Aviation Maintenance Technician Handbook

The Aviation Maintenance Technician Handbook provides crucial insights into the materials and components essential for aircraft upkeep and safety. Understanding these nonmetallic materials, their properties, and proper handling procedures is fundamental for any aspiring aviation maintenance technician. This guide delves into plastics, various types of rubber, seals, and composite materials, ensuring comprehensive knowledge for students and professionals.

Understanding Nonmetallic Materials in Aviation Maintenance

The landscape of aircraft construction has significantly evolved since the mid-20th century. While wood, fabric, and even aluminum once dominated, modern aircraft increasingly rely on nonmetallic aircraft materials, particularly reinforced plastics and advanced composites. These materials offer superior performance characteristics vital for contemporary aerospace design.

The Role of Plastics in Aircraft Design

Plastics are ubiquitous in modern aircraft, serving diverse functions from structural components to decorative trims and transparent enclosures. They are broadly categorized based on their reaction to heat and their physical form.

Transparent Plastics: Windows, Windshields, and Canopies

Transparent plastics, used in aircraft canopies, windshields, and windows, fall into two main classes:

• Thermoplastics: These materials soften when heated and harden upon cooling. They can be reheated and remolded multiple times without altering their chemical composition. Examples include acrylics.
• Thermosets: These plastics harden when heated and cannot be softened or remolded once fully cured. They undergo an irreversible chemical change upon heating.

Transparent plastics also come in two forms:

• Monolithic (Solid): Single-layer plastic sheets.
• Laminated: Made of transparent plastic face sheets bonded by an inner layer, often polyvinyl butyral. Laminated plastics offer superior impact resistance (shatterproof qualities) and are preferred in many pressurized aircraft.

Stretched acrylic is a notable development, offering enhanced impact resistance, reduced chipping, greater chemical resistance, simpler edging, and fewer issues with crazing and scratching. Proper storage and handling are crucial for these materials, with recommendations including inclined shelving, controlled temperatures, and careful removal of protective paper using gentle heat or aliphatic naphtha.

Reinforced Plastics for Structural Integrity

Reinforced plastic is a thermosetting material widely used in radomes, antenna covers, wingtips, and electrical equipment insulation. Its excellent dielectric properties make it ideal for radomes. High strength-to-weight ratio, resistance to mold, rust, and rot, and ease of fabrication make it suitable for various aircraft parts.

Reinforced plastic components are formed from solid or sandwich-type laminates. Resins, typically contact pressure types (requiring little to no pressure during curing), are used to impregnate glass fabrics. Curing is achieved with a catalyst, usually benzoyl peroxide.

Fiber reinforced materials derive most of their strength from the reinforcement itself. The three primary forms of fiber reinforcements are:

• Particles: Square pieces, like hollow glass spheres (Q-cells).
• Whiskers: Pieces longer than they are wide, often single crystals used to reinforce ceramics and metals.
• Fibers: Individual filaments much longer than they are wide, forming the basis of most composites. They are smaller than human hair and often woven into fabric-like materials.

Laminated Structures: Lightweight Strength

Laminated structures provide significant strength and rigidity. A sandwich structure features a core material between two high-density laminates or solid face sheets. This design offers the same strength as a solid laminate but with significantly less weight, which is critical for aerospace applications.

Core materials can vary widely, from rigid foam and wood to metal or the aerospace preference: honeycomb structures made from paper, Nomex®, carbon, fiberglass, or metal. Adhering to manufacturer's maintenance manual instructions for construction, repair, and testing is paramount to ensure structural integrity.

Rubber and Its Diverse Applications in Aircraft Systems

Rubber in aviation encompasses natural, synthetic, and silicone types, serving to prevent dirt, water, or air entry, retain fluids and gases, and absorb vibrations. Each type possesses unique properties tailored to specific operational demands.

Natural Rubber: Flexibility and Elasticity

Natural rubber offers superior physical and processing properties, including flexibility, elasticity, tensile strength, tear resistance, and low heat buildup from flexing. It's a general-purpose product but has limited suitability in aircraft due to its lower resistance to common deteriorating influences. It swells and softens in most aviation fuels and solvents, and is primarily used for water/methanol systems.

Synthetic Rubbers: Tailored for Performance

Synthetic rubber types are engineered with different compositions to achieve desired properties:

• Butyl: Excellent gas permeation resistance and resistance to deterioration. It resists oxygen, vegetable oils, animal fats, alkalis, ozone, and weathering. It's used with phosphate ester hydraulic fluids (Skydrol™), silicone fluids, gases, ketones, and acetones, suitable for temperatures from -65°F to 300°F.
• Buna-S: Similar to natural rubber in processing and performance, with good water resistance and slightly better aging characteristics. It has poor resistance to gasoline, oil, concentrated acids, and solvents. Commonly used for tires and tubes.
• Buna-N: Outstanding resistance to hydrocarbons and solvents, though it has low resilience in solvents at low temperatures. It offers good resistance up to 300°F and down to -75°F for certain applications. Used for oil and gasoline hoses, tank linings, packings, and seals.
• Neoprene: Superior to natural rubber in resistance to punishment and low-temperature characteristics. It boasts exceptional resistance to ozone, sunlight, heat, and aging. While good for non-aromatic gasoline systems, it has poor resistance to aromatic gasoline. Applications include weather seals, window channels, and oil-resistant hoses.
• Thiokol (Polysulfide Rubber): Offers the highest resistance to deterioration from petroleum, hydrocarbons, esters, alcohols, gasoline, or water. However, it ranks lowest in physical properties like compression set, tensile strength, and abrasion resistance. Used for oil hoses and aromatic aviation gasoline tank linings.
• Silicone Rubbers: Known for excellent thermal stability and flexibility at extreme temperatures (from -150°F to 600°F). While resistant to oils, they react unfavorably to both aromatic and non-aromatic gasoline. Silastic, a well-known silicone, is used for insulating electrical equipment and in certain oil systems due to its dielectric properties.

Seals: Essential for Containing Fluids and Preventing Contamination

Seals are vital components in aircraft systems, preventing fluid leakage, maintaining system integrity against air and dirt, and accommodating various operating speeds and temperatures. No single seal type is universally satisfactory, as selection depends on pressure, fluid type, metal finish, clearance, and type of movement.

Seals are categorized into three main classes: packings, gaskets, and wipers.

Packings: The Mobile Seals

Packings, made from synthetic or natural rubber, function as “running seals” in units with moving parts like actuator cylinders, pumps, and selector valves. They are available as O-rings, V-rings, and U-rings.

• O-Ring Packings: The most common type, effectively sealing in both directions to prevent internal and external leaks. For pressures above 1,500 psi, backup rings are used to prevent O-ring extrusion. When an O-ring is subject to pressure from both sides, two backup rings are installed (one on each side). If pressure is from a single side, one backup ring is placed on the side away from the pressure. O-ring materials are compounded for specific operating conditions, temperatures, and fluids. A common series for MIL-H-5606 fluid at -65°F to +275°F is MS28775. Color coding on O-rings is not a reliable identification method; the part number on the sealed envelope provides the most dependable compound identification. O-rings must be inspected under magnification for surface flaws before installation.
• Backup Rings (MS28782): Made of Teflon™, these rings do not deteriorate with age, are unaffected by system fluids or vapors, and tolerate extreme temperatures. Their dash numbers correlate directly to the O-ring size they support. They are not color-coded and must be identified from packaging labels. Inspection includes checking for surface irregularities, clean edges, parallel scarf cuts, and ensuring helically wound Teflon™ rings do not separate more than 1/4 inch when unsupported.
• V-Ring Packings (AN6225): Unidirectional seals always installed with the open end of the “V” facing the pressure. They require male and female adapters to maintain proper position and specific torque values on the seal retainer for satisfactory service.
• U-Ring Packings (AN6226) and U-Cup Packings: Used in brake assemblies and master brake pumps. They seal pressure in one direction, with the lip facing the pressure. Primarily low-pressure packings for pressures under 1,000 psi.

Gaskets: The Static Seals

Gaskets are used as static (stationary) seals between two flat surfaces. Common materials include asbestos, copper, cork, and rubber.

• Asbestos Sheets: Used for heat-resistant gaskets, such as in exhaust systems, often with a thin copper sheet edge for extended life.
• Solid Copper Washers: Essential for spark plug gaskets, providing a non-compressible yet semi-soft seal.
• Cork Gaskets: Ideal for oil seals between engine crankcases and accessories, and where a gasket must fill uneven or variable gaps due to rough surfaces or expansion/contraction.
• Rubber Sheets: Used where a compressible gasket is needed, but not in contact with gasoline or oil due to rapid deterioration.

O-ring shaped gaskets are also used in fluid systems for actuator cylinder end caps and valves.

Wipers: Protecting Against Contamination

Wipers clean and lubricate exposed piston shaft parts, preventing dirt entry into the system and protecting the piston shaft from scoring. They can be metallic, felt, or a combination of both.

Sealing Compounds for Environmental Protection

Certain aircraft areas are sealed for pressurization, fuel leak prevention, vapor passage blockage, or corrosion protection against weathering. Most sealants are multi-ingredient compounds, precisely proportioned for optimal results.

One-Part Sealants

These sealants are pre-prepared by the manufacturer and ready for direct application. Their consistency can be altered with manufacturer-recommended thinners to suit specific application methods.

Two-Part Sealants

Two-part sealants consist of a base compound and an accelerator, packaged separately to prevent premature curing. Altering the prescribed proportions compromises material quality. They are mixed by combining equal parts by weight. Accurate weighing, often using specialized scales, is crucial. Both components should be thoroughly agitated before mixing. Dry, lumpy, or flaky accelerators must be rejected.

Pre-weighed sealant kits simplify mixing for full batches. The accelerator is added to the base compound and immediately mixed thoroughly, avoiding air entrapment. Overly fast or prolonged stirring generates heat, shortening the sealant's working life. A small test spread can check for flecks or lumps, requiring further mixing or rejection if they persist. The working life ranges from 1/2 hour to 4 hours, requiring prompt application or refrigerated storage.

Curing speed varies with temperature and humidity; ideal conditions are 77°F with 50% relative humidity. Curing can be accelerated by increasing temperature (not exceeding 120°F) using infrared lamps or filtered hot air. Heat should not be applied to faying surface sealant installations until all work is complete and fastenings are in place within the sealant's application limitations. Sealant must be tack-free before applying finish coatings.

Shock Absorber Cord: Cushioning and Support

Shock absorber cord, or bungee cord, is used for cushioning and support. It consists of natural rubber strands encased in a braided cotton cover, treated for oxidation and abrasion resistance. High tension and elongation are achieved by weaving the cover over stretched rubber strands.

There are two types:

• Type I: Straight cord.
• **Type II (