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.

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):

Series	Principal Alloying Element	Typical Application
1xxx	Pure aluminium (99%+)	Electrical conductors, non-structural
2xxx	Copper	High-strength structural (2024-T3 skins)
3xxx	Manganese	Moderate strength, good formability
5xxx	Magnesium	Corrosion-resistant, marine applications
6xxx	Magnesium + Silicon	Medium strength, good corrosion resistance (6061)
7xxx	Zinc	Very high strength (7075-T6 fittings)

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.

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.

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):

121.Assess the corrosion depth and extent using NDT where required

122.Consult the SRM/AMM for allowable limits

123.Remove corrosion using approved methods (hand blending with abrasive paper or approved power tools)

124.Re-inspect to ensure all corrosion is removed

125.Re-protect the surface (chemical conversion coating and primer)

126.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

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:

Code	Material	Characteristics
A	1100-F (pure aluminium)	Soft, non-structural
AD	2117-T4	Most common, work-hardenable
D	2017-T3	Requires heat treatment before driving
DD	2024-T4	High strength, requires refrigeration
B	5056-H32	Corrosion-resistant, for magnesium

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

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:

Defect	Action
Any broken wire in a strand	Replace — control cables are rejected at the first sign of broken wires
Corrosion pitting on wires	Replace — pitting creates stress concentrations
Kinks or bird-caging	Replace if beyond limits
Surface rust removable with abrasive	May be acceptable if within limits
Wear in fairleads	Replace fairlead if worn beyond allowable limit

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

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

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

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:

Level	Description	Example
1	Overview — familiarisation with the subject	Recognise different material types
2	General knowledge — understanding of principles	Explain corrosion types and causes
3	Detailed theory — comprehensive understanding	Perform corrosion assessment and treatment per approved data

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

5. Typical Exam Focus Points

5.1 Frequently Tested Areas

321.Rivet identification — Understanding the AN/MS designation system, particularly material codes (AD = 2117-T4)

322.Control cable inspection criteria — Any broken wire in a control cable strand requires replacement; corrosion pitting is a reject condition

323.Corrosion identification — White/grey powder on aluminium = surface or pitting corrosion; red/brown rust = steel corrosion

324.Tap test interpretation — Dull sound = disbond; sharp sound = good bond

325.Torque application — Self-locking nut torque includes prevailing torque; torque wrench accuracy calculations

326.Self-locking nut reuse — Generally not reused on dynamic components; minimum prevailing torque per AMM

327.Acrylic window care — Only mild soap and water; never solvents

328.Sealant selection — Polysulphide for fuel tanks

329.NDT method selection — MPI for ferromagnetic steel; dye penetrant for non-ferrous

330.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

340.Safety first — Flight control components are safety-critical; when in doubt, replace

341.Approved data — Always work to the AMM/SRM, not general knowledge alone

342.Documentation — Record all findings and actions in the appropriate maintenance records

343.Conservative approach — If a defect is close to the limit, consider the operating environment and criticality

344.Complete corrosion removal — Never paint over corrosion without removing it

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.

Module 6: Materials and Hardware