Module 6: Materials and Hardware

Module 6: Materials and Hardware

1. Overview of Module 6

This module provides the certifying maintenance technician with a comprehensive understanding of the materials used in aircraft construction and the hardware used to assemble and maintain them. It covers the properties, identification, and application of ferrous and non-ferrous metals, composite and non-metallic materials, and the full range of aircraft hardware including fasteners, locking devices, bearings, transmissions, and control cables. The module also addresses corrosion—its causes, types, and control—along with the principles of non-destructive testing and the correct handling of sealants, adhesives, and fluids.

The syllabus is structured to build knowledge from basic material properties (Level 1) through to detailed application and inspection criteria (Level 3). For the A-Piston category, the emphasis is on the materials and hardware typical of light piston-engine aeroplanes, though the principles apply universally.

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2. Aircraft Materials – Ferrous and Non-Ferrous Metals

2.1 Aluminium and its Alloys

Aluminium is the most widely used structural material in aircraft construction due to its excellent strength-to-weight ratio when alloyed and heat-treated.

Key Alloying Elements and Their Effects:

• Copper (e.g., 2024): Increases strength but reduces corrosion resistance. Requires protective cladding or anodising.
• Zinc (e.g., 7075): Provides the highest strength of common aluminium alloys. Used in highly stressed structures.
• Magnesium (e.g., 5056): Improves corrosion resistance and weldability.
• Manganese (e.g., 3003): Adds moderate strength without reducing formability.
• Silicon (e.g., 4043): Improves casting and welding characteristics.

Heat Treatment Conditions:

• -O (Annealed): Soft, ductile, and easily formed. Not used for primary structure because it lacks the required mechanical properties.
• -T3 (Solution heat-treated, cold-worked, naturally aged): Common for 2024 sheet and rivets.
• -T4 (Solution heat-treated, naturally aged): High strength, used for structural sheet.
• -T6 (Solution heat-treated, artificially aged): Maximum strength, used for 7075 extrusions and plate.
• -T62: Heat-treated from annealed condition by the user.

Identification of Aluminium Alloys:

Aluminium alloys are identified by a four-digit number (e.g., 2024, 7075) followed by a temper designation. Clad (Alclad) alloys have a suffix indicating the cladding (e.g., 2024-T3 Alclad).

Corrosion Characteristics:

Aluminium relies on a thin, protective oxide layer. When this is damaged, corrosion can occur rapidly, especially in the presence of electrolytes. The corrosion product is typically a white/grey powder. Aluminium alloys containing copper are particularly susceptible to intergranular corrosion if improperly heat-treated.

2.2 Magnesium and its Alloys

Magnesium is the lightest structural metal (density approximately 1.74 g/cm³), making it attractive for gearbox casings, wheels, and other non-primary structural components.

Critical Properties:

• Extremely flammable, especially in fine particle or swarf form.
• Highly susceptible to galvanic corrosion when in contact with dissimilar metals.
• Requires careful protective treatment (e.g., chemical conversion coating plus paint).

Maintenance Considerations:

• Cleaning: Use aliphatic naphtha or other approved solvents. Avoid chlorinated solvents (e.g., trichloroethylene, carbon tetrachloride) which can react with magnesium and are hazardous.
• Corrosion Removal: Mechanical removal must avoid sparking (fire risk) and must not embed dissimilar metal particles (e.g., steel from wire brushes) that would create galvanic cells. Use non-metallic abrasives or specialised magnesium-safe tools.
• Fire Safety: Magnesium fires are extremely difficult to extinguish. Dry sand, specialised extinguishers (Class D), or cast iron turnings are required. Water must never be used.

2.3 Titanium and its Alloys

Titanium offers a unique combination of properties that make it valuable in specific aircraft applications.

Key Properties:

• Density: approximately 4.5 g/cm³ (between aluminium and steel).
• High strength-to-weight ratio.
• Excellent corrosion resistance, including resistance to seawater and many chemicals.
• Good fatigue properties and high-temperature performance (up to approximately 400°C).
• Low thermal expansion coefficient.

Identification in the Workshop:

• Non-magnetic (unlike steel).
• When ground, produces a brilliant white spark (steel produces yellow sparks).
• Density is approximately 1.7 times that of aluminium.

Applications:

• Engine components (compressor blades, casings).
• Fasteners in highly stressed or high-temperature areas.
• Structural fittings where corrosion resistance and strength are required.

Maintenance Precautions:

• Titanium is difficult to machine; requires specialised tooling and techniques.
• Avoid contact with steel tools during machining to prevent galling.
• Not low-cost; use is justified only where its properties are essential.

2.4 Steels and their Alloys

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

Types of Steel Used in Aircraft:

• Low-carbon steels: For general-purpose, non-structural items.
• Medium-carbon steels: For parts requiring moderate strength and toughness.
• High-carbon steels: For springs, cutting tools, and wear-resistant parts.
• Alloy steels (e.g., chromium-molybdenum, nickel-chromium-molybdenum): For highly stressed structural components such as landing gear, engine mounts, and control system parts. These are typically heat-treated to high strength levels.
• Stainless steels (chromium-nickel): For corrosion-resistant applications such as exhaust systems, control cables, and fasteners.

Protective Treatments for Steel:

• Cadmium Plating: The most common protective coating for high-strength steel fasteners. Cadmium corrodes preferentially (sacrificial anode) to protect the steel. However, the plating process can introduce hydrogen into the steel, leading to hydrogen embrittlement. Baking (e.g., 190°C for 4 hours) is required to drive out hydrogen after plating.
• Zinc Plating: Used for less critical applications. Provides sacrificial protection but is less effective than cadmium in marine environments.
• Chromate Conversion Coating: Applied over cadmium or zinc to enhance corrosion resistance.
• Painting: Provides a barrier against moisture and electrolytes.

Hydrogen Embrittlement:

A significant risk with high-strength steels. Hydrogen atoms diffuse into the metal lattice, causing brittleness and potential catastrophic failure under stress. Sources include acid cleaning, electroplating, and corrosion reactions. Prevention includes baking after plating and avoiding acid contact with high-strength steels.

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3. Corrosion – Types, Causes, and Control

Corrosion is the electrochemical degradation of a metal. It is a primary concern in aircraft maintenance because it can compromise structural integrity and is often hidden from view.

3.1 Types of Corrosion

3.2 Corrosion Control and Prevention

Preventive Measures:

• Protective coatings (paint, anodising, plating, conversion coatings).
• Corrosion Inhibitor Compounds (CICs) applied to internal structures and areas prone to moisture ingress.
• Drain holes to prevent water accumulation.
• Proper sealing of joints and faying surfaces.
• Use of compatible materials to avoid galvanic corrosion.
• Regular inspection and prompt rectification of paint damage.

Corrosion Removal Procedure:

1. Assess the extent: Determine the type and depth of corrosion against manufacturer's allowable limits (AMM/SRM).
2. Mechanical removal: Use non-metallic abrasives (e.g., glass bead, aluminium oxide paper) to avoid embedding dissimilar metal particles. For magnesium, avoid sparking tools.
3. Evaluate remaining thickness: Measure to ensure the component is still within acceptable limits. If below limits, repair or replacement is required.
4. Restore protective finish: Apply chemical conversion coating (e.g., Alodine for aluminium), primer, and topcoat as specified in the AMM.
5. Record the finding: Document the corrosion, its extent, and the action taken in the aircraft logbook.

Important Principles:

• Corrosion must never be simply painted over without removal.
• Wire brushes must not be used on aluminium or magnesium (risk of galvanic contamination).
• The structural integrity of the component must be verified after corrosion removal.
• If corrosion exceeds allowable limits, the component must be repaired or replaced per approved data.

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4. Aircraft Hardware – Fasteners and Locking Devices

4.1 Bolts

AN Standard Bolts:

AN bolts are identified by a part number system that specifies diameter, length, and shank/head configuration.

Part Number Breakdown (e.g., AN6-20A):

• AN: Army-Navy (standard series)
• 6: Nominal shank diameter in 1/16-inch increments (6/16 = 3/8 inch)
• -20: Length in 1/8-inch increments (20/8 = 2.5 inches)
• Suffix Letter: Indicates shank and head drilling:
• A: Drilled head, undrilled shank
• B: Drilled head and drilled shank
• C: Undrilled head, drilled shank
• D: Undrilled head and undrilled shank

Close-Tolerance Bolts:

• Designated as AN173, AN174, or NAS series.
• Shank diameter is machined to a tighter tolerance (typically +0.000/-0.0005 inches) to provide a snug fit in reamed holes.
• Used in applications where precise fit is critical, such as engine mounts, control system attachments, and highly loaded joints.
• The snug fit ensures even load distribution and prevents fretting.

MS (Military Standard) Bolts:

• MS bolts use a different numbering system.
• For metric series bolts (e.g., MS20004-10), the number after the dash indicates the nominal diameter in millimetres (10 = 10 mm).
• For inch-based bolts, the number indicates diameter in 1/16-inch increments.

Bolt Inspection and Serviceability:

• Stripped or damaged threads render a bolt unserviceable; it must be replaced.
• Over-torqued (stretched) bolts must be replaced—stretching reduces the bolt's clamping force and fatigue life.
• Bolts with corrosion, nicks, or scratches in the shank must be rejected.
• The mating threads (nut or tapped hole) must also be inspected for damage.

4.2 Nuts

Types of Nuts:

• Plain nuts (AN315): Require a secondary locking device (cotter pin, safety wire).
• Castle nuts (AN310): Have slots for a cotter pin. Used with drilled shank bolts.
• Self-locking nuts:
• Nylon insert (elastic stop nut): A nylon ring deforms over the bolt threads to provide friction. The insert must be intact; a cracked or damaged insert renders the nut unserviceable.
• All-metal (prevailing torque): The locking feature is an elliptical or slotted portion of the nut that grips the bolt threads. If the nut can be run down the bolt threads by hand for more than two turns, the locking feature has worn and the nut must be rejected.
• Shear nuts: Used in shear applications only, not in tension.

Self-Locking Nut Limitations:

• Limited number of reuses; the AMM specifies a minimum prevailing torque. If the nut fails to meet this torque, it must be replaced.
• Not used where the temperature exceeds the insert material's rating (nylon inserts are limited to approximately 120°C).
• Not used on fasteners that rotate (e.g., control rod end bearings).

4.3 Washers

• Plain washers: Distribute load, provide a bearing surface, and prevent damage to the component surface.
• Lock washers: Provide a locking action by spring tension. Not used on primary structural fasteners.
• Special washers: Used for specific purposes (e.g., sealing washers, insulating washers).

4.4 Locking Devices

Cotter Pins:

• Used with castle nuts to prevent self-loosening under vibration.
• Do not increase clamping force; they are secondary locking devices.
• Installation procedure:

1. Torque the nut to the specified value.
2. If a slot aligns with the bolt hole, insert the cotter pin.
3. If not, loosen the nut (never tighten beyond torque) to the nearest slot that aligns.
4. Bend the cotter pin ends over the nut flats, not over the bolt end.

Safety Wire:

• Used to lock bolts, screws, and other fasteners in critical applications.
• Must be installed so that it tends to tighten the fastener (i.e., the wire pulls in the tightening direction).
• Must be taut, with no kinks or sharp bends.
• Must be replaced whenever it is disturbed.

Lock Washers:

• Provide a spring action to prevent loosening.
• Not suitable for primary structural applications.

4.5 Rivets

Solid Rivets:

• Materials:
• 2117-T3 (AD): The most common aluminium rivet. Work-hardenable and can be driven in the as-received condition. Acceptable substitute for many applications.
• 2024-T4 (DD): Stronger than 2117-T3 but must be heat-treated (solution treated) and driven within a short time (typically 30 minutes) after quenching to retain ductility. If the rivet becomes too hard, it must be re-heat-treated.
• 1100 (A): Pure aluminium, soft and ductile. Used for non-structural applications where corrosion resistance is important.
• Monel, Copper, Steel: Used for specific applications (e.g., high-temperature, high-strength). Must not be used in aluminium structure due to galvanic corrosion risk.
• Head Types:
• Universal (AN470): Rounded head, used for general applications.
• Countersunk (AN426): Dimpled head with a raised centre dot. Used where a flush surface is required.
• Round Head (AN430): Fully rounded head, used for non-structural applications.
• Flat Head (AN442): Flat head, used for non-structural applications.
• Identification: The head type and material are identified by markings on the rivet head (e.g., a dimple and centre dot for AN426, a raised dot for 2117-T3).

Rivet Selection:

• The rivet diameter should be approximately three times the thickness of the thickest sheet being joined, but not less than the sheet thickness. For thin sheets, the diameter is often limited to 2–2.5 times the thickness to avoid distortion.
• The rivet length must be sufficient to form a proper shop head. A general rule is that the length should equal the grip length plus 1.5 times the diameter.

Rivet Installation:

• Solid rivets are installed using a pneumatic rivet gun and a bucking bar to form the shop head.
• The rivet must fill the hole completely; oversize or undersize holes are not acceptable.
• The shop head should be uniform, with no cracks or folds.

Rivet Removal:

• To drill out a damaged rivet, use a drill bit approximately 1/32 inch smaller than the rivet shank diameter. For a 3/16-inch rivet, use a 5/32-inch drill.
• Drill to the depth of the manufactured head, then use a punch to remove the head and drive out the shank.
• Care must be taken not to enlarge or damage the hole.

Blind Rivets:

• Used where access to the back of the joint is not possible.
• Installed using a pulling tool.
• Not used in primary structure unless specifically approved.

4.6 Screws

• Types: Machine screws, self-tapping screws, structural screws (e.g., Hi-Lok, Hi-Lite).
• Identification: Screws are identified by head type (Phillips, slotted, hex), material, and part number.
• Replacement: Always replace fasteners with the correct part number as specified in the manufacturer's documentation (e.g., IPC). Using any available screw of the same size is not acceptable because the material, strength, and coating may be different.

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5. Hoses, Tubing, and Control Cables

5.1 Flexible Hoses

Construction:

• Inner tube: Carries the fluid. Materials include synthetic rubber, PTFE (Teflon).
• Reinforcement: Braided or spiral-wound wire (stainless steel, nylon) to withstand pressure.
• Outer cover: Protects the reinforcement from abrasion and environmental damage.

Identification:

• Hoses are marked with a coloured stripe to indicate the fluid type:
• Yellow stripe: Fireproof or fire-resistant hose.
• Brown or red stripe: Fuel hose.
• Blue stripe: Pneumatic hose.
• Black or grey: Low-pressure hose.
• Specific markings may vary by manufacturer; always consult the AMM.

Inspection and Serviceability:

• Chafing of the outer cover can lead to abrasion of the reinforcing braid and eventual failure.
• Any damage to the reinforcing braid requires replacement, as the hose's pressure rating is compromised.
• Taping or sealing is not an approved repair.
• Chafing against a bracket must be rectified by installing a protective sleeve (chafe guard) and/or re-routing the hose to avoid contact.

5.2 Rigid Tubing

Materials:

• Aluminium alloy (e.g., 2024-T3, 5052-O).
• Stainless steel.
• Titanium.

Inspection Criteria:

• Small dents in rigid tubing may be allowable if they are not in a bend, not on a flare, and within the limits specified in the AMM/SRM (e.g., less than 20% of the tube diameter).
• Hammering out dents is not permitted.
• Cracks, kinks, or corrosion beyond allowable limits require replacement.

5.3 Control Cables

Construction:

• Steel wire rope, typically 7x7 or 7x19 construction.
• The cable is made up of strands, each containing multiple wires.

Inspection Criteria:

• Broken wires: The cable must be replaced if the number of broken wires in any one lay length exceeds 10% of the total wires.
• Wear: A flattened spot without broken wires may be acceptable if it does not reduce the cable diameter below limits.
• Corrosion: Any corrosion requires replacement.
• Kinks, birdcaging, or other damage: Requires replacement.

Cable Terminals:

• Swaged terminals (e.g., Nicopress) must be inspected for cracks and slippage.
• Turnbuckles must be checked for thread engagement and safety-wired.

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6. Transparent Plastics

6.1 Acrylic (Perspex, Plexiglass)

Properties:

• Thermoplastic material.
• Excellent optical transparency.
• Good resistance to weathering and UV degradation.
• Relatively brittle; will crack under repeated bending or impact.
• Susceptible to stress cracking when exposed to aromatic solvents.

Applications:

• Cabin windows, windshields, canopies, landing light lenses.

Maintenance Considerations:

• Cleaning: Use non-aromatic solvents (e.g., mild soap and water, or approved acrylic cleaners). Aromatic solvents (e.g., toluene, xylene) can cause crazing and stress cracking.
• Cracks: Cracks in acrylic windows are not repairable by stop-drilling or sealing; they compromise structural integrity and pressure retention. The AMM/CMM will specify allowable crack lengths; if beyond limits, replacement is mandatory.
• Installation: Follow the AMM for sealant and installation. Use appropriate tools to avoid scratching or stressing the material.
• Handling: Acrylic is easily scratched; protect the surface during maintenance.

6.2 Polycarbonate

Properties:

• Tough, transparent thermoplastic.
• Highly impact-resistant; can be flexed repeatedly without cracking.
• More resistant to cracking than acrylic.
• Susceptible to UV degradation; requires a protective coating.

Applications:

• Windshields, canopies, protective shields.

Maintenance Considerations:

• Similar to acrylic, but more tolerant of flexing.
• Must be protected from UV exposure to prevent yellowing and embrittlement.

6.3 Identification of Transparent Plastics

• Acrylic: Relatively brittle; will crack under repeated bending.
• Polycarbonate: Tough and flexible; can be bent repeatedly without cracking.

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7. Sealants and Adhesives

7.1 Sealants

Types:

• Polysulfide: The most common aircraft sealant. Used for fuel tanks, pressure cabins, and general sealing. Resistant to fuels and many chemicals.
• Silicone: Used for high-temperature applications and electrical sealing. Not fuel-resistant.
• Polyurethane: Used for corrosion protection and abrasion resistance.

Storage and Handling:

• Sealants have a shelf life and must be stored per manufacturer specifications (often refrigerated).
• After mixing, they have a pot life (working time) and must be applied before curing.
• Unused mixed sealant must be discarded; it cannot be stored for later use.
• The application surface must be clean and dry.

Fuel Tank Sealants:

• Specifically formulated to resist degradation by fuel and to prevent leaks.
• They do not provide structural strength or electrical bonding.

7.2 Adhesives

• Used for bonding structural and non-structural components.
• Types include epoxy, acrylic, and polyurethane adhesives.
• Surface preparation is critical for bond strength.
• Adhesives have shelf life and pot life limitations similar to sealants.

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8. Fluids and Lubricants

8.1 Hydraulic Fluids

Types and Identification:

• Mineral-based (MIL-PRF-5606): Red in colour. Used in older systems and some light aircraft.
• Phosphate ester (Skydrol): Purple (or blue/green) in colour. Used in modern commercial and military aircraft. Not compatible with mineral-based fluids.
• Vegetable-based: Used in some light aircraft brake systems.

Compatibility:

• Hydraulic fluids must not be mixed unless specifically approved.
• Phosphate ester fluids attack many paints, sealants, and plastics; special precautions are required.

8.2 Lubricants

• MIL-PRF specifications: Define the performance requirements for lubricants and other materials. Commonly referenced in aircraft maintenance manuals.
• Greases: Used for bearings, hinges, and other moving parts. Types include MIL-PRF-23827 (general purpose), MIL-PRF-81322 (wide temperature range).
• Oils: Used for engines, gearboxes, and hydraulic systems.

Selection:

• Always use the lubricant specified in the AMM.
• Do not substitute unless the substitute is specifically approved.

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9. Non-Destructive Testing (NDT)

9.1 Dye Penetrant Inspection

Principle:

• A penetrant liquid is applied to the surface and allowed to dwell, during which it seeps into surface-breaking cracks and other discontinuities.
• The surface is then cleaned to remove excess penetrant.
• A developer is applied, which acts as a blotter, drawing the penetrant out of the discontinuities to form a visible indication.

Procedure:

1. Clean the surface: Remove all dirt, grease, and other contaminants.
2. Apply penetrant: By spraying, brushing, or dipping.
3. Dwell time: Allow the penetrant to remain on the surface for the specified time (typically 10–30 minutes, depending on the penetrant type).
4. Remove excess penetrant: Wipe off the surface penetrant using a solvent or water, as specified.
5. Dry the surface: The part must be dried before applying developer, because residual solvent can dilute the penetrant and affect the indication.
6. Apply developer: As a thin, even coating.
7. Inspect: Examine the surface for indications (coloured or fluorescent, depending on the penetrant type).

Important Points:

• The developer does not clean the surface; that is the role of the cleaner/remover.
• The developer does not protect against corrosion.
• The developer does not enhance UV visibility; that is the role of the penetrant itself if it is fluorescent.
• High-pressure cleaning can force penetrant out of cracks and must be avoided.

9.2 Other NDT Methods

• Magnetic particle inspection: Used for ferromagnetic materials. Detects surface and near-surface cracks.
• Eddy current inspection: Used for conductive materials. Detects surface and subsurface cracks and corrosion.
• Ultrasonic inspection: Used for detecting internal flaws and measuring thickness.
• Radiographic inspection (X-ray): Used for detecting internal flaws.

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10. General Maintenance Practices

10.1 Cleaning

• Cleaning must be performed using materials and methods specified in the approved maintenance data (AMM/CMM).
• Using unapproved solvents or methods could cause damage or leave residues.
• High-pressure water is not suitable for all components.
• For magnesium alloys, use aliphatic naphtha or other approved solvents. Avoid chlorinated solvents.

10.2 Torqueing

• Torqueing ensures the correct preload (clamping force) is applied, which is critical for joint integrity and fatigue life.
• Over-torquing can damage the fastener or component; under-torquing can lead to loosening.
• Torque values are specified in the AMM.
• If a fastener moves during a torque check (breakaway torque less than specified), the correct procedure is to loosen the fastener completely, inspect threads for damage, and then re-torque to the specified value.

10.3 Defect Assessment and Recording

• Any damage to aircraft materials and hardware must be assessed against approved data (e.g., SRM, AMM).
• A line maintenance certifying staff (Category A) may only perform simple repairs within the limits of approved data.
• If a defect is within the limits given in the SRM or AMM, it is acceptable and the aircraft may be returned to service, with the defect recorded as 'found within limits' in the logbook.
• If a defect is beyond limits, the aircraft must not be returned to service until the defect is rectified per approved data.

10.4 Stop-Drilling

• For non-structural aluminium sheet components, stop-drilling the ends of a crack is an accepted temporary or permanent repair method to prevent crack propagation, provided it is within the limits of the applicable data.
• Stop-drilling is not a recognised repair for acrylic windows or other transparent plastics.

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11. Common Relationships Between Concepts

• Material selection is driven by the required strength-to-weight ratio, corrosion resistance, and cost. Aluminium alloys are used for most structure; steel for high-strength, high-wear applications; titanium where corrosion resistance and high-temperature performance are needed; magnesium for lightweight, non-critical components.
• Corrosion is the primary enemy of aircraft structure. It is controlled by protective coatings, proper material selection, and regular inspection. Corrosion removal must be followed by restoration of the protective finish.
• Fastener selection depends on the application: general-purpose bolts for non-critical joints, close-tolerance bolts for critical joints, rivets for sheet metal structure, and self-locking nuts where vibration is a concern.
• Locking devices (cotter pins, safety wire, self-locking nuts) are secondary locking mechanisms; they do not increase clamping force.
• Sealants and adhesives have limited shelf life and pot life; they must be applied within the specified time and stored per manufacturer instructions.
• NDT methods are used to detect defects that are not visible to the naked eye. The choice of method depends on the material and the type of defect being sought.

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12. Typical Exam Focus Points

1. Corrosion types and identification: Be able to identify corrosion types from descriptions of appearance and location (e.g., white/grey powder on aluminium = surface corrosion; fine cracks with corrosion products = stress corrosion cracking).
2. Corrosion removal procedures: Know the correct sequence (assess, remove, evaluate thickness, restore finish) and the tools/materials to use (non-metallic abrasives for aluminium and magnesium).
3. Fastener identification: Understand AN and MS part number systems, including suffix letters for drilled heads/shanks and the meaning of numbers for diameter and length.
4. Close-tolerance bolts: Know their purpose (precise fit in reamed holes) and typical applications (engine mounts).
5. Rivet materials and identification: Know the properties of 2117-T3 and 2024-T4 rivets, and the head markings for different rivet types.
6. Self-locking nuts: Know the inspection criteria (nylon insert integrity, prevailing torque limits) and when to reject.
7. Hose and tubing inspection: Know the criteria for serviceability (chafe damage to braid requires replacement; dents within limits may be acceptable).
8. Control cable inspection: Know the 10% broken wires rule and other rejection criteria.
9. Transparent plastics: Know the properties of acrylic vs. polycarbonate, and the correct handling/cleaning procedures.
10. Sealants: Know shelf life, pot life, and storage requirements.
11. Hydraulic fluids: Know the colour coding (red = mineral, purple = phosphate ester).
12. NDT principles: Understand the role of the developer in dye penetrant inspection.
13. Torque procedures: Know the correct action if a fastener moves during a torque check.
14. Defect disposition: Know when a defect is within limits (return to service with logbook entry) vs. beyond limits (rectification required).

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13. Key Formulas and Rules of Thumb

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14. Regulatory References

• Regulation (EU) No 1321/2014, Annex III (Part-66): Establishes the basic knowledge requirements for certifying staff.
• Appendix I to Part-66: Defines the Module 6 syllabus and knowledge levels.
• Part-M / Part-145: Govern maintenance and airworthiness management, including defect recording and disposition.
• AMC/GM to Part-66: Provides acceptable means of compliance and guidance material for the syllabus.
• AMM (Aircraft Maintenance Manual): The primary source for approved maintenance procedures, torque values, and inspection criteria.
• SRM (Structural Repair Manual): Provides allowable damage limits and repair procedures for structural components.
• IPC (Illustrated Parts Catalogue): Used to identify correct part numbers for replacement hardware.

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15. Knowledge Level Summary