PDF- -Chapter 3 Aerodynamics of Flight - Chapter 3 Wood





Since the time the Wright brothers built their first airplane of wood and fabric,

there have been major advances in aircraft construction


and later as aluminum in monocoque-type construction,

were a quantum leap forward in terms of the ability to manufacture aircraft quickly and economically


wood was used on early aircraft because of its availability and relatively high strength-to-weight ratio

Because wood is a resilient material when properly maintained,

many older wooden aircraft still exist,

while a few modern designs continue to use wood in select components

This information is only presented as an overview,

information on wood or composite repairs for a specific aircraft should be referenced from the applicable aircraft's structural repair manual

In the event such manuals do not exist,

Acceptable Methods,


and Practices/Aircraft Inspection and Repair,

and approve all major structural repairs through the FAA via Form 337


Although wood was used for the first airplanes because of its favorable strength-to-weight ratio,

it is primarily the cost of the additional hand labor needed for wood construction and maintenance that has caused wood aircraft to become almost entirely superseded by those of all-metal construction


there are still many home-built airplanes that feature wood construction,

commercial designs intended for low-volume production appear using some degree of wood in their structures

excellent shock-resistance qualities

Reputable companies that sell wood for use in aircraft repairs,

stringently inspect and verify that the wood product meets the appropriate FAA specifications

To meet the "Aircraft Sitka Spruce" grade specification,

the lumber must be kiln-dried to a government specification known as AN-W-2

This specification requires that the specific gravity shall not be less than

the slope of the grain shall not be steeper than 1 to 15,

the wood must be sawn vertical-grain (sometimes called edge-grained),

and shall have no fewer than six annular rings per inch

Each of these specification characteristics is discussed in detail later in this section

Most Sitka spruce now comes from British Columbia and Alaska due to the depletion of old growth spruce forests in the United States,

thus making quality spruce valuable and occasionally,

Figure 3-1

This Bellanca Viking incorporates wooden spars in its airframe structure

WOOD SUBSTITUTION Other types of wood are also approved for use in aircraft structures


the wood species used to repair a part should be the same as the original wood whenever possible

If using a wood substitute,

it is the responsibility of the person making the repair to ensure that the wood meets all of the requirements for that repair

If a substitute wood product meets the same quality standards as the original wood,

it is considered an acceptable alternative

For example,

you may substitute laminated wood spars for solid-rectangular wood spars as long as they are manufactured from the same quality wood and they are produced under aviation standards

This section will provide information on the materials,

For a detailed description of the components and function of aircraft structures,

refer to Chapter One of this textbook,

Aircraft Structures and Assembly & Rigging


Wood and adhesive materials used in aircraft repair should meet aircraft (AN) quality standards and be purchased from reputable distributors to ensure such quality

Strict adherence to the specifications in the aircraft structural-repair manual will ensure that the structure will be as strong as the original

WOOD Sitka spruce is the reference wood used for aircraft structures because of its uniformity,

If there is any question about the suitability of a specific piece or type of wood for a repair,

it would be wise to get the approval of the aircraft manufacturer or local FAA inspector before using it on the aircraft

[Figure 3-2] PLYWOOD Structural aircraft-grade plywood is more commonly manufactured from African mahogany or American birch veneers that are bonded together in a hot press over hardwood cores of basswood or


and Transparent Plastic Structures

Species of Wood

Strength Properties as Compared to Spruce

Maximum Permissible Grain Deviation (Slope og Grain)


Excellent for all causes

Considered as standard for this table

Douglas Fir

Exceeds spruce

May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are substantiated

Difficult to work with hand tools

Some tendency to split and splinter during fabrication

Large solid pieces should be avoided due to inspection difficulties

Gluing satisfactory

Noble Fir

Slightly exceeds spruce except 8 percent deficient in shear

Satisfactory characteristics with respect to workability,

May be used as direct substitute for spruce in same sizes providing shear does not become critical

Hardness somewhat less than spruce

Gluing satifactory

Western Hemlock

Slightly exceeds spruce

Less uniform in texture than spruce

May be used as direct substitute for spruce

Gluing satisfactory

Northern White

Properties between 85 percent and 96 percent those of spruce

Excellent working qualities and uniforn in properties but somewhat low in hardness and shock-resisting capacity

Cannot be used as substitute for spruce without increase in sizes to compensate for lesser strength

Gluing satisfactory

White Cedar,

Port Orford

Exceeds spruce

May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are substantiated

Easy to work with hand tools

Gluing difficult but satisfacory joints can be obtained if suitable precautions are taken


Slightly less than spruce except in compression (crushing) and shear

Excellent working qualities

Should not be used as a direct substitute for spruce without carefully accounting for slightly reduced strength properties

Somewhat low in shock-resisting capacity

Gluing satisfactory

Figure 3-2

Only certain species of wood are suitable for aircraft structures

This figure outlines the different types of wood approved for aircraft structural repair along with the characteristics and properties of each type in comparison to the standard,

Sitka spruce

Basswood plywood is another type of aviation-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength

All aviation-grade plywood is manufactured to specifications outlined in MIL-P-6070,

which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies meets specifications

Laminated wood is stronger but less flexible than a piece of solid wood of the same type and size


laminated wood is much more resistant to warping than solid wood,

making it a good substitute for solid wood components such as laminated spars in place of solid spars

Laminated wood is most commonly utilized for components that require curved shapes such as wing-tip bows and fuselage formers

LAMINATED WOOD Laminated wood is constructed of two or more layers of solid wood that are bonded together

The lamination process differs from the plywood process in that each layer of laminated wood is bonded with the grain running parallel with each other


is constructed of wood layers that are bonded with the grain direction at a 90 angle to the previous layer

WOOD ASSESSMENT Aircraft technicians who take on a wooden structural repair must be able to properly assess the wood used

Familiarity with the quality and condition of the wood along with the types of defects inherent to wood products is essential to competent wood assessment

The technician must make certain that the quality of wood meets the original specifications outlined in the aircraft's repair

The following information describes wood characteristics that the maintenance technician must consider for proper wood assessment,

not only for the initial use of a wood product,

but also in the inspection phase of wooden structures


and Transparent Plastic Structures

which stresses the importance of accurate identification of wood defects

Following are several wood defects the technician must be able to identify to properly assess wood quality

The cut of the wood,

and the number of growth rings are factors to examine when determining quality

The way wood is cut affects its shrinkage characteristics and strength qualities

Aviation-quality wood is usually quarter-sawed to reduce the amount of shrinkage over the life of the component

Quarter-sawn wood is cut from quartered logs so that the annual growth rings are at 90 angles to the wide face

The slope of the grain is another factor to consider when assessing wood

The maximum slope of the grain for aviation-grade lumber is 1:15

The slope of the grain is the amount of grain rise over the grain length

In other words,

the grain may not rise more than one inch in a 15-inch section of wood

Figure 3-4

This figure illustrates several wood defects that a technician must be able to identify when evaluating wood condition and quality

Brown rot

An advanced stage of brown is referred to as "cubical rot" that splits the wood along rectangular planes

It develops during drying and is commonly caused by differences in radial and tangential shrinkage or because of uneven shrinkage of the tissues in adjacent portions of the wood

Figure 3-3

According to FAA standards,

a grain slope of 1:15 is the maximum allowable slope allowed in aviation-grade wood

Another factor to consider when assessing wood is the number of growth rings per inch

To accurately calculate the number of rings,

look at the end of the board and count the number of growth rings in one inch

The minimum grain count for softwoods is six rings per inch

Port Oxford white cedar and Douglas fir are exceptions

The minimum grain count for these woods is eight rings per inch

Certain defects are allowed and others disallowed when choosing the appropriate species of wood,

Compression failure

Compression failures vary from pronounced failures to very fine hairlines that require close inspection

This defect is caused from the wood being overstressed in compression due to: natural forces during the growth of the tree,

felling trees on rough or irregular ground,

or rough handling of logs or lumber

Compression wood

it has the appearance of an excessive growth of summerwood

Compression shows little or no contrast in color between springwood and summerwood,

making it a difficult defect to identify

If you have any doubt whether a piece of wood is compression wood or not,

Cross grain Wood in which the direction of the fibers or grain deviate from a line parallel to the sides

Crossed grain may look like diagonal grain,


and Transparent Plastic Structures

Curly grain

The area covered by each curl may vary up to several inches in diameter

may also be referred to as "dote

" Red heart and purple heart are also forms of decay

Dry rot

crumbly rot but especially a wood easily crushed to dry powder in its advanced stage

Hard knots

at least as hard as the surrounding wood,

and shows no indication of decay


extending from the pith to the sap,

which is usually darker in color

This part of the wood contains dead cells that no longer participate in the life processes of the tree

Interlocked grain

then alternate in a spiral direction every few years

Such wood is very difficult to split radially,

although it may split easily in the tangential direction

or that has been surrounded by subsequent stem growth

Mineral streaks

Mineral staining is common in hard maple,

Pin knot clusters Pin knots are knots with diameters less than or equal to 1/2 inch

Several pin knots in close proximity to each other make up a cluster

Pitch pocket

May be empty or may contain liquid or solid resin

Spike knots

Spike knots appear most frequently in quarter-sawn lumber

Spiral grain

Spiral grain is a form of a cross grain

• Shakes A separation or crack along the grain,

the greater part of which may occur at the com mon boundary of two rings or within growth rings • Wavy grain


Certain types of wood defects are permitted in aviation-grade lumber

The following list of permissible wood defects applies to the species of wood listed in figure 3-2

Spiral grain,

or a combination of the two is acceptable providing the grain does not diverge from the longitudinal axis of the material more than specified in col umn 3 of figure 3-2

Acceptable if local irregularities do not exceed limitations specified for spiral and diagonal grain

hard knots up to 3/8 inch in diameter are acceptable providing: (1) they are not projecting portions of I-beams,

along the edges of rectangular or beveled un-routed beams,

or along the edges of flanges of box beams except in lowly stressed portions

and 3) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect

Small clusters are acceptable providing they produce only a small deviation of grain direction

Acceptable in the center portion of a beam providing they are at least 14 inches apart when they lie in the same growth ring and do not exceed 1-1/2 inches in length by 1/8 inch in depth

Acceptable providing that there is no decay indicated anywhere on the wood


While there are certain defects that are allowed in wooden aircraft structures,

there are more that are not acceptable

Choosing a section of wood with non-acceptable defects increases the chance of future structural failure

The following is a list of non-acceptable wood defects

Not acceptable unless they are within the limitations specified in the descrip tion of acceptable cross-grain defects listed pre viously

Not accept able unless they are within the limitations speci fied in the description of acceptable defects listed previously

Not acceptable unless they are within the limitations specified in the descrip tion of acceptable defects listed previously

Not acceptable if they produce a large effect on the direction of the grain

Reject wood that contains this type of defect

Not acceptable unless they are within the limitations specified in the descrip tion of acceptable defects listed previously

Not acceptable if any decay is found

Reject wood contain ing these defects

Reject 'wood that indicates compression wood

Reject wood that contains an obvious compression failure

If there is a ques tion as to whether wood indicates compression failure,

perform a microscopic inspection or toughness test

Reject wood that indicates any form of decay or rot including indications of red heart or purple heart

AIRCRAFT ADHESIVES/GLUES The adhesive used in aircraft structural repair plays a critical role in the overall finished strength of the structure

The maintenance technician must only use those types of adhesives that meet the performance requirements necessary for use in aircraft structures

Not every type of glue is appropriate for use in all aircraft repair situations

Because of its importance,

use each type of glue in strict accordance with the aircraft and adhesive manufacturer's instructions


Most older airplanes were glued with casein glue,

which was a powdered glue made from milk

Casein glue deteriorates over the years after it is exposed to moisture in the air and to wide variations in temperature

Many of the more modern adhesives are incompatible with casein glue

If a joint that has


and Transparent Plastic Structures

been glued with casein is to be re-bonded with a different type of glue,

scrape all traces of the casein away before applying the new glue

The alkaline nature of casein glue may prevent the new glue from curing properly,

thereby compromising the structural integrity

The performance of casein glue is considered inferior to other available products and should be considered obsolete for all aircraft repairs

Plastic resin glue is a urea-formaldehyde resin that is water-,

This type of glue usually comes in a powdered form

Mix it with water and apply it to one side of the joint

Apply a hardener to the other side of the joint,

clamp the two sides together and the adhesive will begin to set

Mix plastic resin glue in the exact proportions specified by the manufacturer,

otherwise the adhesive properties may be impaired

Plastic resin glue rapidly deteriorates in hot,

making it obsolete for all aircraft structural repairs

Any use of this type of glue for aircraft repair should be discussed with the appropriate FAA representative prior to use on certificated aircraft

Resorcinol glue is a two-part synthetic resin glue consisting of a resin and a hardener and is the most water-resistant of the glues used

The glue is ready for use as soon as the appropriate amount of hardener and resin has been thoroughly mixed

Resorcinol adhesive meets the strength and durability requirements of the FAA,

making it one of the most common types of glue used in aircraft wood-structure repair

follow the aircraft manufacturer's recommendations when choosing the type of glue for any structural repair

Phenol-formaldehyde glue is most commonly used in the manufacturing of aircraft-grade plywood

Phenol-formaldehyde glue requires high curing temperatures and pressures making it impractical for use in the field

Epoxy resins are two-part synthetic resins that generally consist of a resin and a hardener mixed together in specific quantities

Epoxies have excellent working properties and usually require less attention to joint quality or clamping pressures as compared to other aircraft adhesives

They penetrate evenly and completely into wood and plywood structures


varying degrees of humidity and temperature affects the joint durability in different epoxies

Only use the recommended epoxy as outlined in the aircraft's repair manual


and Transparent Plastic Structures


The bonding process is critical to the structural strength of an aircraft wooden structure

To ensure the structural integrity of a wood joint,

the bonding process must be carefully controlled

It is imperative to follow the manufacturer's repair procedures in detail when producing a wood joint

Following are the three most important requirements for a strong and durable structural bond

• Preparation of the wood surface prior to apply ing the adhesive

• Utilization of a good quality aircraft-standard adhesive that is properly prepared

tration quotient of the adhesive in these cases only


heavy sanding will change the flatness of the wood and deposit sawdust in its pores

make sure the surfaces are clean and dry before applying adhesive

APPLYING THE ADHESIVE When the wood surfaces are prepared and ready to be glued,

even coat of glue to each surface

following the adhesive manufacturer's recommended procedures,

It is important to observe the orientation of the 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

An acceptable cut of wood has been cut nearly parallel to the direction of grain

• Performing a good bonding technique consistent with the manufacturer's instructions

WOOD PREPARATION It is imperative to properly prepare the wood surface prior to applying any adhesive

The wood surface must be clean,

otherwise the adhesive will not penetrate the wood evenly

Without proper adhesive penetration,

you will not gain a proper glue line,

The glue provides the strength of a properly prepared wood splice-joint

In addition,

wood changes in dimension according to its moisture content,

the pieces of wood to be joined should be kept in the same room for a minimum of 24 hours to equalize the moisture content

Cut the wood to the required bevel with a fine-toothed saw,

then plane or scrape the surface until it is smooth and true

Planer marks,

and other surface irregularities are not permissible

The wood joint must join evenly over the entire bonded surface to produce a strong and durable bond

Do not use sandpaper to smooth the surface

Sanding may round corners and change the flatness of the wood surface resulting in a joint that does not properly meet

Sanding also produces dust that fills the ■wood pores and causes a weak glue line

Roughening the wood surface is also not recommended because it will prevent uniform contact of the wood surface,

which is necessary for strong and durable glue joints

Before applying adhesive to the joint surfaces,

vacuum them to remove anything that remains which may prevent glue penetration

When wood surfaces cannot be freshly machined before bonding,

such as plywood or inaccessible members,

sand them lightly using a very fine grit such as 220

Very light sanding improves the pene-

Figure 3-5

Avoid end-grain joints when gluing wood scarf joints

Make sure the wood is cut with the grain of both pieces as close to parallel as possible

Using end-grain joints increases the chance of future warping

Almost all types of adhesives have four time-periods that are critical to the bonding process

Pot life is the useable life of the glue from the time it is mixed until the time it must be used

Discard the glue once the pot life has expired

Using glue after the pot life has expired or adding thinners to the adhesive will not extend its life

The open-assembly time is the allowable time between the application of the glue and the time the joint is assembled

If the open-assembly time is too long,

the glue will begin to set up on the joint surfaces and the glue line will weaken

Different types of adhesives have varying open-assembly times

Follow the adhesive manufacturer's procedures explicitly when bonding a structure

The closed-assembly time is the allowable length of time between the assembling of the joint and the

application of the clamping pressure

Closed-assembly time allows for the movement of parts to place them in the proper alignment

The pressing time is the period during which the parts are pressed or clamped together and is essentially the adhesive curing period

Pressing time must be sufficient to ensure that the joint is strong enough to withstand manipulation or the machining process

The temperature of the bond line also affects the cure rate of the glue

Each type of glue requires a specific temperature during the curing cycle

Figure 3-6

This chart outlines an example of a manufacturer's recommended bonding times,

and assembly temperature for a specific type of plastic resin glue

Each type of adhesive has specific time-periods and procedures to be followed absolutely

If you waver from the manufacturer's procedures,

you must either discard the wood parts or remove the adhesive,

The structural integrity of the joint will be compromised if the manufacturer's procedures are not followed to the letter

CLAMPING PRESSURE When the joint is connected and properly aligned,

apply pressure to spread the adhesive into a thin,

continuous film between the wood layers

The strength of a glue line is partially dependent upon the correct pressure applied during the curing process

Clamping forces air out of the joint and brings the wood surfaces together evenly

Too little clamping pressure results in thick glue lines and weak glue joints

Too much clamping pressure can squeeze out too much glue weakening the joint

Clamping pressure is accomplished using clamps,

Each type of adhesive requires a specific amount of clamping pressure


follow the adhesive manufacturer's gluing procedures in detail

For example,

the recommended clamping pressure for soft-woods is between 125 and 150 psi and between 150 and 200 psi for hardwoods when using resorcinol glue


and Transparent Plastic Structures


In addition to the amount of clamping pressure,

the method used to apply pressure is also important

Different methods range from the use of brads,

to the use of hydraulic and electrical presses

The choice of clamping method is important to achieving a strong and durable joint

Hand nailing is one method of applying pressure using small nails or screws in the bonding of ribs,

attachment of plywood skins to the wing,


both nails and screws can produce adverse effects such as splitting small parts and creating points where moisture may enter the wood structure causing decay

If you decide to utilize the hand nailing method,

nailing strips are often used to spread the pressure over a larger area and to help in the removal of the nails after the glue has cured

To prevent the nailing strip from sticking to the wood structure,

place a piece of waxed paper between the strip and the structure

The nails or screws used in the hand nailing method may or may not be removed after the adhesive has cured

Nails used for clamping pressure are not intended to hold the structure together for strength purposes

When using nails,

be careful not to crush the wood with heavy hammer blows and do not penetrate all the way through the wood structure

Figure 3-7

Nail strips may be used for clamping pressure on plywood skin during the bonding process

Remove nail strips once the glue is cured

Before applying the finish,

fill the nail holes with a manufacturer's recommended wood filler to prevent any areas at which moisture may enter the structure

Another common method is the use of screw clamps or "C" clamps in conjunction with pressure blocks

Pressure blocks distribute the clamping pressure and protect the members from local crushing

Clamps and pressure blocks apply pressure evenly over the entire glue joint to form a thin,


and Transparent Plastic Structures

Figure 3-8

Apply pressure evenly over the entire joint to avoid gaps between the mating surfaces

An even clamping pressure ensures that the adhesive is squeezed out of the glue-joint uniformly

Insufficient or uneven pressure usually results in thick bond lines that weaken the joint

Apply pressure to a joint for the time recommended by the glue manufacturer

When the clamping pressure is removed,

clean and inspect the joint and remove any glue that has been squeezed from the joint


To effectively inspect wood structures,

be familiar with methods of inspection and the equipment used to examine them

be able to identify the types of defects that are common to wood structures,

as well as the failure modes that are unique to them

Most wood damage is caused by conditions such as moisture,

Because wood is an organic material,

it is subject to mildew and rot unless protected from moisture

Keep wood airplanes in well-ventilated hangars and take special care to ensure that all of the drain and ventilation holes remain open

If a ventilation hole becomes obstructed,

changes in air temperature will cause moisture to condense inside the structure,

which will cause the wood to deteriorate

TYPES OF DETERIORATION The maintenance technician must be able to identify wood deterioration to determine the airworthiness of a wood structure

Along with the list under Wood Defects discussed earlier in this section,

the following are several of the more common types of wood deterioration

Wood decay results from the attack and growth of fungus upon wood products

Decay is indicated by softness,

Musty or moldy odors also indicate wood decay

• Splitting or cracking of a wood member may occur due to the varying shrinkage rates of bonded wood members,

or due to an outside force applied to the structure

Wood splits often result when different types of woods are bonded together

For example,

bonding a mahogany ply wood doubler to a spruce member may produce a split

As the spruce dries,


which shrinks at a lower rate,

holds the spruce firmly in place

The induced stress in the spruce member exceeds its cross-grain strength,

• Bond failure is most commonly due to an improper bonding process or prolonged exposure to moisture

Using the wrong type of glue,

not fol lowing the manufacturer's bonding procedures or improper wood preparation can all lead to bond failure of the wood joint

• Finish failure is the breakdown of the protective finish applied to the wood structure to prevent decay

Finish failure results from long-term expo sure to water,

• Stress damage is caused by excessive impact,

or aerodynamic loads imposed upon the wood structure

Over-tightening of fittings can also cause wood crush and possible bending of the metal fittings

Some applications use steel bush ings to prevent the bolts from being tightened to a point where the wood is crushed

Such bushings also add bearing strength to the assembly

INSPECTION METHODS When inspecting a wood structure aircraft,

Before beginning the inspection,

remove all of the inspection and access panels to facilitate the drying of the wooden structures

One of the first steps is to check the moisture content of the wood using a moisture meter

If the moisture content is high,

dry the wood structures before inspecting further

Wooden structures of the aircraft need to be dry to be able to effectively determine the condition of the bonded joints

The following are several inspection methods and associated equipment employed for inspecting wooden structures


Use moisture meters to determine the moisture content of the wood structure

The moisture content of any wooden member is an important factor in its structural integrity

Wood that is too wet or too dry may compromise the strength and integrity of the structure

A moisture meter reads the moisture content through a probe that is inserted into a wooden member

Use a correction card to correct for temperature and the type of wood being tested


The wood structure may be inspected for structural integrity by tapping the suspect area with a light plastic hammer or screwdriver handle

Tapping should produce a sharp,

solid noise from a solid piece of wood

If the wood area sounds hollow or feels soft,


If soft,

hollow wood is found during the tap test,

probe the suspect area with a sharp metal tool to determine whether the wood is solid


the wood structure should feel firm and solid when probed

If the area feels soft and mushy,

wood has rotted and disassembly of the structure is necessary to repair or replace the damaged area

Use prying to determine whether a bonded joint shows signs of separation

When prying a joint,

be cautious not to use too much force,

otherwise you may forcibly separate it

Light prying is sufficient to check the integrity of a joint

If there is any movement between the wood members of the joint,

a failure of the bond is confirmed

Repair or replace the bonded structure if a failure has occurred


Smell is a good indicator of musty or moldy areas

When removing the inspection panels,

be aware of any odors that may indicate damage to the wood structure

Odor is an essential indicator of possible wood deterioration

Musty and moldy odors reveal the existence of moisture and possible wood rot


Visual inspection techniques are used to determine any visible signs of damage

Both internal and external visual examinations are imperative to a complete inspection of the wood structure

External Visual Inspection

Many airplanes that have an external skin made of thin mahogany plywood are covered with lightweight cotton or polyester fabric to increase both the strength and smoothness of its surface

A thorough inspection is required to ensure that the fabric covering has not pulled loose or torn away from the wood

A split or tear in the fabric could be an indication of internal damage to the wooden structure


any known surface damage requires a careful inspection of the internal structure

Minor bulging in the panels of a very light plywood structure may be acceptable

Refer to the aircraft manufacturer's repair manual for detailed specifications


large bulges or any indication of the skin loosening requires careful examination to deter-


and Transparent Plastic Structures

mine the source and extent of the damage

It is possible for the layers of plywood to separate,

which is indicated by a slight hump in an otherwise smooth skin

Tap the suspected area with a coin

If the tapping produces a dull thudding noise rather than a solid ringing sound,

it is possible that the plywood has delaminated

Determine the extent of the damage and repair or replace the skin

Internal Visual Inspection

The most likely place for wood deterioration to begin is the lowest point inside an aircraft's structure while the airplane is in its normal ground attitude

Dirt collects at these low points and holds the moisture against the wood until the protective coating is penetrated,

Since wood is an organic material,

it is subject to mildew and rot unless it is adequately protected from moisture

For the best protection,

treat wood structures with a rot-inhibiting sealer,

after the sealant has dried for a specified length of time,

cover the entire structure with good quality varnish

Open and examine the internal structure if there is any reason to suspect glue failure or wood rot

This may entail creating inspection openings or even removing part of the skin

If any opening must be made,

use procedures that are approved by the aircraft manufacturer or by a local FAA inspector

When the area of suspected damage is accessible,

carefully scrape away all of the protective coating and examine the wood and glue lines

Be suspicious of any stains in the wood

Stains usually accompany decay and wood rot

Perform a probe test in the suspected area with a sharp point,

If the wood pulls up in a chunk,


it is usually an indication that the wood is sound

Remove and replace any wood that shows signs of decay

Carefully check all of the glue lines for any indication of separation

Inspect glue lines with a magnifying glass,

and then try to slip a thin feeler-gauge blade into any portion of the glue line that seems to be separated

If the blade inserts into the crack,

the joint is not sound and must be repaired using the methods recommended by the aircraft manufacturer

To determine whether the glue failed or if the joint was forced apart,

examine the surfaces of the damaged joint

If the joint separated and the glue surface showed an imprint of the wood but no wood fibers attached to the glue,


if something physically forced the joint apart,

pieces of wood would be attached to the glue surface

If there are any wood screws in the area where decay is suspected,

remove them and check to see if they


and Transparent Plastic Structures

show any signs of corrosion or water stains

Replace the screw if the old one shows no indication of corrosion and the wood shows no sign of decay in or around the screw hole

Replace it with a screw of the same length but of the next larger size

Be sure that the replacement screw is made of the material specified in the aircraft's illustrated-parts manual

Wood spars utilize reinforcement plates made of birch plywood that are glued to the ends of any splices,

and the strut attachment fittings

Carefully inspect these plates to ensure that they have not separated from the spar

If a glue line failure is indicated between the spar and the plate,

remove the plate and all traces of the glue then install a new plate

Shake the wing to detect any looseness between the struts and the wing spar

Any movement indicates possible elongation or wear at the bolthole

In this case,

remove the bolts and carefully examine them and the boltholes for wear,

An elongated bolthole or any cracks near them require that you splice in a new section of spar or replace the entire spar

The manufacturer's repair manual outlines acceptable tolerances

If a wooden structure has been subjected to any unusual strain or extreme loads,

carefully inspect the main load-carrying members for any indication of compression failure on the side that carried the compressive load

A compression failure usually appears as a fine line across the grain,

the fibers in the wood have actually been ruptured

Replace any wood that shows this type of failure


The basic criterion for any aircraft repair is that the repaired structure must not only be as strong as the original structure,

but the rigidity of the structure and the aerodynamic shape must also be equivalent

Materials used for the repair of a wooden structure should be the same as the original unless they have become obsolete

If substitutions are made,

they must produce a repair that meets the basic requirements of the manufacturer and the FAA

WING SPAR REPAIRS There are several types of wooden spars that are likely to be encountered in aircraft construction

Each type of spar is unique in design and requires specific repair procedures

Reference the aircraft manufacturer's repair manual for specific repair requirements

Some of the most common wood spars include solid spars,

laminated spars with rectangular cross sections,

and externally routed spars with cross sections resembling I-beams

The I-beam spar is routed to reduce weight while still providing adequate strength requirements

You may also encounter built-up box spars that utilize upper and lower flanges of solid spruce with webs of plywood

Other types include built-up I-beam spars with spruce webs and flanges,

as well as internally routed box-spars made of two rectangular pieces of spruce glued together then routed to reduce weight

Figure 3-9

A technician can encounter several types of wooden wing spars that require different repair techniques


If an inspection reveals a longitudinal crack in a solid wood spar,

repair it by carefully scraping away the finish on both sides of the spar and gluing reinforcing plates of spruce or plywood on each side of it

Reinforcing plates should be one-fourth as thick as the spar and extend beyond each end of the crack for at least three times the thickness of the spar

Bevel the ends of the reinforcing plates with a 5:1 taper to within 1/8-inch of the thickness of the plate,

and attach the plates with glue,

Nails compromise the structure and produce moisture collection points,

thus increasing the chance of wood decay

[Figure 3-10] Splice or reinforce a wing spar at any point except under the attachment fittings for the wing root,

None of these fittings may overlap any part of a splice

If a splice will interfere with any of these fittings,

you will have to change the design of the repair so that the spar can be repaired without interfering with the fittings

Regardless of the spar type,

allow no more than two splices on a single spar

Attachments for minor fittings,

such as those for drag or anti-drag wires or compression members,

are allowed to pass through a spar splice with certain restrictions

One restriction is that the reinforcement plates for the splice are not allowed to interfere with the proper attachment or alignment of fittings

These fittings include pulley support brack-


and Transparent Plastic Structures

or control surface support brackets

Do not alter the location of these fittings in any way

A second restriction dictates that the reinforcement plates may overlap drag or anti-drag wire or compression member fittings if the reinforcement plates are located on the front face of the front spar or on the rear face of the rear spar

In these situations,

longer length bolts to attach the components

If a solid,

or internally routed spar is damaged on either its top or bottom edge,

providing that all of the damage can be removed without exceeding certain limits

Clean out the damaged material to a depth of no more than one-fourth of the spar thickness

Once the damage is removed,

taper the ends of the area to a 5:1 slope then insert and glue a spruce block


glue spruce or plywood reinforcing-plates to each side of the spar,

making them one-fourth the thickness of the spar and tapered to a 5:1 slope

[Figure 3-11] Splice solid or rectangular wood spars using a scarf repair that requires a taper of 1:10 or 1:12

Glue reinforcement plates to the end of the splice

If you decide to splice the spar without completely disassembling the wing,

take special care to prepare the spar and the repair material

Cut the spar and the new material to the proper scarf angle

Once the cut is prepared,

put the two scarfed ends together and clamp them to a back-up board that is

Figure 3-10

A lack of strength in a solid wood spar caused by a longitudinal split can be restored by the addition of reinforcement plates on each side of the spar


and Transparent Plastic Structures

Figure 3-11

Repair of a damaged edge of a solid wood wing-spar requires replacement of the damaged material

After the plug is placed in the damaged area,

glue reinforcement plates to both sides of the spar to increase its strength

the same width as the spar and thick enough to give good,

Be sure that the new material is perfectly straight and aligned with the original spar,

then clamp it securely with cabinetmaker's parallel clamps or "C" clamps

Once secured,

pass a fine-toothed crosscut saw through the scarf joint to remove material that does not match properly

Blow out all of the sawdust,

loosen the clamps on the new material,

and then tap the ends of the spar pieces to butt the two tightly together

Tighten the clamps again and make another cut to straighten both sides of the scarfed joint so the two pieces of wood match exactly

The strength of a scarf joint depends heavily on making sure the bevel cuts match precisely

To ensure a tight glue joint,

use a very sharp plane or chisel to make a perfectly smooth surface with open pores

Do not use sandpaper to smooth the surface because sawdust will clog the pores and not allow the glue to properly adhere to the wood,

[Figure 3-12] Spread the properly mixed glue on each prepared surface,

being sure that the spar is in correct edge alignment

When the glue has cured for the proper time,

remove the clamps and pressure blocks and inspect the glue line,

carefully cutting away any glue that squeezed from the joint

Once inspected

Figure 3-12

Clamping the two scarfed spar pieces together and making another scarf cut ensures a perfectly matched joint

If the initial cut produces rough or chipped wood,

use a planer to smooth the surface

glue reinforcement plates over each end of the scarf

Make these plates one-fourth the thickness of the spar from solid spruce or plywood

Extend the reinforcement plates across the spar at six times the spar thickness on each side of the scarf line

Taper the ends of the plates to prevent an abrupt change in the cross sectional area of the repaired spar

Because the alignment of the fittings and attachments to a spar are critical,


and Transparent Plastic Structures

Figure 3-13

A splice for a solid wood wing-spar requires reinforcement plates on each end of the splice

not to drill the new boltholes in the spar until the splice is completed

[Figure 3-13] Routed I-beam spars are spliced in much the same manner as solid rectangular spars

The exception is that the reinforcement plates installed on a routed I-beam spar must be one-half the thickness of the spar web and contoured to fit into the routed portion of the spar

A built-up I-beam spar repair requires a 10:1 to 12:1 scarf joint between the original spar and the new material

This type of repair requires that you place solid spruce filler-blocks in between the spar flanges for added support

It also requires plywood reinforcement plates,

one-half the spar web thickness,

to be glued to the spar flanges and filler blocks to make a box-type repair at the splice

Figure 3-14

Repair to built-up I-beam wood spars includes filler blocks and plywood reinforcement plates


and Transparent Plastic Structures

Figure 3-15

The splice for a built-up wood-box spar is critical

This type of spar carries the heaviest loads of all wood spars

Built-up box spars carry the greatest loads of any of the wooden spars

For this reason,

the built-up box spar repair is the most critical,

and therefore requires the use of approved drawings as proper guidelines

The typical built-up box spar repair consists of removing portions of the webs from both sides of the spar and cutting the flanges to a 10:1 to 12:1 taper

Then spliced in new flanges and spruce reinforcement plates,

one-half the thickness of the flanges,

and install them on the inside of the spar

[Figure 3-15] When replacing the webs on repaired box-spars,

install spruce filler blocks that are the same thickness as the flanges between the flanges

Scarf the undamaged portion of the web to a taper of 10:1 and install a filler-block at a point centered under each scarf joint

Stagger the scarf cuts in the two webs along the spar rather than directly across from each other to improve the strength of the webs

At this point,

glue and nail the filler-block in place

Cut the new web section to an exact fit,

then glue and nail it in place

When the glue has cured and all of the excess glue is removed,

glue and nail a plywood cover strip over the end of the splice

[Figure 3-16] WING RIB REPAIRS Wood wing ribs are usually made of spruce strips that have a cross section of approximately 1/4 to 5/16-inch

These small strips of wood accept the air loads from the covering of the wing and transmit them into the spars

Figure 3-16

The splice for the web of a built-up wood box spar incorporates both filler blocks and plywood cover strips

When manufacturing wing ribs,

soften the upper and lower cap strips with steam before bending them over a form

Cap strips are the upper and lower surfaces that attach to the supporting web

These strips carry the bending loads of the wing and provide a surface for attaching the wing skin

When dry,

place them in a jig and cut all of the cross members to fit between them

Cover each


and Transparent Plastic Structures

intersection between a vertical member and a cap strip with a gusset made of mahogany plywood

Glue the gussets to the strips and secure them with brads to provide the pressure needed to make strong glue joints

Slip the completed ribs over the spars and assemble and square up the wing truss with the drag and anti-drag wires adjusted to the proper tension


If a cap strip is broken between two of the upright members,

cut the strip to a taper 10 to 12 times its thickness

Then cut a new piece of the same type material with a matching taper

Cut a reinforcing block of spruce the same size as the cap strip and 16 times as long as its width,

and glue it to the inside of the cap strip

Then cover both the cap strip and the reinforcement with plywood faceplates that are glued to the strip and held with brads

Figure 3-18

Add splice plates to repair a cap strip broken over an upright member

When cutting splice plates,

ensure that the grain is parallel to the grain of the cap strips to reduce the chance of warpage

Figure 3-17

Use a rib cap-strip splice to repair a cap strip broken between two upright members

If the damage is located above one of the upright members,

cut the cap strip with a 10:1 to 12:1 taper with the center of the cut over the upright member

Then splice a new piece of cap-strip material into the structure

The upright member serves as the reinforcement so no block is needed under the splice

Put splice plates of thin plywood on each side of the splice so that none of the joints depends upon end-grain gluing

[Figure 3-18] It is sometimes necessary to replace only the leading or trailing edge portion of a rib,

so cap strips may be cut at a spar

When the cap strips are cut over a spar,

and glue gussets of plywood the same size as the original to each side of the rib

Figure 3-19

Use original size gussets on a rib-cap splice that is located over a spar


and Transparent Plastic Structures


The trailing edge of a wing is the area most likely to be damaged by moisture collecting and causing the wood to rot

All wings must incorporate drainage grommets at the lowest part of each rib bay to drain accumulated moisture

Drainage grommets also ventilate the compartment to prevent condensation


grommets will clog with dirt and not allow adequate drainage


moisture will collect around the wood structures producing an environment ripe for decay

If there is any movement when you flex the trailing edge,

cut away the trailing edge fabric and examine the edge structure

If the rear end of the rib has rotted,

cut it away and cut a spruce block to fit the removed rib section

Then cut reinforcing plates of plywood,

and fasten new trailing edge finishing materials to the repaired rib

Be sure to treat the wood repair with a rot-resistant sealer before re-covering the structure

When replacing corroded metal sections of the support structure,

protect the new metal parts with a corrosion inhibiting primer such as zinc chromate

Most wooden wings utilize metal compression members


wooden compression members are used on certain aircraft

When it is necessary to repair wooden compression members,

use a 10:1 to 12:1 scarf joint

Glue reinforcing plates to each side of the splice that are made of the same material as the strip and 12 times its thickness

Then cover the entire repair with plywood to form a boxed rib

Figure 3-21

Compression rib repairs require reinforcement of the entire splice with plywood to restore the original strength

Figure 3-20

Repair to the trailing edge of a wood wing rib requires replacement of the damaged portion with a wood block

Install a larger gusset to reinforce the structure

PLYWOOD SKIN REPAIRS Aircraft that incorporate plywood skins normally carry a large amount of stress from the flight loads


make repairs to plywood skins in strict accordance with the recommendations of the aircraft manufacturer

If you repair plywood skin exactly as described by the manufacturer or by Advisory Circular 43

Acceptable Methods,


Aircraft Inspection and Repair,

the FAA will most likely approve the repair

If the repair cannot be made according to the approved data,

contact the district agent of the FAA for approval of the proposed method before beginning the work


and Transparent Plastic Structures

Figure 3-22

Use a splayed patch to repair small holes in thin plywood skin

Use circular or elliptical plywood patches in plywood skin repair to avoid the stress concentrations developed by abrupt changes in the cross-sectional areas of square or rectangular patches

Following are several types of plywood patches approved for aircraft applications


Small holes in thin plywood skin may be repaired by a splayed patch

Use this type of patch if the skin is less than or equal to 1/10-inch thick and the hole can be cleaned out to a diameter of less than 15 thicknesses (15T)

[Figure 3-22] To fabricate a splayed patch,

tape a small piece of scrap plywood over the center of the damage

Use it as a rest for the point of a drafting compass,

Draw one circle to form the trim size of the hole,

For the other circle,

the size of the outside of the patch can be no more than 5T beyond the edge of the hole

To produce the patch,

remove the inner circle with a sharp knife,

taper the edges evenly from the outer circle to the edge of the hole

Cut the patch plug from the same material as the original skin and taper it to fit the hole exactly

Apply glue to the tapered edges of the hole and to the taper cut on the patch

Put the patch in place,

aligning the face grain of the patch with the face grain of the skin

Once installed,

place a piece of vinyl plastic or waxed paper over the patch

With a pressure plate cut from scrap plywood that is just slightly larger than the patch,

apply pressure and allow the glue to cure

After the glue has cured,

and finish the repair to match the rest of the surface

SURFACE PATCH If an airplane's plywood skin is damaged,

repair it with a surface patch covered with aircraft fabric and finish it to match the rest of the airplane

This does not produce the best looking repair,

but its simplicity and economy of time and labor make it a suitable repair for most working-type airplanes

[Figure 3-23] PLUG PATCH Make a perfectly flush patch in a section of plywood skin by trimming the damage to a round or oval shape

Put a doubler inside the structure for support,

then glue the plug patch to the doubler

[Figure 3-24] SCARFED PATCH The most difficult type of patch to make on plywood skin is the scarfed patch


because it makes the least change in skin thickness or rigidity,

it is preferred for most stressed wood skin repairs


and Transparent Plastic Structures

Figure 3-23

Use surface patches to repair larger holes and damage in plywood skins

Make the patch of the same material as the damaged skin and run its face grain in the same direction as that of the skin


and Transparent Plastic Structures







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