Jeppesen 07B - Aircraft Non-metallic Structural Materials

Wooden structures require a great deal of handwork and, therefore, are extremely expensive.

What is the most likely reason why far fewer aircraft today are made of wood?

Solid wood.

This type of wood is used for some aircraft wing spars and is made of one solid piece cut from a log.

Quarter sawing.

When planks are cut from a log in this fashion, the annual rings cross the plank at an angle greater than 45 degrees.

This reduces the chance of warpage as compared to planks that are cut straight across, or plain sawed.

Why is it that most solid wood used on aircraft is cut from a tree by quarter sawing?

Laminated wood.

This type of wood is made of two or three pieces of thin wood glued together with the grain running in the same direction.

Each of the laminations can be spliced so large components can be made from pieces of wood that are not large enough for a solid spar.

Which statement is true when using laminated wood for aircraft spars?

As long as they are both of the same quality and neither has a grain deviation that exceeds the allowable limits.

On what condition does the FAA allow solid and laminated spars to be used interchangeably?

Plywood.

This type of wood consists of three or more layers of thin veneer glued together so the grain of each successive layer crosses the others at an angle of 45 degrees or 90 degrees.

Hardwood.

This species of wood come from deciduous trees having broad leaves.

Softwood.

This species of wood come from coniferous trees with needle-like or scale-like leaves.

Sitka spruce.

It is the most common wood used in aircraft structures.

Sitka spruce.

It is such a high-quality wood that the FAA has chosen this as the reference wood for aircraft construction.

It is relatively free from defects, has a high strength-to-weight ratio, and is available in large sizes.

What is the most likely reason why Sitka spruce is so widely used?

Douglas fir.

The strength properties of this wood exceed those of spruce but it is much heavier. Furthermore, it is more difficult to work than spruce, and has a tendency to split.

Noble fir.

This type of fir is slightly lighter than spruce and is equal or superior to spruce in all properties except hardness and shock resistance. It is often used for structural parts that are subject to heavy bending and compression loads such as spars, spar flanges, and cap strips.

Northern white pine.

This wood possesses excellent working and gluing characteristics, but its strength is slightly lower than that of spruce.

Balsa.

It is an extremely light wood that is grown in South America. Because it lacks structural strength, it is often sliced across its grain for use as a core material for sandwich-type panels that require light weight and rigidity.

Mahogany.

This hardwood is heavier and stronger than spruce. Its primary use in aircraft construction is for face sheets of plywood used as aircraft skin

Birch.

It is a heavy hardwood with very good shock resistant characteristics. It is recommended for the face plies of plywood used as reinforcement plates on wing spars and in the construction of wooden propellers.

Regardless of the species of wood used for aircraft structure, it must have a straight grain.

Which statement is true regarding the grain deviation of wood used for aircraft?

The grain must not slope more than one inch in 15 inches.

A maximum deviation of 1:15 is allowed for woods used in aircraft. What does this mean?

You can see the grain direction by putting a bit of free-flowing ink on the wood.

What practice is done to compensate for the difficulty to determine exactly which way the grain of wood runs?

Knot.

It identifies a point where a branch grew from the tree trunk in a piece of wood.

Spike knots weaken a board and are not acceptable in wood for aircraft construction or repair.

Which statement is true about spike knots?

Conditions acceptable in aircraft wood for hard knots up to 3/8 inch at their maximum dimension.

These knots must not be in the projecting portion of I-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of the flanges of box beams, except in low-stress areas.

Conditions acceptable in aircraft wood for hard knots up to 3/8 inch at their maximum dimension.

These knots must not cause a grain divergence at the edge of the board or in the flanges of a beam more than 1:15.

Conditions acceptable in aircraft wood for hard knots up to 3/8 inch at their maximum dimension.

They must be in the center third of the beam and no closer than 20 inches to another knot or other defect.

If they do not cause excessive grain deviation.

On what conditions are small pin-knot clusters acceptable in aircraft wood?

Acceptable only in the center portion of a beam, provided they are at least 14 inches apart and measure no more than 11/2 inches by 1/8 inch by 1/8 inch deep.

Pitch pockets weaken a piece of wood slightly. On what conditions make these pockets acceptable?

It is represented by the heavier and darker rings.

How are summer woods identified?

Wood having extra wide summer-growth rings indicates the wood was subjected to compressive stresses during its growth, and is denser and weaker than wood that grew normally.

Which statement is true about compression wood?

Compression failure.

This failure occurs when a tree falls and must not be confused with compression wood.

Irregular, thread-like lines across the grain.

If a tree falls across a log, some of the wood fibers on the tree's lower side may buckle. This buckling severely weakens the wood and is identified by:

Check.

It is a crack that runs across the annual rings of a board and occurs during the seasoning process.

Shake.

It is a crack or separation that occurs when two annual rings separate along their boundary.

Split.

It is a lengthwise separation of the wood caused by the wood fibers tearing apart.

As a rule, stains that uniformly discolor the annual rings are evidence of decay.

Which statement is true about stains?

Decay.

It is caused by fungi that grow in damp wood, and is prevented by proper seasoning and dry storage.

Pick at a suspected area with the point of a knife. Sound wood will splinter, while a knife point will bring up a chunk of decayed wood.

Which is the simplest way of identifying decayed wood?

Inserting a knife point into the area and prying a small piece out. If the wood is good, it splinters, while decayed wood crumbles.

Which is the simplest way of detecting dry rot?

Using kiln dried wood with a moisture content of 20 percent or less, and protecting it from the elements.

How is dry rot prevented?

Replacement of the affected material.

What is the only way to repair a wood with dry rot?

Resin.

Plastic is also referred to as:

Thermoplastic resins.

These plastics are hard when in their normal state, but become soft and pliable when heated. When softened, these plastics can be molded and shaped, and retain their shape when cooled.

They are types of transparent thermoplastic materials that are used for aircraft windshields and side windows.

Which of the following statements is true about cellulose acetate and acrylic?

It has a tendency to shrink and turn yellow.

Early aircraft used cellulose acetate plastic because of its transparency and light weight. However, what is the major drawback of cellulose acetate which caused it to be almost completely replaced for aircraft use?

Cellulose acetate.

This thermoplastic resin can be identified by its slightly yellow tint and when it is burned, it produces a sputtering flame and dark smoke.

When acetone is applied to cellulose acetate it softens.

Which statement is true when identifying cellulose acetate with an acetone test?

Acrylic.

These thermoplastic resins are identified by such trade names as Lucite or Plexiglas, or in Britain by the name Perspex.

Acrylic.

This thermoplastic resin is stiffer than cellulose acetate, more transparent, colorless, burns with a clear flame and produces a fairly pleasant odor.

Thermosetting resins.

These plastics usually have little strength in themselves and are generally used to impregnate linen, paper, or glass cloth.

It is impregnated with polyester resin and molded into a desired form.

Both fiberglass cloth and mat have a great deal of strength for their weight, but lack rigidity. To convert fiberglass into a useful structural material,

A styrene monomer is added to polyester resin to thin it and make it more workable.

Polyester resin has a complex molecular structure that, in its pure form, is thick and unmanageable. For this reason,

Inhibitors are added to resins to extend their working time.

A mixture of polyester and styrene eventually hardens into a solid mass if left alone. What is usually done to prevent this?

A catalyst must be introduced to suppress the inhibitors and initiate the curing process.

If left alone, a mixture of polyester and styrene eventually hardens into a solid mass. To prevent this, inhibitors are added to resins to extend their working time. However, when the resin is to be used,

Addition of a measured amount of an accelerator.

The resin's curing time can be appreciably shortened by,

The amount of accelerator needed depends on the ambient temperature and the thickness of the resin layer.

Which statement is true when adding a controlled amount of accelerator to the resin?

A chemical reaction between the catalyst and accelerator generates heat within the resin.

When does the actual cure of polyester resin occur?

A thick layer of resin cures more rapidly than a thin layer.

Which statement is true about the resin's curing time?

Epoxy.

This resin has a low percentage of shrinkage, high strength for its weight, and the ability to adhere to a wide variety of materials.

Epoxy resins require a hardener, or curing agent.

If polyester resins require a catalyst,

One ounce of catalyst is used with 64 ounces of resin.

What ratio is required for polyester resins to mix with their catalysts?

One quart of curing agent with four quarts of resin.

What ratio is required for epoxy resins to mix with their hardeners?

Increase the viscosity of resin.

What is the purpose of a thixotropic agent?

Microballoons.

These hollow glass or phenolic balls range in diameter from about 10 to 300 microns, or about the size of fine sand. These are among the more familiar thixotropic agents used in aviation maintenance.

A thixotropic agent is carefully added to make a paste.

After an epoxy or polyester resin is properly mixed,

Phenol-formaldehyde (urea-formaldehyde).

This thermosetting resin is used to impregnate linen cloth to make control cable pulleys. These pulleys have high strength but do not wear the control cable.

Once cured, this material is stiff and acts as an exceptionally good electrical insulator, but it is quite heavy.

Which statement best describe a phenol-formaldehyde resin?

Glass fiber.

It greatly enhances the strength and durability of thermosetting resin.

The fibers are woven into a cloth.

What is done to glass fiber if it is going to be used for high strength requirements?

The fibers are gathered into a loose mat which is saturated with resin and molded into a desired shape.

What is done to glass fiber if it is going to be used where cost is of greater importance than strength?

Ceramic fibers.

It is a form of glass fiber designed for use in high-temperature applications.

Ceramic fibers.

This glass fiber can withstand temperatures approaching 3,000°F, making it an effective laminate for use around engines and exhaust systems. However, it is heavy weight and expensive.

Kevlar fibers.

It is one of the most commonly used cloth reinforcing fabrics. In its cloth form, it is a soft yellow organic fiber that is extremely light, strong, and tough.

Its great impact resistance makes it useful in areas where damage from sand or other debris can occur.

Why is Kevlar used in areas which include around the landing gear and behind propellers?

It is difficult to work and does not perform well under compressive loads.

What is the disadvantage of using Kevlar fibers?

Graphite fibers.

These are manufactured by heating and stretching Rayon fibers. This produces a change in the fiber's molecular structure that makes it extremely lightweight, strong, and tough.

Fiberglass-reinforced honeycomb.

A high-strength structural material used to provide an electrically transparent, streamlined housing for radar antenna is made of

Most wood cores consist of balsa wood that has been cut across its grain and the end-grain slabs bonded between two face sheets of fiberglass or metal.

Which statement is true about wood cores?

Metal-faced honeycomb.

This type of core is used where abrasion resistance is important.

Synthetic or natural rubber.

These packings are generally used as "running seals" in units that contain moving parts, such as actuating cylinders, pumps, and selector valves.

O-ring.

These packings effectively seal in both directions and are used to prevent both internal and external leakage.

O-ring.

This is the most commonly used seal in aviation.

Prevent the O-ring from being extruded.

Why are backup rings used with O-rings in installations subject to pressures above 1,500 psi?

It is not a reliable or complete means of identification.

Which statement is true about color coding on some O-rings?

O-rings are available in individual hermetically sealed envelopes labeled with all pertinent data.

What is the solution to the problem regarding the lack of reliability of color coding for O-ring identification?

Inspect it for cuts, abrasions, or surface imperfections that could affect its performance.

What should be the practice done before installing an O-ring?

Two backup rings must be used.

For applications such as actuators that subject a seal to pressure from two sides,

A single backup ring is generally adequate.

When an O-ring is subject to pressure on only one side,

They do not deteriorate with age, are unaffected by any system fluid or vapor, and tolerate temperature extremes well in excess of those encountered in high-pressure hydraulic systems.

Which statement best describes backup rings that are commonly made of Teflon?

Any Teflon backup ring can be used to replace any other Teflon backup ring if it is of proper overall dimension required to support the applicable O-ring.

Which statement is true regarding servicing of Teflon backup rings?

Ensure that the surfaces are free from irregularities, that the edges are clean and sharp, and that scarf cuts are parallel.

When inspecting backup rings,

Make sure that the coils do not separate more than 1/4 inch when unrestrained.

When checking Teflon spiral backup rings,

V-ring.

These packings are one-way seals that are installed with the open end of the "V" facing the pressure.

V-ring.

These packings are coded as AN6225.

V-ring.

Each of this packing consists of a male and female adapter that hold the packings in the proper position after installation.

U-ring.

These packings are coded as AN6226.

U-ring.

These packings are primarily low-pressure packings that are used with pressures less than 1,000 psi.

U-ring packings and U-cup packings.

These packings are used in brake assemblies and brake master cylinders.

They seal only in one direction so their concave surfaces must face toward the pressure.

Which statement is true about U-ring and U-cup packings?