Module 6: Materials and Hardware

Overview

Module 6 of the EASA Part-66 syllabus provides the fundamental knowledge required for aircraft maintenance certifying staff to understand the materials used in aeroplane construction, the hardware employed in assembly, and the degradation mechanisms that affect them. This module forms the basis for understanding why aircraft are built the way they are, why specific materials are selected for particular applications, and how to identify, inspect, and maintain these materials and components throughout their service life.

The module encompasses aircraft materials—both metallic and non-metallic—corrosion mechanisms and protection, fasteners and locking devices, composite materials, sealants, and non-destructive testing methods. A thorough understanding of these topics is essential for making sound engineering judgements during maintenance, repair, and certification activities.

Section 1: Aircraft Metallic Materials

1.1 Ferrous Alloys

Steel remains a critical material in aircraft construction, particularly for highly stressed components such as landing gear, engine mounts, and control system linkages. The primary alloying element in steel is carbon, with additional elements such as chromium, nickel, and molybdenum providing specific properties.

Classification of Steels:

Low-carbon steels (up to 0.3% carbon): Used for general-purpose fittings, brackets, and non-structural components. They are readily weldable and machinable.
Medium-carbon steels (0.3%–0.6% carbon): Heat-treatable, used for structural components requiring moderate strength.
High-carbon steels (0.6%–1.0% carbon): Used for springs, cutting tools, and wear-resistant components.
Alloy steels: Contain additional elements such as chromium, nickel, and molybdenum to enhance hardenability, toughness, and corrosion resistance.
Corrosion-resistant steels (CRES): Also known as stainless steels, these contain at least 11% chromium, which forms a self-healing oxide layer. Common grades include 302, 304, 316, and precipitation-hardening grades such as 17-7PH.

Heat Treatment of Steels:

The heat treatment of steel involves controlled heating and cooling cycles to achieve desired mechanical properties:

Annealing: Heating to a critical temperature followed by slow cooling to soften the material, relieve internal stresses, and improve machinability.
Normalising: Heating above the critical temperature followed by air cooling to refine grain structure.
Hardening: Heating to the austenitising temperature followed by rapid quenching (in water, oil, or air) to produce martensite, a hard, brittle structure.
Tempering: Reheating hardened steel to a temperature below the critical point to reduce brittleness while maintaining hardness and increasing toughness.

Hardness Testing:

Verification of heat treatment is commonly performed using hardness testing. The Rockwell hardness test is the most widely used method in aircraft maintenance, employing either a diamond cone (for hard materials) or a steel ball indenter (for softer materials). The depth of penetration is measured and converted to a hardness number. Other methods include Brinell and Vickers tests, each with specific applications depending on material and component geometry.

Identification of Steel Grades:

Steel fasteners are identified by radial lines on the bolt head. The number of lines indicates the strength grade according to SAE J429 or ASTM specifications. For example, a grade 5 bolt has three radial lines, while a grade 8 bolt has six. This identification system is critical for ensuring correct replacement fasteners are used during maintenance.

1.2 Aluminium Alloys

Aluminium is the most widely used structural material in aircraft construction due to its excellent strength-to-weight ratio, corrosion resistance, and formability.

Wrought Aluminium Alloys:

The designation system for wrought aluminium alloys uses a four-digit number:

1xxx series: Commercially pure aluminium (99% minimum), used for non-structural applications.
2xxx series: Copper as the principal alloying element. These alloys are heat-treatable and provide high strength. Common aerospace alloys include 2024 (aluminium-copper-magnesium) and 2017. They are used for wing skins, fuselage structures, and other highly stressed components.
3xxx series: Manganese as the principal alloying element. Non-heat-treatable, used for moderate-strength applications.
5xxx series: Magnesium as the principal alloying element. Non-heat-treatable, with good corrosion resistance and weldability.
6xxx series: Magnesium and silicon as principal alloying elements. Heat-treatable, with good formability and corrosion resistance. 6061 is commonly used for fittings and structural components.
7xxx series: Zinc as the principal alloying element. These are the highest-strength aluminium alloys, including 7075 (aluminium-zinc-magnesium-copper), used for highly stressed structural components such as wing spars and upper wing skins.

Heat Treatment of Aluminium Alloys:

Heat-treatable aluminium alloys are processed through:

Solution heat treatment: Heating to a temperature typically between 460°C and 500°C to dissolve alloying elements into solid solution.
Quenching: Rapid cooling to retain the supersaturated solid solution.
Ageing (precipitation hardening): Natural ageing at room temperature or artificial ageing at elevated temperatures to precipitate fine particles that strengthen the alloy.

The temper designation follows the alloy number, such as -T3 (solution heat-treated, cold-worked, and naturally aged), -T4 (solution heat-treated and naturally aged), and -T6 (solution heat-treated and artificially aged).

Overheating Effects:

Exceeding the solution heat treatment temperature causes eutectic melting at grain boundaries, resulting in:

Loss of strength
Reduced corrosion resistance
Intergranular cracking
Surface blistering

This condition is irreversible and requires re-heat treatment or component replacement.

Rivet Materials:

Solid rivets are commonly manufactured from 2117-T4 aluminium alloy, which provides good ductility for driving while achieving adequate strength after installation. The cold working during rivet driving, combined with natural ageing, results in a -T3 condition. 2024-T4 rivets are stronger but less ductile and more prone to cracking during driving; they are typically driven in the fully annealed condition and allowed to age naturally. Substitution of rivet materials is not permitted without engineering approval.

1.3 Titanium Alloys

Titanium offers an exceptional combination of properties for aerospace applications:

High strength-to-weight ratio
Excellent corrosion resistance
Compatibility with carbon fibre composites (no galvanic corrosion)
Good high-temperature performance (up to approximately 600°C)

Common titanium alloys include Ti-6Al-4V (6% aluminium, 4% vanadium), which is used for structural components, fasteners, and engine parts.

Machining Considerations:

Titanium work-hardens rapidly and has poor thermal conductivity. Drilling and machining require:

Low spindle speeds to prevent work hardening
High feed rates (pressure) to cut effectively
Cobalt or carbide cutting tools rather than standard high-speed steel
Generous coolant application to control heat generation

Failure to observe these parameters results in rapid tool wear, work hardening of the material, and potential damage to the component.

1.4 Magnesium Alloys

Magnesium is the lightest structural metal, approximately one-third lighter than aluminium. However, it has significant limitations:

Poor corrosion resistance, particularly in salt-laden environments
High susceptibility to galvanic corrosion
Flammability in finely divided form

Magnesium alloys are used in limited applications such as gearbox housings and some non-structural components. Corrosion of magnesium appears as white powder with lifting of surface layers (exfoliation). Cleaning must be performed carefully using chromate-based chemical treatments; mechanical methods such as wire brushing can cause galvanic corrosion and damage the soft material.

1.5 Material Identification

Correct identification of aircraft materials is essential for maintenance. Methods include:

Material certification documentation
Manufacturer markings and part numbers
Chemical analysis (for definitive identification)
Electrical conductivity testing (can differentiate some alloys but is not definitive)

Colour, hardness, and spark testing are not reliable methods for identifying aluminium alloys. Spark testing is only applicable to ferrous materials.

Section 2: Corrosion

2.1 Corrosion Mechanisms

Corrosion is the deterioration of a material due to chemical or electrochemical reaction with its environment. It is a significant concern in aircraft maintenance, as corrosion can compromise structural integrity and lead to catastrophic failure if undetected.

Electrochemical Corrosion:

Corrosion requires four elements:

An anode (where metal is lost)
A cathode (where reduction occurs)
An electrolyte (such as water containing dissolved salts)
An electrical path between anode and cathode

The galvanic series ranks metals according to their electrochemical potential. When dissimilar metals are in contact in the presence of an electrolyte, the more anodic metal corrodes preferentially.

2.2 Types of Corrosion

Surface (General) Corrosion:

This form of corrosion appears as a uniform attack over a surface area. On aluminium alloys, it typically presents as white/grey powdery deposits with pitting. Surface corrosion is often the initial stage of more severe corrosion and must be assessed against allowable damage limits in the SRM.

Pitting Corrosion:

Pitting is a localised form of corrosion producing small cavities or pits. It is often initiated by local breakdown of the protective oxide layer. Pitting can be difficult to detect visually and may require NDT methods such as eddy current inspection to determine depth and extent.

Intergranular Corrosion:

This corrosion attacks along grain boundaries, often due to precipitation of intermetallic phases during improper heat treatment. It is not visible on the surface initially and requires NDT methods such as eddy current testing for detection. Intergranular corrosion can lead to:

Loss of strength
Exfoliation (lifting of surface layers)
Stress corrosion cracking

Exfoliation Corrosion:

A form of intergranular corrosion where corrosion products push apart the grain layers, causing the material to lift and flake. It is commonly found on aluminium alloys and magnesium alloys, appearing as a white powder with layered lifting.

Stress Corrosion Cracking (SCC):

SCC occurs when a sustained tensile stress (residual or applied) acts on a material in a corrosive environment. The cracking is typically intergranular and can occur at stress levels well below the yield strength. SCC is distinct from fatigue, which results from cyclic loading. High-strength aluminium alloys (particularly 7xxx series) and high-strength steels are susceptible.

Galvanic Corrosion:

Galvanic corrosion occurs when dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially. Greenish-white corrosion products on steel bolts in contact with aluminium structures are characteristic of galvanic corrosion. Prevention methods include:

Insulation between dissimilar metals (e.g., cadmium-plated washers)
Surface protection (anodising, plating, painting)
Drainage to prevent electrolyte accumulation

Fretting Corrosion:

Fretting corrosion occurs when two surfaces in contact experience small oscillatory movements, leading to wear and oxidation. It commonly occurs at:

Riveted joints
Bolted connections
Bearing surfaces

The characteristic red/brown oxide powder (on steel) or black powder (on aluminium) distinguishes fretting from other corrosion forms.

Filiform Corrosion:

Filiform corrosion occurs under paint films, appearing as thread-like filaments. It is a form of oxygen concentration cell corrosion that requires a breached paint film for initiation.

2.3 Corrosion Protection

Anodising:

Anodising is an electrolytic process that thickens the natural oxide layer on aluminium. The component is made the anode in an acid electrolyte, and oxygen is released at the surface, forming a thick, porous aluminium oxide layer.

Key characteristics of anodised coatings:

Excellent corrosion resistance
Good paint adhesion (porous structure)
Slightly increased wear resistance (especially hard anodising)
Electrical insulation properties

Common anodising processes include chromic acid anodising (MIL-A-8625 Type I) and sulphuric acid anodising (Type II). Hard anodising (Type III) provides a thicker, harder coating for wear-resistant applications.

Chromate Conversion Coating (Alodine):

Chromate conversion coatings are applied to aluminium surfaces to provide:

Corrosion resistance (passivation of the surface)
Enhanced adhesion for subsequent primer and paint application

The process involves chemical treatment with a chromate solution, producing a thin, protective layer. It is not a primer itself but is typically applied before primer application.

Cadmium Plating:

Cadmium plating is applied to steel fasteners and fittings for corrosion protection. Cadmium is anodic to steel, meaning it corrodes preferentially, protecting the underlying steel. Additional benefits include:

Lubricity (reduced friction during installation)
Compatibility with aluminium structures (reduced galvanic corrosion)

White corrosion products on cadmium plating indicate the cadmium is corroding sacrificially—this is a self-limiting protective layer. Reddish-brown discolouration indicates rust forming on the underlying steel, meaning the plating has been breached and requires attention.

Aluminising:

Aluminising (aluminium diffusion coating) provides oxidation and corrosion protection at high temperatures (up to approximately 900°C). It is used on engine exhaust components and other high-temperature applications where cadmium and zinc coatings would fail.

Primers and Paints:

Primers are applied to clean, prepared surfaces to:

Promote adhesion of subsequent topcoats
Inhibit corrosion

Epoxy primers and chromated primers are standard for aluminium alloy structures. The primer must be compatible with both the substrate and the topcoat system.

2.4 Corrosion Inspection and Assessment

When corrosion is detected, the initial action is to assess the extent of damage against allowable limits specified in the SRM. This includes:

Determining corrosion type
Measuring depth and extent
Comparing against allowable damage limits
Deciding between blending, repair, or replacement

Blending is only permitted within SRM limits. Replacement is required when corrosion exceeds allowable limits or affects structural integrity.

Section 3: Non-Metallic Materials

3.1 Polymers: Thermoplastics and Thermosets

Thermoplastics:

Thermoplastics soften when heated and can be re-shaped and re-melted repeatedly. This reversibility is their defining characteristic. Common thermoplastics in aircraft include:

Acrylic (Perspex, Plexiglas): Used for windows and canopies due to excellent optical clarity and light transmission.
ABS (Acrylonitrile Butadiene Styrene): Used for fairings and non-structural components.
Polycarbonate: High impact strength, used for some glazing applications.
Nylon: Used for bearings, bushings, and cable guides.

Thermosets:

Thermosets undergo an irreversible chemical cross-linking reaction during curing. Once cured, they cannot be re-melted or re-shaped. Common thermosets include:

Epoxy: Used as matrix resin in composites and as adhesive.
Polyester: Used in some composite applications.
Phenolic: Used for interior panels due to fire resistance.

3.2 Acrylic Materials (Perspex/Plexiglas)

Acrylic materials are widely used for aircraft windows and canopies due to their:

Excellent optical clarity
Light weight
Good weathering resistance

Crazing:

Crazing is the formation of fine surface cracks on acrylic materials, most commonly caused by contact with solvents such as acetone, gasoline, or cleaning agents that attack the surface. Other causes include:

Excessive stress (stress cracking)
UV degradation (causes yellowing, not crazing)

Handling and Installation:

Acrylic windows are supplied with a protective film that must remain in place until all adjacent work (drilling, sealing, riveting) is completed. Removing the film early exposes the pane to surface scratches and chemical attack.

Repair Considerations:

Cracked acrylic windows must be replaced, not repaired. Cracks reduce structural integrity and can propagate under pressurisation loads. Stop-drilling is not an approved permanent repair for windows.

3.3 ABS (Acrylonitrile Butadiene Styrene)

ABS is a thermoplastic used for fairings and non-structural components. Critical considerations include:

Susceptibility to attack by certain solvents, ketones, and esters
Stress cracking when exposed to incompatible chemicals
Requirement for compatible adhesives and paints only

3.4 Composite Materials

Composite materials consist of a matrix (resin) reinforced with fibres. The matrix transfers loads between fibres and protects them from environmental damage, while the fibres provide strength and stiffness.

Fibre Types:

Carbon fibre: High strength and stiffness, low weight, used for primary structure.
Glass fibre: Lower cost, good electrical insulation, used for fairings and secondary structure.
Aramid (Kevlar): High impact resistance, used for leading edges and ballistic protection.

Resin Systems:

Epoxy: Most common aerospace resin, excellent mechanical properties and adhesion.
Polyester: Lower cost, used in less demanding applications.
Phenolic: Fire-resistant, used for interior panels.

Cure Monitoring:

The glass transition temperature (Tg) is directly related to the degree of cure of thermosetting resins. A fully cured resin has a characteristic Tg; a lower Tg indicates under-cure. Tg testing verifies that the cure cycle (time/temperature) was adequate.

Composite Defects:

Delamination: Separation between layers, detected by tap testing (hollow sound) or ultrasonic inspection.
Resin degradation: White, chalky appearance on the surface indicates UV degradation of the resin.
Moisture ingress: Causes blistering and can lead to core corrosion in honeycomb structures.
Fibre breakage: Internal damage, not visible on the surface.

Vacuum Bagging Process:

The vacuum bagging process consolidates composite laminates during curing:

Release film: Prevents sticking of the bag to the laminate.
Peel ply: Woven fabric applied to the surface before curing; after cure, it is peeled off, leaving a clean, textured surface ideal for secondary bonding.
Bleeder cloth: Absorbs excess resin from the laminate. Without it, excess resin remains on the surface, causing a resin-rich and uneven finish.
Vacuum bag: Applies pressure to consolidate the laminate.

Honeycomb Structures:

Honeycomb sandwich structures consist of a honeycomb core bonded between two face sheets. Core materials include aluminium, Nomex (aramid paper), and fibreglass.

Core Splicing:

When splicing new honeycomb core into existing core:

The new core must match the original in material, cell size, foil thickness, and orientation (ribbon direction).
The adhesive film must have a cure temperature compatible with the existing structure.
Potting foam is used for edge closures, not for core splicing.

Core Corrosion:

Moisture ingress through cracks or damaged face sheets is the primary cause of corrosion in aluminium honeycomb cores. Regular inspection for water ingress is essential.

Lightning Strike Protection:

Composite structures often incorporate a conductive mesh or foil (copper or aluminium) for lightning strike protection. Conductivity tests verify that the LSP system is continuous and effective, ensuring lightning currents are safely conducted away. This test does not measure structural integrity, cure state, or moisture content.

Allowable Damage:

Small, shallow dents that do not affect the core or the bond may be within allowable damage limits and can be left un-repaired if they do not exceed SRM limits. Core damage, delamination, and punctures require repair.

Section 4: Fasteners

4.1 Solid Rivets

Solid rivets are the most common permanent fasteners in aircraft structures. They consist of a manufactured head and a shank; during installation, the tail is deformed to form a shop head.

Rivet Materials:

2117-T4 aluminium: Most common, good ductility, driven in T4 condition, naturally ages to T3 after installation.
2024-T4 aluminium: Higher strength but less ductile, more prone to cracking during driving.
Monel: Corrosion-resistant, used for specific applications.
Titanium: Used where high strength and corrosion resistance are required.

Rivet Identification:

Rivet head markings indicate the material:

Plain head: 1100 or 3003 aluminium
Raised dot: 2117-T4 aluminium
Raised double dash: 2024-T4 aluminium
Raised cross: 5056 aluminium
Depressed dot: 7050-T73 aluminium

Rivet Selection:

Critical factors in rivet selection include:

Grip length: Total thickness of materials being joined. The rivet length must be sufficient to form a proper shop head.
Diameter: Determined by the application and structural requirements.
Head type: Universal, countersunk, or round head depending on aerodynamic and structural requirements.
Edge distance: Minimum distance from rivet centre to material edge.

Rivet Installation:

The manufactured head is placed on the accessible side; the bucking bar forms the shop head on the opposite side.
A new rivet must fit the hole without free play; a hammer tap or light pressure should be required.
Rivets must be visually perfect before installation—any surface damage creates stress concentrations.
The shank expands to fill the hole during driving, providing a tight fit.

Rivet Removal:

To remove a damaged rivet without enlarging the hole:

Drill out the head using a bit slightly smaller than the shank.
Use a pin punch to drive out the remaining shank.
Never use a larger bit, which would enlarge the hole.

Rivet Substitution:

Rivet material substitutions are not permitted without engineering approval. The SRM or AMM specifies the exact rivet material for each application.

4.2 Bolts and Nuts

Bolt Identification:

Steel bolts are marked with radial lines on the head to denote strength grade:

Grade 5: Three radial lines
Grade 8: Six radial lines

Bolt material can also be identified by head markings (e.g., "AN" for Air Force-Navy standard, "MS" for Military Standard).

Hi-Lok Fasteners:

Hi-Lok fasteners consist of a threaded pin and a collar that is swaged using a wrench. Key characteristics:

Consistent clamp-up without the need for a torque wrench
The collar has a break-off hex that shears at a predetermined torque
Used for structural joints
Requires access to both sides of the structure

Lockbolts:

Lockbolts are installed by swaging a collar onto the bolt using a special hydraulic tool. They do not require torque, cotter pins, or welding.

Self-Locking Nuts:

Self-locking nuts incorporate a locking feature—either a nylon insert or deformed (non-circular) threads—that provides friction on the bolt threads.

All-metal type: Locking feature is effective as soon as the nut is fully engaged.
Nylon insert type: The nylon collar grips the bolt threads.
Torque specifications account for the locking torque; no extra torque or backing off is needed.
Lubrication is not recommended as it may alter the locking action.
If a self-locking nut has loosened, the locking feature is likely worn or damaged—the nut must be replaced.

Shear Pins:

A shear pin is designed to break at a specific shear load, acting as a mechanical fuse to protect the system from overload.

4.3 Quick-Release Fasteners

Dzus Fasteners:

Dzus fasteners are quarter-turn quick-release fasteners designed for rapid removal of non-structural panels. They are not used for primary structure.

4.4 Temporary Fasteners

Clecos:

Clecos are spring-loaded temporary fasteners used to align and hold sheet metal parts during drilling and riveting. They are removed before final assembly.

4.5 Installation Techniques

Wet Installation:

Wet installation involves applying a corrosion-inhibiting sealant (e.g., polysulfide) to the fastener or hole before installation. The primary purpose is to:

Seal the joint against fluid ingress (fuel, water)
Prevent corrosion

Wet installation does not significantly increase fatigue life (that is achieved by hole preparation and interference fit) and does not act as a lubricant (torque values are usually adjusted for wet installation).

Torque Seal (Torque Stripe):

Torque seal is a paint-like material applied across the fastener and surrounding structure after torquing. If it cracks or is disturbed, it indicates possible loosening. It does not provide mechanical locking.

Section 5: Sealants and Adhesives

5.1 Sealant Types

Polysulfide Sealants:

Polysulfide sealants are the most common sealants for aircraft integral fuel tanks. Key properties:

Excellent fuel resistance
Two-part systems (base and accelerator)
Room-temperature cure
Require primer for proper adhesion to aluminium

Silicone Sealants:

Silicone sealants are not fuel-resistant and are not suitable for fuel tank applications. They are used for:

Pressurised cabin sealing
Firewall sealing
Electrical component protection

Cyanoacrylate Adhesives:

Cyanoacrylates (super glues) are not suitable for large gaps or structural applications. They are used for small, non-structural bonding tasks.

5.2 Sealant Application

Critical considerations when applying sealants:

Pot life: The working time before the sealant begins to cure.
Primer requirement: Many sealants require a primer for proper adhesion.
Cure time: Varies depending on temperature and humidity.
Application thickness: Must be within specified limits.

5.3 Adhesives

Cold Bonding:

Cold bonding uses a two-part epoxy adhesive that cures at room temperature. Critical requirements:

Surface preparation is essential for bond strength.
Single-part adhesives require heat to cure.
Cyanoacrylates are not structural adhesives.
Pressure-sensitive adhesives are not for structural bonding.

Section 6: Non-Destructive Testing (NDT)

6.1 Dye Penetrant Inspection

Dye penetrant inspection detects surface-breaking discontinuities in non-porous materials.

Solvent-Removable Penetrant System:

For solvent-removable penetrants:

Apply penetrant and allow dwell time.
Remove excess penetrant with a cloth lightly dampened with solvent.
Wipe in one direction to prevent drawing penetrant out of discontinuities.
Apply developer.
Interpret indications.

High-pressure water is used for water-washable penetrants, not solvent-removable systems.

6.2 Magnetic Particle Inspection (MPI)

Magnetic particle inspection detects surface and near-surface cracks in ferromagnetic materials (steel, iron).

Procedure:

Magnetise the component.
Apply magnetic particles (dry or wet).
Particles accumulate at discontinuities, forming indications.
Interpret indications while the component is still magnetised.
Demagnetise after inspection is complete.

MPI is the most appropriate method for detecting surface cracks in steel welds and landing gear components.

6.3 Eddy Current Inspection

Eddy current testing detects surface and near-surface defects in conductive materials. It is commonly used to detect:

Intergranular corrosion in aluminium alloys
Surface cracks
Conductivity variations

Eddy current is particularly useful for detecting subsurface corrosion in aluminium structures.

6.4 Ultrasonic Inspection

Ultrasonic testing uses high-frequency sound waves to detect internal defects. It is effective for:

Delamination in composites
Thickness measurements
Subsurface defects

6.5 Tap Testing

Tap testing is a simple method for detecting delamination in composite structures. A "soft" or "hollow" sound indicates a loss of bond or delamination between layers.

Section 7: Aircraft Painting and Finishing

7.1 Primer Function