Module 6: Materials and Hardware

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:

Heat Treatment of Steels:

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

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:

Heat Treatment of Aluminium Alloys:

Heat-treatable aluminium alloys are processed through:

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:

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:

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:

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:

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:

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:

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:

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:

Fretting Corrosion:

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

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:

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:

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:

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:

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:

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:

Thermosets:

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

3.2 Acrylic Materials (Perspex/Plexiglas)

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

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:

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:

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:

Resin Systems:

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:

Vacuum Bagging Process:

The vacuum bagging process consolidates composite laminates during curing:

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:

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:

Rivet Identification:

Rivet head markings indicate the material:

Rivet Selection:

Critical factors in rivet selection include:

Rivet Installation:

Rivet Removal:

To remove a damaged rivet without enlarging 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:

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:

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.

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:

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:

Silicone Sealants:

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

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:

5.3 Adhesives

Cold Bonding:

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

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:

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:

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:

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:

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

Primers are applied to clean, prepared surfaces to:

Epoxy primers and chromated primers are standard for aluminium alloy structures.

7.2 Paint System Components

A complete aircraft paint system typically consists of:

Section 8: Aircraft Tubing and Hoses

8.1 Hydraulic Tubing

High-pressure hydraulic systems (3000 psi) use corrosion-resistant steel (CRES) tubing for strength and corrosion resistance.

Summary of Key Relationships

Typical Exam Focus Points

References