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Page 1

WOOD, COMPOSITE, AND

TRANSPARENT PLASTIC

STRUCTURES

INTRODUCTION
Since the time the Wright brothers built their first airplane of wood and fabric, there have been major advances
in aircraft construction. Metals, first as steel tubing, and later as aluminum in monocoque-type construction,
were a quantum leap forward in terms of the ability to manufacture aircraft quickly and economically. However,
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 mod-
ern designs continue to use wood in select components. This information is only presented as an overview, and
as such, 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, consult Advisory Circular 43.13-1B,
Acceptable Methods, Techniques, and Practices/Aircraft Inspection and Repair, and approve all major
structural repairs through the FAA via Form 337.

Page 2

AIRCRAFT WOOD STRUCTURES

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entirely superseded by those of all-metal construc-
tion. However, there are still many home-built air-
planes that feature wood construction, and occa-
sionally, commercial designs intended for low-vol-
ume production appear using some degree of wood
in their structures. [Figure 3-1]


Figure 3-1. This Bellanca Viking incorporates wooden spars
in its airframe structure.

This section will provide information on the mate-
rials, inspection, and repair of wood structures. For
a detailed description of the components and func-
tion of aircraft structures, refer to Chapter One of
this textbook, Aircraft Structures and Assembly &
Rigging.

QUALITY MATERIALS
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, strength, and

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 specifi-
cation known as AN-W-2. This specification
requires that the specific gravity shall not be less
than .36, 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 deple-
tion of old growth spruce forests in the United
States, thus making quality spruce valuable and
occasionally, limited in supply.

WOOD SUBSTITUTION
Other types of wood are also approved for use in air-
craft structures. However, 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.

AC 43.13-lB outlines information regarding accept-
able wood species substitutions. 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 com-
monly manufactured from African mahogany or
American birch veneers that are bonded together in
a hot press over hardwood cores of basswood or

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3-34 Wood, Composite, and Transparent Plastic Structures



pre-measured packages described before, cartridges
also eliminate mixing ratio errors. [Figure 3-44]
[Figure 3-45]

epoxy ratios. They eliminate the sticky, messy, and
in some cases, inaccurate hand proportioning of
epoxy resins. [Figure 3-46]




Figure 3-44.The resin and catalyst are divided into separate
containers that are attached on one end. When ready for
use, the partition, which separates the resin from the cata-
lyst, is broken to allow the two to mix. Still within the pack-
age, the resin/catalyst combination is mixed together by
squeezing and kneading the package to thoroughly blend
the mixture. When completely mixed, the package is cut
with scissors and the resin dispensed.


Figure 3-46.This type of epoxy ratio pump offers the advan-
tage of being able to supply adjustable resin ratios. The
resin and catalyst are dispensed through separate tubes so
there is no mixed material in the pump. Mixing resins using
an epoxy ratio pump also helps to increase safety since the
user neither touches the resin or inhales the epoxy resin
fumes during production.




Figure 3-45. To use epoxy cartridges, the seal that separates
the two components must be broken with a plunger. The
materials are then mixed together by moving the plunger in
a twisting and up-and-down motion to thoroughly mix the
resin and catalyst. The label describes how many strokes
are required to give a thorough mix. A needle or syringe
may then be installed onto the end of the cartridge, and the
resin dispensed. Be sure to check the cartridge part num-
ber, shelf life expiration date, and any special instructions.

In addition to prepackaged resin units, epoxy ratio
pumps reduce mixing errors. Epoxy ratio pumps
enable the technician to precisely measure varying

Some resins systems are weighed verses measured
to determine the proper mix ratio. Precision scales
are used to weigh the two parts of the resin. The
scale surfaces should always be clean, and cali-
brated periodically to ensure accuracy. If the type of
resin system used requires refrigerated storage,
allow each part to warm up to room temperature
before weighing and mixing. When cold, a resin will
weigh more than an equal quantity of the same resin
at room temperature.

The resin and catalyst must be mixed thoroughly in
order to achieve maximum strength. Mix resin sys-
tems together in a wax-free container. If a waxed
container is used, the solvents in the resin and cat-
alyst dissolve any wax on the inside of the con-
tainer, which then contaminates the mixture.
Though wax is used for heat control purposes in
some resins, the wax from a container may cause the
repair to cure incorrectly, or possibly not cure at all.
Follow the manufacturer's mixing instructions,
which often entails three to five minutes stirring or
agitation time to completely mix the components.

Resins that are not mixed properly will not cure to
the maximum strength obtainable. If resins are
mixed too quickly, small bubbles may rise into the
air and could get on your skin or in your hair. Do
not be concerned if you have bubbles in the cup
because they will be worked out with a roller or
squeegee during the lay-up process. Vacuum bag-

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Wood, Composite, and Transparent Plastic Structures 3-35



ging, which is a process of applying pressure to a
lay-up, further ensures that no air bubbles remain
in the final composite. If more resin is mixed than
necessary to complete a project, any unused
amount is wasted. If too much resin is mixed, allow
it to cure before throwing it away. In most cases,
resins in a cured condition are not considered haz-
ardous materials for disposal.

A large volume of resin and catalyst causes an
acceleration of the chemical reaction when curing.
When this happens, it starts to cure in the mixing
cup, possibly becoming too thick to work com-
pletely into the fabric. The pot life is also reduced
if large amounts are mixed at one time. If a resin
exceeds its pot life, it must not be used. Smaller
mixed quantities are generally easier to work with
and are more cost effective. One of the best ways
to ensure that a properly prepared batch of matrix
resin has been achieved is to mix enough for a test
sample.

If the work is extensive and takes a long time, the
pot life of the resin mixture may be exceeded if too
much is initially prepared. Find out the length of
the pot life, or working life, of the resin before
preparing a resin batch. Some resin systems have
very short pot lives (15 minutes), while others have
long pot lives (4 hours].

The shelf life of a resin is the time that the product
is still good in an unopened container. Like the pot
life, the shelf life varies from product to product. If
it has expired, the resin or catalyst must be dis-
carded. Using a resin that has exceeded its shelf life
does not produce the desired chemical reaction,
and the strength of the finished product may be
insufficient.

If too much resin is applied to the part, it is called
resin rich. Traditional fiberglass work is used in
nonstructural applications, so extra resin is not that
critical. However, the use of excessive resin in
advanced composite work used for structural appli-
cations is very undesirable. Excessive resin affects
the strength of the composite by making the part
brittle in addition to adding extra weight, which
defeats the purpose for using composites for their
lightweight characteristics.

A resin starved part is one where not enough resin
was applied, which weakens the part. The correct
amount of fiber to resin ratio is important to provide
the structure with the desired strength. In advanced
composite work, a 50:50 ratio is generally consid-

ered acceptable. However, a 60:40 fiber to resin ratio
for advanced composite lay-ups is generally consid-
ered the best for strength characteristics. Actual
ratios utilized should be in accordance with the
manufacturer's instructions.

When working the resin into the fibers, care should
be taken not to distort the weave of the fabric. If too
much pressure is applied when using a brush or
squeegee, the fibers could pull apart, altering the
strength characteristics of the fabric. The curing of
the resins must also be accomplished correctly to
achieve the maximum strength. Be sure to follow
the manufacturer's directions concerning curing
requirements.

CAUTION:If two batches of resin and catalyst are
mixed equally, leaving one batch in a jar and
spreading the other in a thin layer, the one in the jar
will harden rapidly because of the heat trapped by
the glass. This cure rate takes place so quickly that
it will cause minute fractures within the plastic.
The thin sheet, on the other hand, will not cure as
fast because it has a large surface area exposed to
the air, which allows the heat to escape.

SAFETY CONSIDERATIONS
Safety is always important when working with com-
posite materials. Many accidents have occurred
because of the improper usage and handling of
composite materials. Before working with any com-
posite resin or solvent, it is important to know
exactly what type of material you are using and
exactly how to use it.

MATERIAL SAFETY DATA SHEETS (MSDS)
Material safety data sheets contain information on
hazardous ingredients, health precautions,
flamma-bility characteristics, ventilation
requirements, spill procedures, information for
health professionals in case of an accident, along
with transportation and labeling requirements.
MSDS must be available for review in the shop
where the specific material is stored and used.
Review the MSDS and become familiar with the
specific types of materials you come in contact
with before you begin working with the materials.

PERSONAL PROTECTION
Some materials cause allergic reaction and some
people are more sensitive to certain materials
than others. Therefore, it is imperative to protect
your skin from contact with composite matrix
materials. The most effective way to provide skin
protection is by the use of protective
gloves,

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Wood, Composite, and Transparent Plastic Structures 3-67



hardens the surface enough that machining and pol-
ishing can be done without soft spots causing an
uneven surface. [Figure 3-93]


Figure 3-93. Heat treatment of a joint between acrylic pieces

will disperse the solvent through a greater volume of the

plastic and will increase the strength of the joint.

REPAIRS
When windshields and side windows made of
acrylic plastics are damaged, they are usually
replaced unless the damage is minor and not in the
line of vision. Repairs usually require a great deal of
labor, and replacement parts are readily available,
so replacement is normally more economical than
repair.

TEMPORARY REPAIRS
There are times when a windshield is cracked and
must be put in good enough condition to fly to a
location where it can be replaced. In this situation,
make temporary repairs by stop-drilling the ends of

the crack with a number 30 drill to prevent the
crack from growing larger. Drill a series of number
40 holes a half-inch from the edge of the crack and
about a half-inch apart. Lace through these holes
with brass safety wire. Another way to make a tem-
porary repair is to stop-drill the ends of the crack,
and then drill number 27 holes every inch or so
throughout the crack. Use AN515-6 screws and
AN365-632 nuts, with AN960-6 washers on both
sides of the plastic. This will hold the crack together
and prevent further breakage until the windshield
can be properly repaired or replaced. [Figure 3-94]

PERMANENT REPAIRS
Windshields or side windows with small cracks that
affect only the appearance rather than the airwor-
thiness of a sheet may be repaired by first
stop-drilling the ends of the crack with a number 30
drill to relieve the stresses. Then use a
hypodermic syringe and needle to fill the crack
with ethylene dichloride, and allow capillary
action to fill the crack completely. Soak the end of
a 1/8-inch acrylic rod in ethylene dichloride to
soften it and insert it in the stop-drilled hole.
Allow the repair to dry for about half an hour,
then trim the rod flush with the sheet.

POLISHING AND FINISHING
Scratches and repair marks, within certain limita-
tions, can be removed from acrylic plastic. Do not
sand any portion of a windshield that could
adversely affect its optical properties and distort the
pilot's vision. In addition, do not reduce the thick-
ness of windows on pressurized aircraft to the point
where the window would be weakened. The manu-
facturer's service manual will specify the minimum
allowable thickness.


Figure 3-94. When an acrylic windshield is cracked, it can be fixed temporarily with brass safety wire, or with small machine screws

and washers.

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3-68 Wood, Composite, and Transparent Plastic Structures



CLEANING

PROTECTION

WINDSHIELD INSTALLATION

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