects. Applying developer to a wet surface will dilute the penetrant and reduce sensitivity.
Magnetic Particle Inspection (MPI): Used to detect surface and near-surface cracks in ferromagnetic materials (e.g., steel). It is the preferred method for inspecting steel bolts and welds.
Ultrasonic Testing (UT): Used to detect internal defects and measure material thickness.
Eddy Current Testing (ET): Used to detect surface and near-surface cracks in conductive materials, including aluminium.
X-Ray (Radiography): Used to detect internal defects.

2.6 Non-Metallic Materials

Acrylic (Plexiglass): Used for windows and canopies. Susceptible to crazing (micro-cracks) when exposed to solvents, fuels, or improper cleaning chemicals. This can occur around fastener holes where stress is concentrated.
Composite Materials (e.g., Honeycomb): Consist of a core (e.g., honeycomb) bonded to face sheets.
Disbond: The loss of adhesion between the core and face sheet.
Delamination: Occurs within a laminate (separation of layers).
Corrosion-Inhibiting Compounds (CICs): Applied to internal structures to penetrate crevices and faying surfaces. They are often applied by high-pressure spray or fogging to ensure complete coverage. Safety: These compounds contain solvents; adequate respiratory protection and ventilation are mandatory during application.

2.7 Engine Hardware (Piston Engines)

Crankcase Breather: Vents the internal pressure that builds up due to blow-by gases, preventing oil seal failure and oil leakage.
Crankshaft Main Bearings: Typically plain (sleeve) bearings made of a soft metal alloy (e.g., lead-bronze) that can support high radial loads and provide a large contact area.
Connecting Rod Bolts: Critical high-strength steel bolts. They must be inspected for cracks using magnetic particle inspection.
Cylinder Barrels: Deep scores in cylinder bores are typically beyond repair limits. The only acceptable action per the AMM is replacement of the cylinder barrel.
Spark Plug Threads: In aluminium heads, damaged spark plug threads are commonly repaired using thread repair inserts (Heli-Coil) as approved in the AMM.

2.8 Control Cables

Construction: Typically 7x7 (7 strands of 7 wires) or 7x19 (7 strands of 19 wires) steel wire rope.
Inspection Criteria:
Broken Wires: For 7x19 cables, typical criteria allow up to 5 broken wires in one lay length, or 10 broken wires distributed throughout the cable, before replacement is required. For 7x7 cables, the limits are lower (3 and 6 respectively). Any broken wire in a critical control cable is a rejectable defect.
Flat Spots: Indicate wear from rubbing against a pulley or other surface, often due to misalignment. The cable must be replaced, and the cause (alignment) corrected.
Corrosion: Surface rust that can be removed is not necessarily rejectable, but broken wires are always cause for replacement.

3. Important Formulas, Regulations, and Procedures

Rivet Protrusion Length: For a universal head rivet, protrusion = 1.5 × rivet diameter.
Minimum Bend Radius for Aluminium Tubing: Typically 3 × tube diameter.
ISO Property Class (e.g., 8.8): Tensile Strength (MPa) = First Digit × 100; Yield Strength (MPa) = Tensile Strength × Second Digit.
Regulatory Framework:
Regulation (EU) No 1321/2014, Annex III (Part-66): Governs the certification of maintenance staff.
Appendix I: Defines the basic knowledge syllabus, including Module 6.
Acceptable Means of Compliance (AMC) and Guidance Material (GM): Provide acceptable methods to show compliance with the regulation.
AC 43.13-1B (Acceptable Methods, Techniques, and Practices): Widely accepted as acceptable data for standard practices, including cable inspection criteria.
Key Procedures:
Corrosion Removal: Mechanical removal -> assess depth -> blend (if within limits) -> re-apply protective finish.
Dye Penetrant Inspection: Clean -> Apply penetrant -> Dwell -> Remove excess -> Dry -> Apply developer -> Inspect.
Torque Check: If a bolt rotates without increasing torque, the threads are stripped. Remove bolt, inspect internal threads, and repair with a thread insert if necessary.

4. Common Relationships Between Concepts

Material Properties and Application: The ductility of 2117-T4 rivets is why they are used for riveting. The high-temperature resistance of nickel-chromium alloys is why they are used for exhaust manifolds. The bearing properties of bronze are why it is used for gears.
Corrosion and Material Selection: The high anodic nature of magnesium dictates that it must be isolated from dissimilar metals to prevent galvanic corrosion. Cadmium plating on steel provides sacrificial protection but is unsuitable for high-temperature areas due to LME.
Fastener Fit and Application: A close-tolerance (Class 5) bolt is used where precise alignment is needed, while a free-fit (Class 3) bolt is for general use. A shear pin is specifically designed for shear loads only.
NDT Method and Material: Magnetic particle inspection is only for ferromagnetic materials (steel). Dye penetrant is for surface cracks in non-porous materials. Eddy current is for conductive materials.
Hose Condition and Serviceability: Crazing of the outer cover, chafed braid, or a leaking swaged fitting all lead to the same conclusion: the hose assembly must be replaced. No field repair is permitted.
Cable Damage and Replacement: Broken wires, flat spots, and kinks all necessitate cable replacement. The cause of the damage (e.g., pulley misalignment) must also be corrected.

5. Typical Exam Focus Points

Corrosion: Identifying the type of corrosion from a description (e.g., white powder on aluminium = surface corrosion; under a gasket = crevice corrosion). Know the correct initial action (e.g., remove corrosion, assess depth).
Material Properties: Definitions of ductility, toughness, hardness, and elasticity. Know which material is ferrous vs. non-ferrous.
Fasteners: The difference between Class 3 and Class 5 fits. The purpose of self-locking nuts and how to check their prevailing torque. The correct action if a bolt rotates during torquing (stripped threads). The use of cotter pins and lock washers. The correct rivet length calculation.
Fluid Lines and Hoses: The minimum bend radius for tubing. The correct action for a chafed hose (replace). The primary reason for respecting bend radius (protect fitting). The correct action for a leaking B-nut (replace washer/re-flare).
NDT: Which method is used for which material and defect type (e.g., MPI for steel bolts, DPI for surface cracks). The correct sequence of steps in DPI, especially the drying step before developer application.
Control Cables: The maximum number of broken wires allowed in a 7x19 cable (5 in one lay, 10 distributed). The action for a flat spot (replace cable and correct cause).
Heat Treatment: The condition of 2117-T4 rivets and why they must be driven promptly. The process to restore a 2024-T4 temper after overheating.
Safety: The primary safety precaution when applying corrosion-inhibiting compounds (respiratory protection). The danger of using cadmium-plated fasteners in high-temperature areas (LME).