Aircraft Materials and Hardware
ATA 20 – Standard Practices – Airframe
Metals
Ferrous Materials
Ferrous materials contain iron as their main component. Examples include:
- Steel (used for loaded parts due to its strength)
- Cast iron
- Cast aluminum
Properties of Ferrous Metals
- Elasticity: Deformation disappears when the applied force is removed.
- Hardness: Resistance to abrasion, penetration, and cutting.
- Brittleness: Tendency to fracture with little bending or deformation (e.g., very hard steel, cast iron, cast aluminum).
Heat Treatment Processes for Steel
- Tempering: Reduces brittleness after hardening.
- Carburizing: Adds carbon to increase surface hardness.
- Annealing: Softens the material by heating and slow cooling.
- Nitriding: Nitrogen reacts with iron to form a hard surface layer.
Hardness Testing
- Rockwell test: Measures hardness by determining the depth of penetration of an indenter.
- Brinell test: Measures hardness by determining the diameter of an indentation made by a ball indenter.
Non-Ferrous Metals
Non-ferrous metals do not contain iron. Examples include:
- Aluminum (good strength-to-weight ratio, used for aircraft structures)
- Titanium (high strength, corrosion resistance, used for high-temperature applications)
- Magnesium (lightweight, but presents a dust hazard)
- Copper (high electrical conductivity)
- Nickel alloys (e.g., Inconel, used for high-temperature applications)
Aluminum Alloys
- 2024-T4: Heat-treated aluminum alloy with good strength and machinability.
- 7075: Heat-treated aluminum alloy with very high strength.
- Alclad: Aluminum alloy core with a pure aluminum surface layer for corrosion protection.
Composites
Types of Composites
- Fibrous composites: Made of fibers embedded in a resin matrix (e.g., fiberglass, carbon fiber).
- Laminated composites: Made of layers of materials bonded together (e.g., plywood, sandwich panels).
Composite Fabrication
- Warp direction: The direction along the length of the fabric.
- Fill direction: The direction perpendicular to the warp direction.
- Wet layup: Dry fabric is impregnated with resin before being laid up.
- Dry layup: Pre-impregnated fabric is used.
Composite Inspection Methods
- Thermography: Detects delaminations and other defects near the surface by measuring temperature variations.
- Ultrasonic inspection: Uses high-frequency sound waves to detect internal defects.
- X-ray inspection: Uses X-rays to detect internal defects.
- Tap testing: Uses a light hammer to detect delaminations by the sound produced.
Fasteners
Bolts
- Aircraft bolts: Typically Class 3 (high strength).
- Standard steel bolts (AN): Marked with an asterisk (*) or cross (+).
- Corrosion-resistant steel bolts (AN): Marked with a single raised dash (-).
- Close tolerance bolts (NAS): Marked with a triangle (△).
Bolt Locking Mechanisms
- Castle nuts and cotter pins
- Self-locking nuts
- Safety wire
Rivets
- Solid rivets: Permanent fasteners.
- Blind rivets: Used when access is limited to one side.
- Pull rivets: A type of blind rivet.
Fittings and Fluid Lines
Types of Fittings
- Flare fittings: Used with flared tubing.
- AN fittings: Common type of aircraft fitting.
Fluid Line Identification
- Color-coded markings
- Tags
Cables and Wiring
Types of Cables
- Control cables: Used to transmit motion.
- Electrical cables: Used to transmit electrical power and signals.
- Coaxial cables: Used for high-frequency applications.
Wiring Practices
- Wire bundles should be secured at regular intervals.
- Bending radii should be observed to prevent damage to wires.
- Proper crimping techniques should be used.
- Connectors should be properly polarized and secured.
Corrosion
Types of Corrosion
- Intergranular corrosion: Occurs along grain boundaries in metals.
- Stress corrosion cracking: Occurs due to the combined effects of stress and a corrosive environment.
Corrosion Prevention
- Protective coatings (e.g., paint, plating)
- Corrosion-resistant materials
- Proper maintenance practices