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.
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:
Alloy Series
Principal Alloying Element
Typical Application
1xxx
Pure aluminium (99%+)
Electrical conductors, cladding
2xxx
Copper
Wing skins, structural members (2024-T3)
5xxx
Magnesium
Fuel tanks, marine applications
6xxx
Magnesium + Silicon
Fuselage frames, fittings (6061-T6)
7xxx
Zinc
High-strength structural components (7075-T6)
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):
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)
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:
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.
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.
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:
Specification
Insulation
Temperature Rating
Application
M22759/16
Tefzel (ETFE)
150°C
General-purpose, most common
M22759/11
Teflon (PTFE)
260°C
High-temperature areas
M22759/32
Tefzel, lightweight
150°C
Weight-critical installations
Kapton
Polyimide
200°C
Special applications (known failure modes)
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.
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.
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:
Application
Maximum Resistance
Primary bonding (antenna bases)
2.5 milliohms
Secondary bonding (general)
10 milliohms (0.01 ohm)
General equipment bonding
100 milliohms
For antenna installations, the typical maximum allowable resistance is 10 milliohms. A reading of 5 milliohms is well within limits.
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)
2.8 Batteries and Hazardous Materials
Nickel-Cadmium (Ni-Cd) Batteries
Ni-Cd battery electrolyte is alkaline (potassium hydroxide). In case of spillage:
227.Neutralise the electrolyte with a weak acid (boric acid or vinegar)
228.Clean the area with water
229.Inspect for damage to surrounding structure
Dry wiping is insufficient; the alkaline residue will continue to corrode the structure.
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:
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
Summary
Module 6 provides the foundational knowledge of materials and hardware essential for B2 avionics maintenance. The key themes are:
346.Material properties determine their application and maintenance requirements
347.Corrosion is the primary degradation mechanism, requiring systematic prevention and treatment
348.Fasteners and locking devices must be correctly selected, installed, and torqued
349.Electrical wiring and connectors require careful handling, proper termination, and shielding for signal integrity
350.Bonding and earthing are critical for safety and avionics performance
351.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.
