Module 6: Materials and Hardware

Module 6: Materials and Hardware — EASA Part-66 B2 Study Material

1. Module Overview

Module 6 of the EASA Part-66 syllabus (Appendix I) covers the fundamental knowledge of aircraft materials, hardware, and their applications in maintenance. For the B2 avionics category, this module emphasises the relationship between materials science and electrical/electronic systems, including corrosion protection, electrical wiring, connectors, bonding, and shielding. The module is structured into several sub-topics:

• 6.1 Aircraft Materials — Ferrous and non-ferrous metals, composites, and their properties
• 6.2 Corrosion — Types, prevention, and treatment
• 6.3 Fasteners — Bolts, nuts, rivets, and locking devices
• 6.4 Pipes and Hoses — Types and applications
• 6.5 Springs, Bearings, and Transmissions — Basic principles
• 6.6 Electrical Cables and Connectors — Wiring, shielding, and termination
• 6.7 Aircraft Hardware — Clamps, ties, and mounting devices
• 6.8 Non-Metallic Materials — Plastics, sealants, and adhesives
• 6.9 Fibre Optics — Principles and aircraft applications

The knowledge levels range from Level 1 (overview) to Level 3 (detailed theory with practical application). For B2 certifying staff, particular emphasis is placed on electrical hardware, bonding, and corrosion prevention as they relate to avionics installations.

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2. Key Concepts Explained in Detail

2.1 Aircraft Metallic Materials

Aluminium Alloys

Aluminium is the primary structural material in aircraft construction due to its favourable strength-to-weight ratio. The four-digit designation system identifies the principal alloying elements:

The temper designation (e.g., -T3, -T6) indicates the heat treatment and mechanical working process. For example, 2024-T3 has been solution heat-treated, cold-worked, and naturally aged to achieve its characteristic strength.

Key properties for maintenance:

• 2024-T3: High fatigue resistance, excellent for wing skins
• 7075-T6: Highest strength, but more susceptible to stress corrosion cracking
• 6061-T6: Good corrosion resistance, easily welded, used for fittings

Titanium Alloys

Titanium offers an exceptional combination of properties for aerospace applications:

• High strength-to-weight ratio (comparable to steel at approximately 60% of the weight)
• Outstanding corrosion resistance, particularly in saltwater environments
• Excellent high-temperature performance (up to approximately 600°C)
• Good fatigue resistance

Common applications include engine components, landing gear parts, and fasteners in corrosion-prone areas. Titanium's compatibility with carbon fibre composites (no galvanic corrosion) makes it valuable in modern aircraft structures.

Steel Alloys

Aircraft steels are classified by their carbon content and alloying elements:

• Low-carbon steels: Used for general hardware, cable terminals
• Medium-carbon steels: Springs, shafts, and gears
• High-carbon steels: Cutting tools, bearings
• Alloy steels: Chromium-molybdenum (chrome-moly) for structural tubing, nickel-chromium for high-temperature applications

Corrosion protection of steel is critical. Common protective coatings include:

• Cadmium plating (sacrificial protection)
• Zinc plating
• Red oxide primer (iron oxide-based, characteristic red-brown colour)
• Paint systems

Corrosion of Metals

Galvanic Corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially. The galvanic series ranks metals from anodic (active) to cathodic (noble):

Anodic (corrodes) → Magnesium, Zinc, Cadmium, Aluminium, Steel, Tin, Lead, Copper, Silver, Gold ← Cathodic (protected)

Sacrificial anodes such as cadmium are deliberately used to protect more noble metals. Cadmium plating on steel fasteners corrodes preferentially, protecting the underlying steel.

Pitting Corrosion is localised attack that produces small pits on aluminium surfaces, appearing as white or grey powdery deposits. It commonly occurs where protective coatings are breached, allowing moisture and contaminants to reach the bare metal.

Intergranular Corrosion attacks the grain boundaries of aluminium alloys, particularly 7075-T6. It can spread deep into the material while the surface appears only lightly etched or blistered. This is particularly dangerous because it can lead to sudden failure without visible warning. Detection requires eddy current or ultrasonic inspection.

Corrosion Prevention Methods:

• Isolation of dissimilar metals using non-conductive washers (neoprene, nylon)
• Protective plating (cadmium, nickel, zinc)
• Primer systems (red oxide for steel, zinc chromate for aluminium)
• Sealants to prevent moisture ingress
• Drainage provisions to prevent water accumulation

Corrosion Treatment Procedure:

1. Remove corrosion products mechanically (abrasive pads, glass bead blasting)
2. Assess residual material thickness
3. Determine if repair or replacement is required
4. Apply chemical treatment (conversion coating)
5. Re-apply protective finish

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2.2 Composite and Non-Metallic Materials

Thermosetting Plastics

Thermosetting plastics undergo an irreversible chemical cross-linking reaction during curing. Once cured, reheating does not soften them — they will degrade or char before melting. This is a fundamental distinction from thermoplastics.

Common thermosets in aircraft:

• Epoxy resins (primary matrix for structural composites)
• Phenolic resins (interior panels, due to fire resistance)
• Polyester resins (general-purpose composites)

Composite Structures

Modern aircraft use fibre-reinforced composites extensively:

• Glass Fibre Reinforced Plastic (GFRP): Radomes, fairings
• Carbon Fibre Reinforced Plastic (CFRP): Primary structures, control surfaces
• Aramid (Kevlar): Impact-resistant areas, engine cowls

Sandwich constructions combine composite face sheets with honeycomb cores (Nomex, aluminium) to provide high stiffness with low weight.

Damage Types in Composites:

• Delamination: Separation of layers, often caused by impact or moisture ingress
• Dents: Typically caused by low-energy impacts (dropped tools, hangar equipment)
• Cracks: Matrix cracking from overstress or impact
• Fibre breakage: High-energy impact damage

Inspection of Composite Damage:

Before deciding on a repair scheme, the full extent of damage must be mapped using:

• Visual inspection
• Tap testing (acoustic response)
• Ultrasonic inspection
• Thermography

The damage extent must be compared against the manufacturer's allowable damage limits (CMM/SRM). Repairs must follow approved procedures.

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2.3 Fasteners and Locking Devices

Solid Rivets

Solid rivets are the primary permanent fasteners in aircraft structures. The rivet is installed by forming a shop head on the blind side.

Standard shop head dimensions:

• Diameter: 1.5 × shank diameter
• Height: 0.5 × shank diameter

Rivet materials must match the structure to prevent galvanic corrosion. Aluminium rivets are identified by head markings (e.g., dimple for 2117, raised dot for 2024).

Blind Rivets (Pop Rivets)

Blind rivets are designed for installation where access to the back side is limited. The setting tool pulls a stem through the rivet body, forming the shop head. The stem then breaks off at a predetermined point.

Bolts and Nuts

Aircraft bolts are manufactured from corrosion-resistant steel, alloy steel, or titanium. The bolt grade is indicated by head markings (e.g., X for 125 ksi, XXXX for 160 ksi).

Torque Application:

• Torque values are specified in the AMM or approved documentation
• Use a calibrated torque wrench
• Apply torque smoothly, not jerkily
• Torque values are typically for dry threads unless otherwise specified
• Lubrication reduces friction, requiring lower torque for the same clamping force

Over-torquing can damage threads, crush locking inserts, or overstress the bolt. Under-torquing can lead to loosening under vibration.

Self-Locking Nuts

Self-locking nuts provide vibration resistance without additional locking devices:

Nylon Insert (Elastic Stop Nut):

• A nylon collar deforms around the bolt threads, creating friction
• Limited temperature range (typically up to 120°C)
• Over-tightening can crush or deform the nylon insert, causing loss of locking action

All-Metal Locknuts:

• Use distorted threads or a slotted hexagonal design
• Suitable for high-temperature areas where nylon inserts would fail
• Can withstand temperatures exceeding 250°C

Locking Devices

Cotter Pins:

Inserted through a hole in the bolt and engaged with the slots of a castle nut. The legs are bent to physically prevent rotation.

Spring Washers:

Wave washers and split washers exert continuous spring force to maintain preload and resist loosening under vibration.

Lock Wire (Safety Wire):

Wire threaded through holes in fasteners and twisted to prevent rotation. Must be installed so that loosening of the fastener tightens the wire.

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2.4 Electrical Cables and Wiring

Wire Types and Selection

Wire Sizing:

In the EASA Part-66 environment, wire sizes are specified in square millimetres (mm²) per European standards. The cross-sectional area must provide a current rating above the expected load, considering:

• Voltage drop over the wire length
• Ambient temperature
• Bundling derating factors
• Installation conditions

Common Aircraft Wire Types:

PVC is not used in aircraft due to flammability concerns and low temperature rating.

Minimum Bend Radius:

The standard minimum bend radius for wires is 10 times the wire diameter. For a 2 mm wire, the minimum bend radius is 20 mm. This prevents damage to insulation and conductor.

Cable Shielding

Purpose of Shields:

Cable shields (braided or foil) surround the conductors to:

• Block electromagnetic interference (EMI)
• Provide a return path for signals
• Prevent radiation of signals from the cable

Shield Termination:

Proper shield termination requires a low-impedance path to ground:

• 360-degree clamp to the backshell maintains shield effectiveness
• Twisting into a pigtail creates a high-impedance connection and is not recommended for high-frequency signals
• Shields are grounded (bonded) at specified points, often at both ends for RF applications

Twisted Pair Cables

Twisted pair cables are used for data transmission (ARINC 429, CAN bus) because the twisting cancels electromagnetic interference and reduces crosstalk between conductors. This is critical for reliable signal integrity in avionics systems.

Coaxial Cables

Coaxial cables consist of:

• Inner conductor (centre)
• Dielectric insulation
• Braided shield (outer conductor)
• Outer jacket

The braided shield provides a return path for signals and shields against external EMI.

Fibre Optic Cables

Aircraft fibre optic cables use polyurethane jackets due to:

• Excellent resistance to abrasion
• Resistance to fuels and hydraulic fluids
• Wide temperature range capability

PVC is not used due to weight and flammability concerns. PTFE and nylon have specific uses but are not the primary jacket material for avionics fibre optics.

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2.5 Electrical Hardware and Connectors

Connectors

Aircraft electrical connectors are precision components that must maintain electrical integrity under vibration, temperature extremes, and environmental exposure.

Connector Types:

• Circular bayonet (quick-disconnect)
• Threaded coupling
• Rectangular (rack and panel)
• D-subminiature (avionics)

Connector Pin Identification:

The correct method to identify a terminal is to refer to the aircraft's wiring diagram (AMM or WDM), which specifies the pin number for each wire function. Colour codes are not standard for pin identification.

Connector Maintenance:

• Use the correct extraction tool for removing relays and components from sockets
• Verify secure locking of bayonet connectors by applying slight torque in the lock direction
• Check continuity between backshell and connector shell using a low-resistance ohmmeter

Ferrules

Ferrules are small metal tubes crimped onto the end of stranded wire to:

• Prevent fraying
• Provide a solid connection point for screw terminals
• Ensure reliable, repeatable connections in avionics racks and junction boxes

Cable Ties and Clamps

Nylon cable ties bundle wires and cables, providing mechanical support and preventing movement that could cause chafing or damage.

Clamps secure harnesses to structure, prevent chafing, and dampen vibration. Nylon is the most common clamp body material due to being lightweight, non-conductive, and flame-retardant.

Ferrite Beads

Ferrite beads or cores placed around cables attenuate high-frequency noise and EMI. They act as a low-pass filter, blocking high-frequency signals while allowing DC and low-frequency signals to pass. They are used on signal cables to prevent interference with sensitive avionics.

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2.6 Bonding and Earthing

Purpose of Bonding

Bonding straps connect metal parts to:

• Equalise electrical potential between components
• Prevent static charge accumulation
• Provide a path for lightning currents to flow safely to structure
• Ensure proper operation of avionics systems

Bonding Straps

Bonding jumpers are typically made of tinned copper braid. Tinning provides:

• Protection against oxidation and corrosion
• Improved solderability
• Critical in fuel tank environments where moisture and fuel vapours accelerate corrosion

Bonding Jumper Inspection:

A broken strand reduces the cross-sectional area and can lead to overheating or failure. Any broken strands, corrosion, or damage that reduces effectiveness makes the jumper unserviceable. Slight discolouration is acceptable; natural sag is normal.

Bonding Resistance Limits

Standard aircraft bonding requirements specify maximum resistance values:

For antenna installations, the typical maximum allowable resistance is 10 milliohms. A reading of 5 milliohms is well within limits.

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2.7 Sealing Materials

Sealants in Aircraft

Sealants serve multiple purposes:

• Provide fluid-tight barriers
• Maintain pressure integrity in pressurised bulkheads
• Prevent moisture ingress that can lead to corrosion
• Provide aerodynamic smoothness

Pressure Bulkhead Sealing:

Sealants on pressure bulkheads are critical for maintaining cabin pressurisation and preventing moisture ingress. The sealant must maintain its properties over the aircraft's operational temperature range and resist ageing.

Sealant Types:

• Polysulphide (fuel tank sealants)
• Silicone (high-temperature applications)
• Polyurethane (general purpose)

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2.8 Batteries and Hazardous Materials

Nickel-Cadmium (Ni-Cd) Batteries

Ni-Cd battery electrolyte is alkaline (potassium hydroxide). In case of spillage:

1. Neutralise the electrolyte with a weak acid (boric acid or vinegar)
2. Clean the area with water
3. Inspect for damage to surrounding structure

Dry wiping is insufficient; the alkaline residue will continue to corrode the structure.

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3. Important Formulas and Regulations

3.1 Wire Sizing and Current Rating

The current-carrying capacity of a wire depends on:

• Cross-sectional area (mm²)
• Insulation temperature rating
• Ambient temperature
• Bundling factor (multiple wires in a bundle derate the capacity)

3.2 Torque Calculations

Torque with Extension Tools:

When using a crow's foot extension or adaptor, the effective torque changes:

\[ T_{actual} = T_{indicated} \times \frac{L_{wrench} + L_{extension}}{L_{wrench}} \]

Where:

• \( T_{actual} \) = actual torque applied to fastener
• \( T_{indicated} \) = torque wrench reading
• \( L_{wrench} \) = distance from wrench centre to handle grip
• \( L_{extension} \) = length of extension

3.3 Bonding Resistance Measurement

Bonding resistance is measured using a low-resistance ohmmeter (milliohmmeter) with a four-wire (Kelvin) connection to eliminate lead resistance.

3.4 Regulatory References

Part-66 (Regulation (EU) No 1321/2014, Annex III):

• Appendix I defines the basic knowledge syllabus
• Module 6 covers Materials and Hardware
• Knowledge levels: Level 1 (overview), Level 2 (general), Level 3 (detailed)

Part-145 (Maintenance Organisation Approvals):

• Requires approved data for all maintenance
• Mandates the use of approved parts with traceability

Acceptable Means of Compliance (AMC) and Guidance Material (GM):

• AMC 20-3: Electrical wiring interconnection system (EWIS) guidance
• Provides acceptable methods for demonstrating compliance

AC 43.13-1B (FAA Advisory Circular):

• Accepted methods for aircraft maintenance
• Widely referenced for wiring practices, bonding, and corrosion control

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4. Common Relationships Between Concepts

4.1 Corrosion and Material Selection

The selection of materials and protective finishes is driven by corrosion considerations:

• Dissimilar metal contact requires isolation (non-conductive washers)
• Sacrificial protection uses more anodic metals to protect cathodic metals
• Surface protection (plating, priming, painting) prevents electrolyte contact

4.2 Electrical Bonding and Corrosion

Bonding requirements must be balanced against corrosion prevention:

• Tinned copper braid provides corrosion resistance while maintaining conductivity
• Cadmium-plated steel provides sacrificial protection while maintaining electrical continuity
• Stainless steel washers prevent galvanic corrosion at bonding points

4.3 Fastener Selection and Electrical Continuity

Fasteners serve dual roles in aircraft:

• Mechanical fastening
• Electrical bonding (when required)

The choice of fastener material affects both structural integrity and electrical performance.

4.4 Shielding and Grounding

Cable shielding effectiveness depends on:

• Shield material and coverage
• Termination method (360-degree vs. pigtail)
• Grounding points (single-ended vs. double-ended)
• Continuity through connectors and backshells

4.5 Temperature Ratings and Material Selection

Temperature considerations affect material selection across the aircraft:

• Nylon insert locknuts limited to approximately 120°C
• All-metal locknuts required for high-temperature areas
• Tefzel wire rated to 150°C
• Teflon wire rated to 260°C
• Titanium retains strength at elevated temperatures

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5. Typical Exam Focus Points

5.1 Corrosion (High Priority)

• Galvanic corrosion: Understand the galvanic series and sacrificial protection
• Cadmium plating: Most common sacrificial coating for steel
• Pitting corrosion: White/grey powdery deposits on aluminium
• Intergranular corrosion: Dangerous, spreads below the surface, requires non-destructive testing
• Corrosion on control cables: Always requires replacement, never cleaning
• Corrosion treatment: Removal before re-protection is mandatory

5.2 Fasteners and Locking Devices

• Cotter pins: Function in castle nut assemblies
• Self-locking nuts: Nylon insert vs. all-metal (temperature limitations)
• Over-torquing effects: Crushing nylon inserts, thread damage
• Torque application: Calibrated wrench, smooth application, dry vs. lubricated threads
• Rivet dimensions: Shop head diameter 1.5× shank, height 0.5× shank

5.3 Electrical Wiring

• Wire types: M22759/16 (Tefzel) is most common, 150°C rating
• Wire sizing: Metric (mm²) per European standards
• Bend radius: Minimum 10× wire diameter
• Twisted pairs: EMI cancellation for data transmission
• Shield termination: 360-degree clamp preferred, pigtail not recommended
• Insulation repair: Approved methods only (heat-shrink, self-adhesive tape)

5.4 Bonding and Earthing

• Bonding purpose: Equalise potential, static discharge, lightning protection
• Resistance limits: 10 milliohms for antenna bonding (general limit)
• Bonding jumper defects: Broken strands make unserviceable
• Tinned copper: Preferred for corrosion resistance

5.5 Connectors and Hardware

• Relay extraction: Use proper tool, avoid damage
• Bayonet connectors: Verify positive locking
• Backshell grounding: Verify continuity with ohmmeter
• Ferrules: Prevent fraying of stranded wire
• Cable ties: Mechanical support, nylon material

5.6 Composite Materials

• Thermosetting plastics: Irreversible curing, do not soften when reheated
• Damage types: Dents from low-energy impacts, delamination
• Inspection: Tap testing, ultrasonic, compare to allowable limits

5.7 Maintenance Practices

• Approved parts: Only use parts listed in approved documentation
• Wire repair: Even inactive wires must be repaired
• Battery spillage: Neutralise alkaline electrolyte with weak acid
• Minimum separation: 6 inches between wiring and fluid lines
• Torque verification: Use calibrated equipment, follow AMM

5.8 Level 3 Knowledge Application

For B2 certifying staff, Level 3 knowledge requires:

• Understanding of the underlying principles
• Ability to apply knowledge in practical scenarios
• Independent decision-making in maintenance situations
• Troubleshooting capability based on material and hardware knowledge

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Summary

Module 6 provides the foundational knowledge of materials and hardware essential for B2 avionics maintenance. The key themes are:

1. Material properties determine their application and maintenance requirements
2. Corrosion is the primary degradation mechanism, requiring systematic prevention and treatment
3. Fasteners and locking devices must be correctly selected, installed, and torqued
4. Electrical wiring and connectors require careful handling, proper termination, and shielding for signal integrity
5. Bonding and earthing are critical for safety and avionics performance
6. Regulatory compliance ensures airworthiness through approved data, parts, and procedures

Mastery of these concepts enables certifying staff to make sound maintenance decisions, identify defects, and perform repairs that maintain the airworthiness of the aircraft.