Module 6: Materials and Hardware

1. Module Overview

Module 6 of the EASA Part-66 syllabus provides the foundational knowledge required for aircraft maintenance certifying staff to understand the materials used in aircraft construction and the hardware used to assemble and maintain them. This module is critical because the safe and airworthy condition of an aircraft is directly dependent on the correct selection, installation, and maintenance of its materials and components.

The module covers a broad spectrum of topics, from the metallurgy of ferrous and non-ferrous alloys to the properties of composite and non-metallic materials. It also details the vast array of hardware—fasteners, seals, cables, and hoses—that are the building blocks of the airframe and its systems. A significant portion of the module is dedicated to the various forms of corrosion and deterioration that affect these materials, along with the inspection and protection methods used to manage them. The overarching theme is the importance of using approved data, following manufacturer's instructions, and understanding the "why" behind standard maintenance practices to ensure continued airworthiness.

2. Key Concepts Explained in Detail

2.1 Aircraft Materials (Module 6.1, 6.2, 6.3, 6.4)

Ferrous Materials (Steels)

Composition and Types: Ferrous materials are iron-based. In aircraft, the most common are various alloy steels, where elements like chromium, nickel, molybdenum, and vanadium are added to enhance strength, hardness, toughness, and corrosion resistance.
Low-Alloy Steels: Often used for structural parts requiring high strength (e.g., landing gear components, engine mounts). They are typically heat-treated to achieve their final properties.
Stainless Steels: Contain a high percentage of chromium (at least 11%) which provides excellent corrosion resistance. Used for applications like exhaust systems, hydraulic lines, and fasteners where corrosion resistance is critical.
High-Carbon Steels: Used for applications requiring high hardness and wear resistance, such as springs, cutting tools, and bearing races.
Heat Treatment: The properties of steel are significantly altered by heat treatment processes. Key processes include:
Annealing: Softens the steel, relieving internal stresses and improving machinability.
Normalising: Refines the grain structure after forging or rolling.
Hardening and Tempering: Involves heating to a high temperature, rapid quenching (cooling), and then re-heating to a lower temperature (tempering) to reduce brittleness while maintaining hardness.
Identification and Marking: Steels are often identified by a numbering system (e.g., SAE/AISI). For example, 4130 steel indicates a low-alloy steel with specific amounts of chromium and molybdenum.
Corrosion: The primary enemy of steel is oxidation, which produces red rust (iron oxide). This is a direct chemical reaction with oxygen and moisture, leading to a loss of material and structural integrity. Protective finishes like cadmium plating or paint are essential.

Non-Ferrous Materials (Aluminium, Titanium, Magnesium)

Aluminium Alloys: Aluminium is a lightweight metal, but pure aluminium is too soft for structural use. It is alloyed with elements like copper, zinc, magnesium, and manganese to increase its strength.
Wrought Alloys: Designated by a four-digit system (e.g., 2024, 6061, 7075). The first digit indicates the principal alloying element.
2024-T3: Copper is the main alloying element. It has a high strength-to-weight ratio and is widely used in aircraft skins, webs, and other structural components.
7075-T6: Zinc is the main alloying element. It is one of the highest-strength aluminium alloys, used in highly stressed structural parts like wing spars and fuselage bulkheads.
Heat Treatment: Aluminium alloys are strengthened by a process called solution heat treatment followed by ageing (precipitation hardening). The "T" designation (e.g., T3, T6) indicates the specific heat treatment condition. For example, T6 means solution heat-treated and artificially aged. Machining these alloys in their heat-treated state does not require further heat treatment, provided excessive heat is not generated.
Corrosion: Aluminium is protected by a natural, thin oxide layer. However, when this is breached, it is susceptible to corrosion, which appears as a white, powdery deposit (aluminium oxide/hydroxide). This is a serious issue that can lead to pitting, exfoliation, and loss of structural strength.
Titanium Alloys: Titanium offers an excellent strength-to-weight ratio and superior corrosion resistance compared to steel and aluminium. It is used in high-temperature areas (e.g., engine nacelles, firewalls) and highly stressed components.
Critical Issue: At elevated temperatures (above approximately 300°C), titanium absorbs oxygen from the atmosphere. This forms a brittle, oxygen-enriched layer known as "alpha case". This layer is prone to cracking and significantly reduces the material's mechanical properties. Surface discolouration from heat exposure is a sign that this may have occurred, and the component must be replaced as the damage cannot be detected or removed by surface inspection.
Magnesium Alloys: These are the lightest structural metals. They are used in some castings, such as gearbox housings, where weight savings are critical.
Critical Issue: Magnesium is highly susceptible to corrosion, and its corrosion products are highly flammable, especially when ground or machined into fine particles. Special precautions are required when handling and cleaning magnesium components to prevent fires.

Composite and Non-Metallic Materials (Module 6.3, 6.6)

Composite Materials: These consist of a reinforcement material (fibres) embedded in a matrix material (resin). The combination provides properties superior to either component alone.
Reinforcement Fibres: Common types include:
Glass Fibre: Good strength and low cost, used in fairings and radomes.
Carbon Fibre: High strength and stiffness with low weight, used in primary and secondary structures like flight control surfaces and wing components.
Aramid (Kevlar): High toughness and impact resistance, used in areas prone to impact damage like engine cowls and fan blades.
Matrix Resins: The resin binds the fibres together, transfers loads between them, and protects them from the environment.
Thermosetting Resins (e.g., Epoxy, Polyester): These undergo an irreversible chemical cross-linking reaction during curing. Once cured, they cannot be melted or reshaped by heating. This is the most common type in aircraft structures.
Thermoplastic Resins (e.g., PEEK, PPS): These do not undergo a chemical cross-linking reaction. They can be repeatedly melted and reshaped upon heating, making them potentially recyclable and repairable by heat.
Forms: Composites are available as pre-impregnated fabrics ("prepreg") or as dry fabrics that are infused with resin during the manufacturing process.
Defects: Common defects include delamination (separation of layers), disbonds (separation from a core or substrate), and porosity (air pockets). These are often detected using ultrasonic testing.
Repair Materials: In composite repairs, a "peel ply" is a release fabric placed on the surface of a layup. After curing, it is peeled off, leaving a clean, rough surface ideal for bonding additional plies. It does not add structural strength.
Other Non-Metallic Materials:
Plastics (e.g., ABS, Acrylic): Used for non-structural items like fairings, window trim, and interior panels. They can be susceptible to cracking and crazing. Damage limits are often defined in the SRM.
Wood and Fabric: Used in some older or light aircraft structures. Their maintenance requires specific skills and materials.

2.2 Aircraft Hardware (Module 6.2, 6.5, 6.7, 6.8)

Fasteners

Bolts and Screws: These are threaded fasteners used to join components.
AN (Air Force-Navy) Standards: A common US military standard for aircraft hardware. The "AN" prefix is part of the part number (e.g., AN3-5A). AN bolts are made of corrosion-resistant steel or cadmium-plated alloy steel and are identified by the markings on the head.
NAS (National Aerospace Standard) Standards: These are higher-strength fasteners often used in critical applications.
Identification: Bolts are identified by the material, head type, and part number. For example, a bolt with no markings may be a low-strength bolt, while a bolt with a raised dash or asterisk may indicate a higher-strength material.
Cadmium Plating: A common protective finish for steel bolts. It provides sacrificial corrosion protection and controls friction during torquing. If the plating is worn or damaged, the bolt must be replaced, as the exposed steel is vulnerable to corrosion and the torque characteristics are altered.
Rivets: Permanent fasteners used extensively in airframe construction.
Solid Rivets: Installed by deforming the tail (shop head) to clamp the materials together. The rivet diameter should match the hole size, and the length must be sufficient to form a proper shop head (typically 1.5 times the diameter protruding above the material stack).
Rivet Materials: Common materials include 2117-T4 (aluminium alloy), 2024-T4 (higher strength), and Monel (corrosion-resistant steel). Substituting one material for another is not permitted without approval, as they have different driving characteristics and strength.
Installation Defects: A shop head that is off-centre or not concentric with the manufactured head is a defect. Re-striking is not allowed as it can damage the rivet; it must be drilled out and replaced.
Drilling Out Rivets: To remove a rivet without damaging the surrounding material, use a drill bit slightly smaller than the rivet shank to drill off the manufactured head, then punch out the remaining shank.
Special Fasteners:
Hi-Lok Fasteners: Consist of a pin and a collar. The collar is tightened with a hex driver until it shears at a predetermined torque, providing a consistent preload. They are used in areas where access for a wrench is limited.
Self-Locking Nuts: These have a prevailing torque feature (a deformed thread or a nylon insert) that prevents them from loosening under vibration. They can be reused only if they still meet the minimum prevailing torque specified in the manufacturer's documentation. If they can be turned by hand after installation, they must be replaced.
Castellated Nuts and Cotter Pins: Used in critical applications like axle nuts. The nut is torqued to the specified value. If the slot does not align with the hole in the bolt, the nut may be loosened up to 30 degrees (one hex flat) to align the slot. It is never acceptable to overtighten beyond the specified torque. Cotter pins must always be new.
Locking Devices: In addition to cotter pins, other locking methods include:
Lockwire: Must be installed with a slight tension (no slack). The direction of the wire must be such that any tendency for the fastener to loosen will increase the tension in the wire, thus preventing rotation. Reuse is not permitted.
Lock Washers: Used in less critical applications.

Torque

Purpose: Torque wrenches are used to apply a specific preload to a fastener. This preload is critical for joint integrity. Over-torquing can damage the fastener or the material, while under-torquing can lead to loosening under vibration.
Torque Wrench Extensions: When using an extension (crowfoot) at 90 degrees (perpendicular) to the wrench handle, the effective lever arm length remains unchanged, so the torque reading is accurate. Only when the extension is in line with the handle (axial extension) does the length change, requiring recalculation.

Cables and Pulleys

Control Cables: Used in flight control systems. They are made of carbon steel or corrosion-resistant steel.
Inspection: Cables must be inspected for broken wires, corrosion, and wear.
Broken Wires: The general rule is that if broken wires are found in any one rope lay (the length of one complete helix of the outer wires), the cable must be replaced. The AMM may provide specific allowable limits (e.g., a maximum number of broken wires per lay length). If within limits, the defect should be recorded for monitoring.
Corrosion: For carbon steel cables, replacement is required if the diameter is reduced by more than 10% due to corrosion or wear.
Pulleys: Cables run over pulleys to change direction. Pulley bearings have wear limits specified in the AMM. If within limits and the pulley operates smoothly, it may remain in service.

Seals and Hoses

Seals: Used to prevent leakage of fluids or gases.
O-Rings: Used for static or low-speed dynamic applications.
U-Cup Seals: Specifically designed for dynamic applications like reciprocating piston rods. The lip presses against the rod, providing a seal that works in both directions.
Gaskets: Used for static joints between flat surfaces.
Hoses: Used to convey fluids in systems where flexibility is required.
Shelf Life: Hoses have a limited shelf life from the date of manufacture. Installation beyond the shelf life is not permitted unless the manufacturer provides an extension.
Inspection: A chafed outer cover that exposes the wire braid compromises the hose's integrity and can lead to failure. Taping or sleeving is not an approved repair unless specified in the AMM. Replacement is required.

2.3 Corrosion (Module 6.4)

Corrosion is the deterioration of a material due to a chemical or electrochemical reaction with its environment. It is a major threat to aircraft structural integrity.

Forms of Corrosion:
Surface Corrosion: Appears as general roughening, pitting, or a powdery deposit. On aluminium, this is a white powder (aluminium oxide/hydroxide). On steel, it is red rust (iron oxide).
Pitting Corrosion: Localised attack that creates small holes or pits in the surface. It can be difficult to detect and can significantly reduce strength.
Galvanic Corrosion: Occurs when two dissimilar metals are in contact in the presence of an electrolyte. The more active metal corrodes preferentially.
Intergranular Corrosion: Attack along the grain boundaries of a metal. It can occur without visible surface signs and can severely weaken the material.
Exfoliation Corrosion: A form of intergranular corrosion that occurs in extruded or rolled aluminium alloys, causing the layers to separate and flake.
Stress Corrosion Cracking: Cracking that occurs due to the combined action of tensile stress and a corrosive environment.
Filiform Corrosion: A form of corrosion that occurs under a protective coating, appearing as a network of fine filaments.
Corrosion Control:
Prevention: Protective coatings (paint, plating, anodising) and corrosion-inhibiting compounds (CICs) are used to prevent corrosion.
Inspection: Regular inspections are essential to detect corrosion early.
Removal: Corrosion is removed by blending or sanding, following the SRM. After blending, it is essential to perform a non-destructive test (typically dye penetrant) to confirm that all corrosion products and any associated cracks have been removed.
Repair: If the remaining thickness is below the minimum allowable, the structure must be repaired or replaced.

3. Important Formulas, Regulations, and Procedures

Regulations:
Part-66 (Regulation (EU) No 1321/2014, Annex III): The regulation that governs the certification of maintenance staff. This module (Module 6) is part of the basic knowledge requirements.
Part-145 (Regulation (EU) No 1321/2014, Annex II): The regulation that governs the approval of maintenance organisations. It requires that all maintenance be performed using approved data (e.g., AMM, SRM, SB) and that defects be managed according to the MEL/CDL.
Part-21: The regulation that governs the certification of aircraft and aeronautical products. It is relevant when a modification or repair requires design approval.
Procedures:
Defect Management: In line maintenance, a defect may be deferred if it is covered by the Minimum Equipment List (MEL) or Configuration Deviation List (CDL) and the associated procedures are followed. If a defect is not within limits, the aircraft must be grounded.
Approved Data: All maintenance actions, including repairs and replacements, must be performed in accordance with approved data from the aircraft manufacturer (AMM, SRM, IPC) or the design authority.
Shelf Life: Components like hoses and seals have a limited shelf life. They must not be installed if they are beyond their shelf life.
Component Protection: Protective caps and plugs on components must remain in place until the moment of connection to prevent contamination.

4. Common Relationships Between Concepts

Material Properties and Application: The choice of material is directly related to its properties. For example, aluminium alloys are used for their high strength-to-weight ratio, steel for its high strength and hardness, and titanium for its high-temperature performance and corrosion resistance.
Corrosion and Protection: The susceptibility of a material to corrosion dictates the type of protective finish required. For example, steel bolts are cadmium-plated, and aluminium skins are painted or anodised.
Fastener Type and Application: The type of fastener used is related to the application. Solid rivets are used for permanent joints, bolts for demountable joints, and Hi-Loks for high-strength applications with limited access.
Damage Limits and Airworthiness: The SRM defines allowable damage limits for various structures. If damage is within these limits, the aircraft may be returned to service, but the defect must be recorded. If damage exceeds the limits, repair or replacement is required.
Inspection and NDT: The type of defect being sought dictates the NDT method used. For example, magnetic particle inspection is used for surface cracks in ferromagnetic steel, while ultrasonic testing is used for delamination in composites.

5. Typical Exam Focus Points

Material Identification and Properties: Be able to identify common materials (e.g., 2024-T3, 7075-T6, 4130 steel) and their typical applications.
Corrosion Types and Prevention: Understand the different forms of corrosion, their appearance, and the methods used to prevent and remove them.
Fastener Identification and Installation: Know the different types of fasteners, their markings, and the correct procedures for installation, including torque and locking.
Non-Destructive Testing (NDT): Understand which NDT method is most appropriate for detecting specific defects in specific materials.
Approved Data and Procedures: Emphasise the importance of using approved data (AMM, SRM) and following standard maintenance practices.
Defect Management: Understand the procedures for dealing with defects, including the use of MEL/CDL and the requirements for recording and reporting.
Safety Precautions: Be aware of specific safety hazards, such as the flammability of magnesium corrosion products and the dangers of handling phosphate ester hydraulic fluid.
Shelf Life and Storage: Know the requirements for storing and handling materials like tyres, hoses, and seals.