Module 6: Materials and Hardware

Module 6: Materials and Hardware — EASA Part-66 B3 Category Study Material

1. Module Overview

Module 6 of the EASA Part-66 syllabus (Appendix I to Regulation (EU) No 1321/2014, Annex III) covers aircraft materials and hardware, forming the foundational knowledge required for certifying staff working on light aeroplanes and helicopters. For the B3 category (light piston-engine aeroplanes and helicopters), this module provides essential understanding of:

• Aircraft materials — ferrous and non-ferrous metals, composites, and non-metallic materials
• Corrosion — types, detection, and treatment
• Fasteners — rivets, bolts, nuts, and their identification systems
• Non-destructive testing (NDT) — methods and applications
• Inspection techniques — including tap testing and visual inspection criteria
• Maintenance practices — torque application, safetying, and approved data usage

The knowledge levels range from Level 1 (overview) for general material properties to Level 3 (detailed theory) for fastener identification, corrosion treatment, and inspection criteria. This module is fundamental to safe maintenance practice, as it underpins almost every maintenance task performed on aircraft structures and systems.

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2. Key Concepts Explained in Detail

2.1 Aircraft Metallic Materials

Aluminium Alloys

Aluminium is the primary structural material in most light aircraft and helicopters. Pure aluminium is soft and weak, so it is alloyed with other elements to achieve the required mechanical properties.

Wrought Aluminium Alloy Designation System (4-digit):

Key alloys for B3 aircraft:

• 2024-T3 — Al-Cu-Mg alloy, solution heat-treated and naturally aged. This is the most common alloy for wing and fuselage skins due to its excellent fatigue resistance and high strength-to-weight ratio. It has moderate corrosion resistance and is typically supplied with an alclad (pure aluminium) coating.
• 7075-T6 — Al-Zn-Mg-Cu alloy, solution heat-treated and artificially aged. Higher strength than 2024 but lower corrosion resistance and poorer fracture toughness. Used for highly stressed fittings and structural members.
• 6061-T6 — Al-Mg-Si alloy with good corrosion resistance and weldability. Used for non-structural components, fittings, and some helicopter components.
• 5052-H32 — Al-Mg alloy with excellent corrosion resistance. Used for fuel tanks and non-structural components.

Heat Treatment Conditions (Temper Designations):

• -T3 — Solution heat-treated, cold-worked, and naturally aged
• -T4 — Solution heat-treated and naturally aged
• -T6 — Solution heat-treated and artificially aged
• -H32 — Strain-hardened and stabilised

Important consideration: 2024-T3 rivets (designation 'D') must be heat-treated and stored refrigerated before use, whereas 2117-T4 rivets (designation 'AD') are work-hardenable and can be driven in the as-supplied condition. This is why 'AD' rivets are preferred for general airframe work.

Steel Alloys

Steels are used where high strength, hardness, or wear resistance is required.

• 4130 (Chromium-Molybdenum steel) — The most common alloy steel in light aircraft construction. Used for tubular fuselage frames, engine mounts, and landing gear. It is weldable and heat-treatable to moderate strength levels.
• 4340 (Nickel-Chromium-Molybdenum steel) — Higher strength than 4130, used for highly stressed components such as landing gear struts and rotor head components.
• Stainless steels — Corrosion-resistant steels containing chromium (minimum 10.5%). Types 302, 304, and 321 are used for control cables, fasteners, and exhaust components. They are not magnetic and cannot be inspected by magnetic particle methods.

Corrosion protection for steel: Cadmium plating is commonly applied to steel fasteners and small components. This provides sacrificial corrosion protection — the cadmium corrodes preferentially, protecting the underlying steel.

Titanium Alloys

Titanium offers an excellent strength-to-weight ratio and corrosion resistance. It is used in helicopter rotor heads, fasteners, and firewalls. Titanium is non-magnetic and has a relatively low density (approximately 4.5 g/cm³ compared to 7.9 g/cm³ for steel).

Nickel and Cobalt Alloys

Used for high-temperature applications such as exhaust systems and turbine components. Inconel and Monel are common trade names.

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2.2 Non-Metallic Materials

Composite Materials

Composites consist of a reinforcement (fibres) embedded in a matrix (resin). They offer high strength-to-weight ratios and excellent fatigue resistance.

Fibre types:

• Carbon fibre — Very high strength and stiffness, low weight. Used in rotor blades, tail booms, and control surfaces.
• Glass fibre (E-glass, S-glass) — Lower cost, good electrical insulation. Used in fairings, radomes, and secondary structures.
• Aramid (Kevlar®) — High impact resistance, used in leading edges and ballistic protection.

Matrix types:

• Polyester resin — Low cost, used in non-structural applications.
• Epoxy resin — Superior mechanical properties and environmental resistance. The most common matrix for aerospace composites.
• Phenolic resin — Fire-resistant, used in cabin interior panels.

Honeycomb core construction: Sandwich structures consist of thin face skins bonded to a lightweight honeycomb core (aluminium, Nomex®, or paper). This provides high bending stiffness with minimal weight. The core may be hexagonal, flex-core, or over-expanded.

Inspection of composite structures — the tap test: A coin or purpose-made tapping hammer is used to lightly tap the surface. A sharp, ringing sound indicates a good bond between skin and core. A dull, low-frequency sound indicates a disbond or delamination. This is a simple but effective method for detecting skin-to-core disbonding.

Transparent Plastics

Acrylic (Plexiglas®, Perspex®) — The most common material for windows and windshields. Available in:

• Cast acrylic — Higher strength and better optical quality
• Stretched acrylic — Molecular chains aligned for improved strength and crack resistance

Critical maintenance points for acrylic:

• Use only approved cleaners (mild soap and water or approved plastic cleaner)
• Never use solvents such as acetone, MEK, or toluene — these cause crazing (fine surface cracks) and can lead to catastrophic failure
• Protective film should remain in place during handling and installation to prevent scratching
• Cracks around fastener holes require assessment against the AMM — stop drilling is only permitted if specified by the manufacturer

Rubber and Elastomers

Used for seals, hoses, and vibration isolation.

• Natural rubber — Good elasticity but poor oil and weather resistance
• Neoprene (polychloroprene) — Good oil and weather resistance
• Nitrile (Buna-N) — Excellent oil and fuel resistance
• Silicone — Wide temperature range but poor fuel resistance
• PTFE (Teflon®) — Excellent chemical resistance, wide temperature range, fire-resistant. Used for hydraulic hoses in engine compartments.

Sealants

Sealants are used for fuel tanks, pressurised cabins, and corrosion protection.

• Polysulphide sealants (e.g., PR-1422) — The standard for integral fuel tanks. Excellent fuel and solvent resistance. Two-part systems that cure at room temperature.
• Silicone sealants — Good temperature resistance but not fuel-resistant.
• Polyurethane sealants — Good mechanical properties but may not meet fuel tank specifications.

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2.3 Corrosion

Corrosion is the electrochemical degradation of metals. It is a primary concern in aircraft maintenance because it can compromise structural integrity.

Types of Corrosion

1. Surface (Uniform) Corrosion

• Appears as a white/grey powdery deposit on aluminium alloys
• Often the first stage of corrosion, caused by breakdown of the protective oxide layer
• Can be removed by blending if within allowable limits

2. Pitting Corrosion

• Localised attack producing pits or holes in the surface
• Appears as white/grey powdery deposit with pitting
• Creates stress concentrations that can initiate fatigue cracks
• May be blendable if within manufacturer's allowable limits

3. Galvanic (Bimetallic) Corrosion

• Occurs when dissimilar metals are in electrical contact in the presence of an electrolyte
• The more anodic metal corrodes preferentially
• Prevented by insulation (e.g., cadmium plating, primer, or plastic washers)

4. Intergranular Corrosion

• Attack along grain boundaries
• Often caused by improper heat treatment or sensitisation
• Difficult to detect visually — may appear as surface blistering or exfoliation
• Requires immediate and aggressive treatment

5. Exfoliation Corrosion

• A form of intergranular corrosion where corrosion products force layers apart
• Appears as flaking or lifting of the surface
• Common in extruded aluminium sections

6. Stress Corrosion Cracking

• Combined effect of tensile stress and corrosive environment
• Produces cracks that can propagate rapidly
• Particularly dangerous in high-strength aluminium and steel alloys

7. Filiform Corrosion

• Appears as worm-like threads under paint films
• Common on aluminium and steel surfaces with inadequate surface preparation

Corrosion Detection and Assessment

Visual inspection — Look for:

• Surface discolouration
• White/grey powder on aluminium
• Red/brown rust on steel
• Blistering or lifting paint
• Pitting or etching of the surface

Non-destructive testing — Used to determine the extent of corrosion:

• Eddy current for subsurface corrosion
• Ultrasonic for thickness measurement
• Radiography for internal corrosion

Corrosion removal procedure (aluminium alloys):

1. Assess the corrosion depth and extent using NDT where required
2. Consult the SRM/AMM for allowable limits
3. Remove corrosion using approved methods (hand blending with abrasive paper or approved power tools)
4. Re-inspect to ensure all corrosion is removed
5. Re-protect the surface (chemical conversion coating and primer)
6. Apply topcoat

Important principle: Corrosion must be completely removed before re-protection. Painting over corrosion without removal will not stop the corrosion process.

Corrosion Protection Systems

Primers:

• Zinc chromate primer — The traditional corrosion-inhibiting primer for aluminium and steel. Provides corrosion inhibition and a key for topcoat adhesion.
• Epoxy primers — Modern alternative with superior adhesion and corrosion resistance.

Anodising:

• Electrochemical process that produces a controlled oxide layer on aluminium
• Enhances corrosion resistance
• Provides an excellent key for paint adhesion
• Does not significantly affect hardness or weight

Plating:

• Cadmium plating — Sacrificial protection for steel components
• Nickel-cadmium plating — Used on steel and aluminium for corrosion protection, particularly on helicopter rotor blade leading edges
• Chromium plating — Hard, wear-resistant coating for hydraulic cylinders

Cladding (Alclad):

• Pure aluminium layer metallurgically bonded to the surface of aluminium alloy sheet
• Provides sacrificial protection — the pure aluminium corrodes preferentially

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2.4 Fasteners

Rivets

Solid rivets are the primary permanent fastener for aircraft structures.

Rivet identification — AN (Army-Navy) system:

The AN470AD4-6 designation breaks down as:

• AN — Army-Navy standard
• 470 — Universal head type (MS20470 is the equivalent MS standard)
• AD — Material designation
• 4 — Diameter in 1/32 inch (4/32 = 1/8 inch = 3.2 mm)
• 6 — Length in 1/16 inch (6/16 = 3/8 inch = 9.5 mm)

Material designations:

Head types:

• Universal (AN470) — The most common, combines countersunk and round head features
• Countersunk (AN426) — For flush surfaces
• Round head (AN430) — For interior applications

Rivet installation:

• Hole diameter should be 1/32 inch (0.8 mm) larger than the rivet diameter for standard applications
• Rivet length should be sufficient to form a shop head of 1.5 times the rivet diameter
• Rivets must be driven with the correct bucking bar to form a properly shaped shop head

Bolts and Screws

Bolt vs. screw: In aviation terminology, a bolt is installed with a nut and is used for structural connections. A screw is threaded into a tapped hole or captive nut.

Bolt identification — AN system:

The AN3-5A designation breaks down as:

• AN — Army-Navy standard
• 3 — Diameter in 1/16 inch (3/16 inch = 4.8 mm)
• 5 — Length in 1/8 inch (5/8 inch = 15.9 mm)
• A — Material code (A = 2024-T4 aluminium, C = corrosion-resistant steel, no letter = cadmium-plated alloy steel)

Bolt head markings:

• A — 2024-T4 aluminium
• B — 2024-T4 aluminium (larger sizes)
• C — Corrosion-resistant steel
• DD — 2024-T4 aluminium (high strength)
• No marking — Cadmium-plated alloy steel (8740)

Nut types:

• Castellated nut — Slots for cotter pin, used with drilled bolts
• Self-locking nut — Nylon insert or all-metal locking feature
• Plain nut — Requires separate locking device
• Shear nut — Lower height, for shear applications

Self-locking nuts:

• Rely on prevailing torque from the locking element
• Nylon insert type (elastic stop nut) — Limited temperature range
• All-metal type — For high-temperature applications
• Reuse criteria: Generally not reused on dynamic components. The AMM specifies a minimum prevailing torque for reuse. If the nut can be turned by hand on the bolt, it must be replaced.

Torque application:

• Torque values are specified in the AMM
• For self-locking nuts, the specified torque is the running torque — the final applied torque must be the sum of the specified torque plus the prevailing torque
• Torque wrench accuracy must be considered (e.g., ±4% accuracy on 25 Nm gives a range of 24–26 Nm)
• Never exceed the maximum torque to achieve cotter pin alignment — replace the nut or change washers if permitted

Safetying Methods

Cotter pins:

• Used with castellated nuts
• The pin must be inserted through the nut slots and bolt hole
• Ends must be bent back against the nut, not over the bolt threads

Safety wire:

• Double-twist method — Used for turnbuckles and most applications
• Single-wire method — For small screws in a closed pattern
• Wire must be installed so that it tightens when the fastener attempts to loosen
• Maximum length between attachments: 75 mm (3 inches) for standard applications

Lock washers:

• Split washers, shakeproof washers
• Limited use in aircraft — generally not approved for primary structure

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2.5 Control Cables

Control cables are critical flight control components that require careful inspection.

Cable construction:

• 7×7 — Seven strands of seven wires (flexible)
• 7×19 — Seven strands of nineteen wires (more flexible)
• 1×19 — Single strand of nineteen wires (rigid, for straight runs)

Inspection criteria for control cables:

Cable wear limits:

• For cables up to 3.2 mm (1/8 inch) diameter: maximum 4 broken wires in one lay
• For larger cables: limits may be higher (5 or 6 broken wires)
• However, for flight control cables, any broken wire is cause for replacement — this is the conservative and safe approach

Fairleads and pulleys:

• Fairleads guide cables through structures
• Wear limits are specified in the AMM
• A worn fairlead must be replaced if the groove depth exceeds the allowable limit

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2.6 Non-Destructive Testing (NDT)

NDT methods are used to detect defects without damaging the component.

Dye penetrant inspection:

• Used for surface cracks in non-ferrous and non-porous materials
• Sequence: Clean surface → Apply penetrant → Allow dwell time → Remove excess penetrant → Apply developer → Inspect
• Cannot detect subsurface defects

Magnetic particle inspection (MPI):

• Used for surface and near-surface cracks in ferromagnetic materials (iron, steel, nickel)
• Not suitable for aluminium, titanium, or stainless steel (non-magnetic)
• Most sensitive method for ferromagnetic steel components

Eddy current:

• Used for surface and subsurface crack detection in conductive materials
• Can detect corrosion and measure coating thickness
• Requires skilled operators

Ultrasonic:

• Used for thickness measurement and subsurface defect detection
• Can detect corrosion, disbonds, and delaminations

Radiography (X-ray):

• Used for internal inspection
• Can detect internal corrosion and cracks

Tap test:

• Simple method for bonded structures
• A sharp, ringing sound indicates good bond
• A dull, low-frequency sound indicates disbond or delamination

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2.7 Lightning Strike Inspection

Lightning strikes can cause significant damage to aircraft structures.

Most vulnerable areas:

• Extremities (blade tips, wing tips, tail)
• Leading edges
• Antennas and protruding components

For rotor blades:

• The blade tip is a point of high electric field concentration
• The leading edge is directly exposed to lightning attachment
• Both areas must be inspected after a reported strike

Inspection procedure:

• Visual inspection for burn marks, pitting, or holes
• Check for delamination or disbonding in composite structures
• Tap test the affected area
• Refer to the AMM for specific inspection requirements

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3. Important Formulas and Regulations

3.1 Torque Calculations

Torque wrench accuracy:

Acceptable range = Specified torque ± (Specified torque × Accuracy %)

Example: 25 Nm with ±4% accuracy:

Tolerance = 25 × 0.04 = 1 Nm
Acceptable range = 24 to 26 Nm

Self-locking nut torque:

Final torque = Specified torque + Prevailing torque

3.2 Rivet Sizing

Rivet diameter: Specified in 1/32 inch increments

Diameter (mm) = Designation number × 0.794 mm

Example: AN470AD4-6

Diameter = 4 × 1/32 inch = 1/8 inch = 3.2 mm
Length = 6 × 1/16 inch = 3/8 inch = 9.5 mm

3.3 Regulatory References

• Regulation (EU) No 1321/2014, Annex III (Part-66) — Defines the licensing requirements and syllabus
• Appendix I to Part-66 — Contains the basic knowledge syllabus for each module
• AMC (Acceptable Means of Compliance) — Provides guidance on how to comply with the regulation
• GM (Guidance Material) — Additional explanatory material
• AC 43.13-1B — Acceptable Methods, Techniques, and Practices (FAA document, widely referenced)
• AMM (Aircraft Maintenance Manual) — Manufacturer's approved data for specific aircraft
• SRM (Structural Repair Manual) — Manufacturer's approved repair data

3.4 Knowledge Levels

The Part-66 syllabus defines three knowledge levels:

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4. Common Relationships Between Concepts

4.1 Material Selection and Application

• Aluminium alloys are used where weight is critical and strength requirements are moderate
• Steel alloys are used where high strength, hardness, or wear resistance is required
• Titanium bridges the gap — high strength with moderate weight
• Composites offer excellent fatigue resistance and weight savings but require careful inspection for disbonds

4.2 Corrosion and Protection

• Corrosion requires: metal + electrolyte + oxygen (and often a galvanic couple)
• Protection methods: barrier protection (paint), sacrificial protection (cadmium, alclad), and inhibition (primers)
• The type of corrosion determines the treatment:
• Surface corrosion → blend and re-protect
• Pitting → blend if within limits
• Intergranular → aggressive treatment, often replacement
• Galvanic → remove dissimilar metal contact

4.3 Fastener Selection and Application

• Material compatibility: avoid galvanic corrosion between fastener and structure
• Strength requirements: shear vs. tension applications
• Temperature requirements: nylon insert nuts are limited to approximately 120°C
• Accessibility: determines whether a bolt or screw is used

4.4 Inspection and Defect Assessment

• Visual inspection identifies potential defects
• NDT confirms and quantifies defects
• Manufacturer's data (AMM/SRM) defines allowable limits
• If within limits → monitor at specified intervals
• If beyond limits → repair or replace per approved data

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5. Typical Exam Focus Points

5.1 Frequently Tested Areas

1. Rivet identification — Understanding the AN/MS designation system, particularly material codes (AD = 2117-T4)
2. Control cable inspection criteria — Any broken wire in a control cable strand requires replacement; corrosion pitting is a reject condition
3. Corrosion identification — White/grey powder on aluminium = surface or pitting corrosion; red/brown rust = steel corrosion
4. Tap test interpretation — Dull sound = disbond; sharp sound = good bond
5. Torque application — Self-locking nut torque includes prevailing torque; torque wrench accuracy calculations
6. Self-locking nut reuse — Generally not reused on dynamic components; minimum prevailing torque per AMM
7. Acrylic window care — Only mild soap and water; never solvents
8. Sealant selection — Polysulphide for fuel tanks
9. NDT method selection — MPI for ferromagnetic steel; dye penetrant for non-ferrous
10. Approved data usage — Always consult AMM/SRM before repair decisions

5.2 Common Exam Traps

• Confusing rivet material codes — Remember: AD = 2117-T4 (most common), D = 2017-T3, DD = 2024-T4, B = 5056
• Over-tightening to align cotter pin holes — Never exceed maximum torque; replace the nut
• Painting over corrosion — Corrosion must be removed before re-protection
• Using solvents on acrylic — Causes crazing and cracking
• Ignoring prevailing torque — Self-locking nuts require total torque = specified + prevailing
• Assuming all NDT methods work on all materials — MPI only works on ferromagnetic materials
• Reusing self-locking nuts — Default is replacement, especially on dynamic components

5.3 Key Principles to Remember

1. Safety first — Flight control components are safety-critical; when in doubt, replace
2. Approved data — Always work to the AMM/SRM, not general knowledge alone
3. Documentation — Record all findings and actions in the appropriate maintenance records
4. Conservative approach — If a defect is close to the limit, consider the operating environment and criticality
5. Complete corrosion removal — Never paint over corrosion without removing it

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Summary

Module 6 provides the essential knowledge base for aircraft maintenance certifying staff. Understanding material properties, corrosion mechanisms, fastener systems, and inspection techniques is fundamental to safe maintenance practice. The key to success in the examination is not just memorising facts, but understanding the relationships between materials, their applications, and the maintenance practices that keep aircraft airworthy.

Remember: the AMM and SRM are the ultimate authority for any maintenance task. This module provides the underlying knowledge that allows you to interpret and apply that approved data correctly.