Knowledge Levels 1, 2, 3: Basic Concepts

Knowledge Levels

• Basic knowledge for categories A, B1, and B2 is indicated by knowledge level indicators 1, 2, or 3.
• Category C applicants must meet either B1 or B2 basic knowledge levels.

Knowledge Level 1

• The applicant should be familiar with the basic elements of the subject.
• The applicant should be able to describe the subject using common words and examples.
• The applicant should be able to use typical terms.

Knowledge Level 2

• A general knowledge of the theoretical and practical aspects of the subject is needed.
• The applicant should understand theoretical fundamentals.
• The applicant should give a general description using examples.
• The applicant should use mathematical formulae with physical laws.
• The applicant should read and understand sketches, drawings, and schematics.
• The applicant should apply knowledge practically using detailed procedures.

Knowledge Level 3

• A detailed knowledge of the theoretical and practical aspects of the subject is needed.
• The applicant should know the theory of the subject and its interrelationships.
• The applicant should give a detailed description using theoretical fundamentals with examples.
• The applicant should understand and use relevant mathematical formulae.
• The applicant should read, understand, and prepare related sketches, drawings, and schematics.
• The applicant should apply knowledge practically using manufacturer's instructions.
• The applicant should interpret results from sources and measurements and apply corrective action.

Wooden Airframe Construction

• Modern ultra-light and home-built aircraft are still constructed using wood.
• Historic aircraft including the De Havilland Tiger Moth are still used.
• The Hughes H4 Flying Boat (Spruce Goose) and the De Havilland Mosquito are notable wooden aircraft.

Wooden Structures

• The three basic types of wooden airframe construction are truss, monocoque, and semi-monocoque.
• There can be variations of each type.

Truss Description

• A truss joins structural members to form a rigid structure.

Pratt Truss Fuselage Construction

• Early aircraft used the Pratt truss with wooden longerons as main structural members.
• Wood struts held the longerons apart.
• Piano wire stays crossed each bay and were tensioned using brass turnbuckles.
• Struts carry compressive loads, stays carry tension loads in a Pratt truss.

Warren Truss Fuselage Construction

• The Warren truss uses longerons separated by diagonal members that can carry both compressive and tensile loads.
• Longerons are primary load-bearing components separated by horizontal and vertical struts forming box structures.
• Each box is triangulated and strengthened by diagonal struts.
• Stringers are secondary components giving shape to the fuselage connected to bulkheads or formers.
• Primary components are longerons, horizontal struts, vertical struts, and diagonal struts.
• Secondary components are stringers, formers, and bulkheads.

Truss Wing Components and Construction

• The major components of a wooden truss-type wing construction are leading edge, front/main spar, rear spar, compression struts, ribs, drag wires, and anti-drag wires.
• The leading edge is sheet aluminium alloy, plywood or composite to give shape and protect the wing from erosion and impact damage.
• Front and rear spars are the primary load-bearing components.
• Compression struts or ribs prevent spars from being pulled together and crushing the ribs, supporting compression loads between spars, at drag and anti-drag wire attachment points, forming bays.
• Ribs give shape to the wing and support the non-load-bearing skin can be fabric or plywood.

Drag and Anti-Drag Wires

• Drag wires oppose drag loads to push the wing reward, opposite the direction of flight.
• Anti-drag wires support the wing, drag and anti-drag wires triangulate the structure, giving rigidity.

Monocoque Construction

• Monocoque is a structural system supporting loads through the external skin, lacking a load-carrying internal frame.
• The skin, bulkheads, and formers carry all flight and landing loads.

Semi-Monocoque Construction

• Semi-monocoque construction's external skin supports most of the load, unlike truss construction.
• Semi-monocoque construction has formers, frame assemblies, and bulkheads, the skin is reinforced by longitudinal members.
• The semi-monocoque fuselage is constructed primarily of aluminium alloy, although steel and titanium are found in high-temperature areas.
• Primary bending loads are taken by the longerons, supplemented by stringers.
• Wooden semi-monocoque aircraft exist, primarily with metal or composite skins.

Common Aircraft Wood

• Sitka spruce is favored for aircraft structures due to its uniformity, strength, and shock resistance.
• To meet 'Aircraft Sitka Spruce' specifications, the wood must be kiln-dried to a specific gravity (SG) of no less than 0.36 and the slope of the grain should be no steeper than 1:15.
• The wood grain must be sawn vertical grain (edge-grained) with no fewer than six annular rings per inch.

Wood Substitution

• In the Oceania region (Australia and Papua New Guinea), hoop pine, Queensland maple, bunya pine, and klinki pine are equivalent in quality to Sitka spruce.
• Acceptable wood specifications are outlined in AMM, SRM, or AC 43.13-1B/2A.

Laminated Wood

• Laminated wood consists of two or more layers of solid wood bonded together with parallel grains.
• Plywood is constructed with grain direction at a 90° angle to the previous layer.
• Laminated wood is stronger but less flexible than an equivalent piece of solid wood.
• With greater resistance to warping than solid wood, making it a good substitute, typically found in curved shapes like wing-tip bows and fuselage formers.

Plywood

• Structural aircraft-grade plywood is veneers bonded together in a hot press.
• Veneers are laid with alternating grain directions at 90° giving greater rigidity on the face veneers.
• For greater rigidity in all directions 45° offsets are slightly stronger but more expensive to produce.

Storing Wood

• Wood should be stored flat with boards placed between layers for air circulation.
• The wood storage area should be well ventilated, dry and out of direct sunlight.

Wood Moisture Content

• Strength increases as wood strength increases.
• Kiln drying ensures a controlled drying rate, while insects and mildew are killed.
• A moisture content of 15% +/-2% is normally acceptable.

Wood Density

• Density of timber is found by weighing a sample of known volume and multiplying the result to pounds per cubic foot/kilograms per cubic meter.

Wood Brittleness

• Brittleness is determined by using an Izod-type impact test machine.
• A notched test piece should resist a load of at least 5 ft-lb.

Wood Splitting

• A splitting test confirms the grain direction and run using a blunt chisel made away from the edge.
• The maximum acceptable grain slope is 1:15

Grain Structure

• Springwood/earlywood grows early, its light in color, with large fibres and thin walls.
• Summerwood/latewood grows later, its dark in colour, with small fibres and thick walls, creating annual rings.

Timber Terminology

• Sapwood consists of the living cells of the tree.
• Heartwood consists of dead cells of the tree, with a pith or hole in the center.
• Knots are where limbs have grown.
• Checks are longitudinal splits extending across the grain.
• Shakes are defects by a parting of the annular rings.

Effects of Shrinkage

• Greatest shrinkage of wood occurs tangentially across fibres, parallel to growth rings
• Moderate shrinkage occurs radially across fibres and perpendicular to growth rings.
• Negligible shrinkage occurs in the longitudinal direction parallel to fibres.

Sawn Wood

• Aviation-quality wood is quarter-sawn or rift sawn to reduce shrinkage.
• Wood must be quarter sawn so the annual rings form a 45° to 90° angle with the face of the board.
• It needs no fewer than 6 annular rings per inch.
• The maximum slope of the grain is 1:15 or the grain may not rise more than 1 in. in a 15-in. length of wood.

Aircraft Glues

• Most joints are made with glue, wood screws and bolts are used for critical joints in a timber airframe.
• Glue plays a critical role in the overall strength of the structure.
• Choose glue in accordance with the AAMS, SRM, and AC43-13-1B.

Health and Safety When Using Glues

• Glues present some toxic potential to users from contact with skin or eyes, inhalation, or ingestion.
• Use manufacturer's instructions and Safety Data Sheet (SDS).
• Appropriate skin, eye and respiratory protection should be worn whenever indicated.
• Maintain shop cleanliness for health, fire safety and product performance.
• Shop personnel should maintain awareness of others is maintained and proper shop ventilation is used.

Glue Bonding Process

• Most glue types have four time periods crucial to the bonding process.
• Pot life refers to the glue's useable life, affected by temperature and humidity.
• Adding thinners does not extend pot life, it can seriously compromise the strength of the glue.
• Open-assembly time is the allowable time between glue application and joint assembly.
• Closed-assembly time is the allowable time between joint assembly and applying clamping pressure.
• Pressing time is when parts are pressed/clamped together, requiring temperature range-curing cycle, while temperature affects the bond line's cure rate.

Types of Glues

• Common glues in aircraft structure are casein and synthetic resins.
• Modern glues should be considered first and Casein is generally considered inferior to other products.
• Synthetic-resin glues are plastic resin, resorcinol, hot-pressed phenol, and epoxy.

Casein Glue

• Deteriorates over the years after exposure to air moisture and to temperature.
• Many modern glues are incompatible with casein glue.
• Scrape all traces of casein away before applying the new glue as alkaline nature of casein glue may prevent new glue from curing properly, thereby compromising structural integrity.
• The performance is considered inferior to available products.
• Not used for all aircraft repairs.

Plastic Resin Glues (Urea-Formaldehyde)

• The glue is normally applied to one surface, the hardener to the other.
• When mixed, the glue has a pot life of 4 to 5 hours, with a setting time of 1.5 to 4 hours.
• The maximum strength, is attained within in 3-5 days, has good fuel and temperature resistance, while its reliability has been questioned in a tropical environment, its use is banned for primary structure in Australia.

Resorcinol Glue

• A two-part synthetic resin glue has resin and hardener, its the most water-resistant of the glues and is mixed prior to use.
• Once its cured, it has high resistance to temperatures and chemical attack.
• The resulting dust may cause a severe allergic reaction when the glued wood is sanded.

Epoxy Resins

• Two-part synthetic resin glues, having excellent working properties require less attention to joint quality or clamping pressures compared to other aircraft glues, they penetrate evenly and completely into wood and plywood structures.

Wood Preparation

• Proper preparation is imperative, with the wood surface is be clean, dry, and free of oil, grease or wax.
• Without proper penetration, the glue line is intermittent.
• The wood should be kept in the same room for a minimum of 24 hr to equalise the wood moisture content.
• Cut and plane until smooth and true.
• Planer marks and chipped surfaces are not permitted.
• Do not use sandpaper as it rounds corners.
• Vacuum after.

WHS When Preparing Wood Surfaces

• Machining existing structures and applying glues means PPE should avoid dust and fume inhalation.
• Appropriate skin, eye and respiratory protection should be worn.
• Shop cleanliness is essential for health, fire safety and product performance.
• Shop personnel should maintain awareness of others in the work area.
• Dust extraction should be considered essential to control dust and fumes.
• Avoid electric sanders near dope, paint and glues to lower fire hazards.

Applying the Glue

• A smooth, even coat of glue should be applied to each wood surface that is prepared and ready to be glued.
• In the case of plastic resin glues, the glue is applied to one surface and the hardener to the mating surface.
• Follow the manufacturer's instructions for mixing and application.
• The orientation of wood grain to avoid applying glue to the end grain
• End grain is wood that is cut at a 90° angle to the direction of the grain.
• In an acceptable cut of wood, the cut is nearly parallel to the direction of the grain.

Joint Clamping Pressure

• Applying pressure spreads glue into a thin continuous film.
• Clamping forces remove air from the joint and brings wood surfaces together.
• Too little clamping pressure leads to thick glue lines.
• Too much clamping pressure squeezes out too much glue.
• Types of glue require different amounts of clamping pressure.
• Follow gluing procedures in detail.
• Recommended clamping pressure for soft woods is 125 to 150 psi.
• Recommended clamping pressure for hardwoods is 150 to 200 psi when using resorcinol glue.

Methods of Applying Pressure

• Applying pressure is important.
• Methods can range from brads, nails, small screws, to mechanical, hydraulic, electrical presses, and pneumatically.
• Screw clamps are used in conjunction with pressure blocks to distribute the clamping pressure and protect the structure.
• Apply pressure to a joint for the time recommended by the glue manufacturer.
• After removed, clean and inspect the joint and remove any glue.

Hand Nailing and Screws

• Hand-nailing applies pressure when bonding ribs, attaching plywood skins to the wing, control surfaces, and fuselage frames.
• Nails and screws produce adverse effects such as splitting small parts and creating moisture entry points.
• Nailing strips are often used to spread pressure and remove nails after glue is cured.
• Prevent gluing by placing waxed paper between nailing strip and wood.
• Use a 4-oz (120-g) cross-pein hammer for nailing brads and small nails with the nails or screws are not intended to hold the structure.

Finishing Wood

• After assembly or repair, protect the timber and joints to prevent drying, becoming brittle, or rotting due to excess moisture.
• Apply an approved varnish or lacquer.
• Adhere to the instructions issued by manufacturer, protect from chemicals that might permeate.
• The best types of varnish to use are epoxy varnishes and polyurethane varnishes - two-part epoxy resins do not react with solvents from fabric coverings as polyurethane varnishes are prone to.
• During repairs avoid excessive surplus glue. Clear away all contaminated surfaces with glue.

Finishing End-Grain Surfaces

• Exposed end grain includes such surfaces as spar butts; skin edges; areas around vent holes, inspection holes and fittings; and exposed scarfed or tapered surfaces to seal out moisture entry.
• Apply 1+ thinned coats of acceptable sealer to ensure penetration, and then many full-strength coats as achieve a smooth, glossy coating.

Finishing Interior Surfaces

• The primary objective of interior finishes is to afford protection of the wood against changes in moisture content from damp air/water.
• Finish ribs, spars, interior of plywood skins & Internal-members, apply 1 thinned-coat (penetration into wood grain) use acceptable-finish, followed-by two full-coats protect built-up box-spars and similar closed structures.

Finishing Exterior Surfaces

• The exterior finish protects the wood against weathering, providing a smooth surface in flight.
• Exterior surfaces require to be more protected because of abrasion, weather & sunlight.
• Prolonged high temperature is detrimental to wood, adhesives and finishes.

Sealing Bolt Holes

• Bolt holes provide vulnerable entry for moisture to decay or split around bolt holes and corrosion of the bolts.
• Seal wood surfaces in bolt holes with a sealing varnish and allowing to dry prior to bolt installation.

WHS (when using wood finishes)

• The most finishers present some toxic potential to users from contact or inhalation.
• Awareness, instructions, and Safety Data Sheet (SDS) and appropriate skin, eye, and respiratory protection should be worn whenever indicated.
• Maintaining, clean shop and awareness of bystards and shop personnel.
• With, dispersed dispersion from the surfaces.

Common Wooden Structure Defects

• Dry rot includes: Contagious, any affected, & by tenderness, discoloration, a mushroom odour.
• Splitting/cracking is by varying shrinkage rates, or an outside force has been applied.
• Bond failure includes most commonly to improper processing, while moisture improper preparation, and using the wrong type.
  • Including, Cohesive bond failure, and adhesive or structural failure.
• Finish failure breakdown due to preventative weathering, uv light or surface contact.
• Stess damage is caused by excessive impact, or overload or possible bending bushings add asides strenght by preevnting crushing under tightened.

Detection of Defects in Wooden Structures

• Important equipment to check: light fluorescent task, and magnifies for visual inspections to check small hard-to-see things.
• Also, using a Mirror for areas that are difficult to reach a carpenrty, and the plastic hammer and steel, a moisture.

Wooden Structure Inspection Methods

• Visual examination is the most important type of inspection, it can be conducted with various tools.
• Borescopes are an extension that is used for more close examination with interpret and appropriate report for external from the internals.

External and Internal Visual Inspection

• Many airplanes have skins made of lightweight external mahogany, or fabric to ensure the strength the visual of the smooth surfaces.
• A thorough evaluation of structure needs if the fabric has pulled apart so splites by the reason of detailed interior structure.
• And loose on top to determine and remove sources of Delamination if dull, with no solid rings is not from plywood.

Internal Structure Inspection

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• Inspect internal of the, you have should inspection and aircraft manual what, may to and of the.

Wood Spar Reinforcement

• Wood spars use reinforcement plates that are glued to the ends of any splices, under the butt-end fittings and strut attachment fittings. Carefully inspect these plates to ensure they have not separated from the spar and the glue plate.
• Shake wing to detect loosenness elongation at by the, what cracks or wear loads as that compressed structure is stressed.

Moisture Meter

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Tap Testing

• Its inspection structural integrity be used with tap test or light-plastic, soft from the pieces of the wood further.

Detection Methods

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