Module 6: Materials and Hardware

1. Module Overview

Module 6 of the EASA Part-66 syllabus provides the certifying staff with a comprehensive understanding of the materials and hardware used in aircraft construction and maintenance. This module is fundamental, as it bridges the gap between theoretical materials science and the practical, hands-on decisions made during maintenance, inspection, and repair.

The module covers the properties, identification, and behaviour of ferrous and non-ferrous metals, composite and non-metallic materials, and the vast array of hardware—from fasteners and bearings to pipes, hoses, and sealants. Crucially, it also addresses the degradation mechanisms that threaten airworthiness, most notably corrosion, and the non-destructive testing (NDT) methods used to detect them. A key theme is the absolute necessity of following approved data (Aircraft Maintenance Manuals, Structural Repair Manuals, Illustrated Parts Catalogues) and the strict control of parts and materials.

2. Key Concepts Explained in Detail

2.1 Aircraft Materials: Ferrous

Ferrous materials are iron-based and are used extensively in aircraft for their high strength and hardness. Their properties can be significantly altered by heat treatment and alloying.

Types of Steel Used in Aviation:
Low-Alloy Steels (e.g., 4130, 4340): These are the most common structural steels. They are heat-treatable to achieve high strength and are used for engine mounts, landing gear components, and critical structural fittings. They are susceptible to corrosion and require protective plating (e.g., cadmium) or painting.
Corrosion-Resistant Steels (Stainless Steels): Alloyed with chromium (and often nickel) to provide corrosion resistance. They are used for exhaust systems, springs, and fasteners. They are generally non-magnetic (austenitic grades) or less magnetic than plain carbon steels.
High-Strength Steels: Used in highly stressed components like drive shafts and landing gear axles. They are extremely susceptible to embrittlement and stress corrosion cracking.
Heat Treatment and Overheating Effects:
Heat treatment processes like hardening, tempering, and annealing are used to tailor the mechanical properties of steel.
Critical Concept: Exposure of high-strength steels to temperatures above their maximum allowable operating temperature (e.g., from a fire or excessive friction) can cause severe metallurgical changes. This often results in over-tempering, leading to a loss of hardness and strength, or in some cases, re-hardening which causes a loss of ductility and increased brittleness. A component that has been overheated must be removed from service, as its structural integrity is compromised.
Identification of Steel Bolts (AN Standard):
AN (Air Force-Navy) steel bolts are identified by markings on the bolt head.
A raised dash or asterisk on the head indicates the bolt is made of corrosion-resistant steel.
The absence of a marking generally indicates a cadmium-plated alloy steel bolt.
Colour coding is not a standard method for identifying AN bolts.

2.2 Aircraft Materials: Non-Ferrous

Non-ferrous materials are valued for their light weight, corrosion resistance, and specific electrical or thermal properties.

Aluminium and its Alloys:
The primary airframe material due to its excellent strength-to-weight ratio.
Anodising: An electrochemical process that creates a thick, hard, and corrosion-resistant oxide layer on the surface. This layer is not a coating but a conversion of the aluminium surface itself. Its primary purpose is corrosion protection. If scratched down to bare metal, the protective layer is lost locally, and corrosion can initiate.
Chemical Conversion Coating (Alodine): A chemical process that produces a thin, stable oxide/phosphate layer. Its main purposes are to enhance paint adhesion and provide corrosion protection for aluminium surfaces before painting. It does not significantly affect conductivity or hardness.
Corrosion: Aluminium alloys are susceptible to various forms of corrosion, including pitting, exfoliation, and fretting.
Titanium and its Alloys:
Used in high-strength, high-temperature, and highly corrosive environments (e.g., rotor blade spars, firewalls, fasteners).
Key Properties: Non-magnetic, lightweight, and highly corrosion-resistant.
Identification: Typically marked with 'Ti' or 'Titanium'. They are not identified by colour codes and are not magnetic, which helps differentiate them from some steels. They are approximately 40% lighter than steel but heavier than aluminium.
Magnesium and its Alloys:
The lightest structural metal. Used for gearbox housings and some non-structural components.
Critical Hazard: Highly susceptible to corrosion and is a flammable metal. When a fastener is seized in a magnesium housing, the use of a torch is strictly prohibited due to the risk of ignition. Removal requires careful methods like penetrating oil and a tap extractor.

2.3 Aircraft Materials: Composite and Non-Metallic

Composite Materials:
Consist of a reinforcement (e.g., carbon, glass, or aramid fibres) embedded in a matrix (e.g., epoxy resin).
Advantages: High strength-to-weight ratio, corrosion resistance, and fatigue resistance.
Defects: The primary defects are delamination (separation of layers), disbond (separation of skin from core), and impact damage (which can be hidden internally).
Damage Assessment: The first step in assessing any damage is to measure and document the damage against the manufacturer's allowable limits. This often involves NDT methods like tap testing or ultrasonic inspection.
Water Ingress: A crack or disbond can allow water to enter the honeycomb core, leading to weight gain, internal corrosion, and freeze-thaw damage. A blade with suspected water ingress and a crack must be replaced, as the full extent of internal damage cannot be assessed.
Transparent Plastics (Acrylic/Plexiglass):
Used for windshields and windows due to their optical clarity, light weight, and ease of forming.
Key Property: Excellent light transmission and formability.
Critical Defect: Cracks are not repairable. Drilling stop holes is not an approved repair for acrylic. Any crack, regardless of size, requires replacement of the window.
Drilling: When drilling acrylic, use a sharp drill bit, low speed, and light pressure to prevent heat build-up, cracking, and chipping.
Rubber and Elastomers:
Used for seals, gaskets, hoses, and flexible couplings.
Degradation: Swelling and blistering of a hose indicates chemical attack from an incompatible fluid. Cracking and hardening indicate age or ozone attack.
Flexible Couplings: Any crack in a rubber flexible coupling element is cause for rejection and replacement. There are no acceptable crack limits for these critical components.

2.4 Corrosion

Corrosion is the electrochemical degradation of a metal. It is a primary threat to airworthiness and a major focus of maintenance inspections.

Types of Corrosion:
Galvanic Corrosion: Occurs when two dissimilar metals are in contact in the presence of an electrolyte. The more active metal corrodes.
Pitting Corrosion: Localised attack that forms small pits or holes. Pit depth is a critical measurement for determining airworthiness.
Fretting Corrosion: Occurs at the interface of two tightly fitting surfaces subjected to repeated small relative motion (vibration). This breaks down the protective oxide layer, leading to accelerated wear and oxidation. It is commonly found at riveted joints and bearing interfaces.
Exfoliation Corrosion: A severe form of intergranular corrosion that occurs along the grain boundaries of aluminium alloys. The corrosion products have a larger volume than the original metal, causing the layers to "bulge" or exfoliate. This indicates deep, hidden structural damage.
Corrosion Removal and Assessment:
First Step: The first action is always to assess the damage against the manufacturer's allowable limits (AMM/SRM).
Removal: Corrosion products must be removed mechanically (e.g., with abrasive paper) or chemically, following the AMM. Power wire brushes are often prohibited on structural skins as they are too aggressive and can remove sound material.
Assessment After Removal: After corrosion removal, the remaining material thickness must be measured and compared to the allowable limits. If the remaining thickness is below the minimum, the component must be replaced.
Example Calculation: If a strut wall is 2.0 mm thick and the manual allows a maximum pit depth of 10% of wall thickness, the allowable pit depth is 0.2 mm. A pit of 0.3 mm exceeds this limit, requiring component replacement.
Corrosion Protection:
Cadmium Plating: A sacrificial coating applied to steel fasteners and components. White corrosion products on a cadmium-plated bolt indicate the plating has been consumed, and the bolt must be replaced.
Corrosion-Inhibiting Compounds (CICs): Applied as a thin film by spraying or brushing to internal structures to prevent moisture ingress. Thick coats can trap moisture and are not acceptable.
Wet Installation: Coating the shank of a bolt with a sealant or CIC before installation to prevent moisture ingress and fretting corrosion.

2.5 Fasteners

Rivets:
Used for permanent structural fastening. The diameter is typically 3 times the thickness of the thickest sheet, and the length must account for the total material thickness plus the amount needed to form the shop head. Selection is always based on approved engineering data (AMM, SRM, or standard charts).
Bolts and Nuts:
AN Bolts: Identified by head markings. A raised dash or asterisk indicates corrosion-resistant steel.
Self-Locking Nuts: Use a nylon insert or an all-metal deformed thread to create friction and resist loosening.
Reuse: Reuse is only permitted if the prevailing torque meets the AMM limits. If the nut turns without reaching the specified torque, the locking feature is worn, and the nut must be replaced.
Installation: Tightened to a specified torque value from the maintenance manual. The torque value accounts for the friction of the locking feature.
Castellated Nuts: Used with a cotter pin for positive locking. If the torque is below the minimum, the nut must be loosened and re-torqued. The cotter pin must be replaced after any torque adjustment.
Hi-Lok Fasteners: A pin and collar system that provides consistent clamp-up and is installed from one side, eliminating the need for a bucking bar.
Torque Application:
Crowfoot Extension at 90°: When an extension is used at 90 degrees to the wrench handle, the effective lever arm length is not extended, so the indicated torque equals the applied torque.
Crowfoot Extension In-Line: The reading must be recalculated using a formula that accounts for the increased lever arm length.
Over-Torquing: Can cause thread distortion, which reduces the bolt's load-carrying capacity. Such bolts must be replaced.

2.6 Pipes, Hoses, and Sealants

Flexible Hoses:
Part Numbers: Hoses are manufactured to specific part numbers (e.g., MS28741-4-12) that define size, material, fitting configuration, and pressure rating. A different dash number may be incompatible. Only the exact part number from the IPC or maintenance manual is acceptable.
Inspection: Look for kinks, chafing, swelling, blistering, and twisting. A twisted hose induces torsional stress on the reinforcement braid and must be replaced.
Swaged Ferrules: A cracked swaged ferrule indicates a loss of structural integrity. The hose assembly cannot be repaired and must be replaced. Re-swaging is not permitted as it must be performed in a controlled environment by the manufacturer.
Sealants:
Fuel Tank Sealants: The primary function is to prevent fuel leakage. They must be resistant to fuel, additives, and environmental degradation. They are not structural materials.

2.7 Non-Destructive Testing (NDT)

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

Dye Penetrant Inspection: Detects surface-breaking discontinuities (cracks, porosity). A continuous line of indications indicates a surface crack.
Magnetic Particle Inspection (MPI): Detects surface and near-surface cracks in ferromagnetic materials (steel). Any linear indication is cause for rejection.
Ultrasonic Testing: Uses high-frequency sound waves to detect internal defects such as delamination in composite or bonded structures.
Eddy Current: Detects surface and near-surface defects in conductive materials.
Tap Testing: A simple acoustic method used to detect disbonds and delaminations in composite structures by listening for a dull sound.

3. Important Formulas, Regulations, and Procedures

Regulatory Framework:
Regulation (EU) No 1321/2014, Annex III (Part-66): Defines the requirements for the certification of maintenance staff.
Part-145: Defines the requirements for maintenance organisations, including the use of approved data.
AMC/GM (Acceptable Means of Compliance/Guidance Material): Provides guidance on how to comply with the regulations.
Key Procedures:
Corrosion Removal: Must follow AMM/SRM procedures. The first step is assessment against allowable limits.
Torque Check: If a nut turns without reaching torque, the locking feature is worn. If torque is below minimum, loosen and re-torque.
Weld Repair: For stainless steel and Inconel, strict surface preparation and preheat are essential to prevent cracking. Epoxy repairs are not acceptable for exhaust components.
Damage Assessment: All damage must be measured and compared to the allowable limits in the AMM/SRM. If within limits, the component is airworthy. If beyond limits, it must be repaired per approved data or replaced.
Formulas:
Allowable Pit Depth: (e.g., 10% of wall thickness). Calculation: Allowable Depth = Wall Thickness × 0.10.
Torque with In-Line Extension: Indicated Torque = (Applied Torque × L) / (L + E) where L is the wrench length and E is the extension length.

4. Common Relationships Between Concepts

Material Properties ↔ Application: The high strength of steel is used for landing gear, the light weight of aluminium for the airframe, the corrosion resistance of titanium for spars, and the optical clarity of acrylic for windows.
Corrosion ↔ Material Degradation: Corrosion is the primary degradation mechanism for metals. The type of corrosion (e.g., fretting, galvanic, pitting) is directly related to the service environment and the materials in contact.
Damage ↔ Airworthiness: The relationship between a defect (dent, crack, corrosion) and its size relative to the allowable limits determines whether a component is airworthy or must be repaired/replaced.
Fastener Condition ↔ Structural Integrity: The condition of a fastener (e.g., worn locking feature, thread distortion, corrosion) directly impacts its ability to carry load and maintain joint integrity.
NDT ↔ Defect Type: The choice of NDT method is directly related to the type of defect and material. Ultrasonic for internal delamination, dye penetrant for surface cracks, and MPI for ferromagnetic materials.

5. Typical Exam Focus Points

Corrosion: Types of corrosion (especially fretting and exfoliation), causes, and the correct sequence of actions for assessment and removal. Calculating allowable pit depth.
Fasteners: Identification of AN bolts (head markings), the reuse criteria for self-locking nuts, torque application (especially the 90° crowfoot rule), and the correct action for a nut that turns at low torque.
Materials Identification: Properties of titanium (non-magnetic, light) and the effects of overheating on high-strength steel.
Damage Limits: The critical decision of whether a defect (dent, crack, disbond) is within or beyond allowable limits and the subsequent action (return to service, repair, or replace).
Non-Repairable Items: Acrylic (Plexiglass) cracks, cracked swaged ferrules, and cracked rubber flexible couplings are always cause for replacement.
Approved Data: The absolute requirement to use the AMM, SRM, IPC, and manufacturer's instructions for all maintenance actions, including parts selection, torque values, and repair schemes.
Hose and Pipe Inspection: The significance of part numbers, the unacceptability of twisting, and the causes of swelling/blistering.