FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 1. Wood Structure | 1. Materials and Practices | 1-10. Clamping Pressure
9/8/98
AC 43.13-1B
curing process. Each type of adhesive requires
a specific temperature during the cure cycle,
and the manufacturer’s recommendations
should be followed.
1-10. CLAMPING PRESSURE.
a. Use the recommended pressure to
squeeze adhesive out into a thin, continuous
film between the wood layers. This forces air
from the joint and brings the wood surfaces
into intimate contact. Pressure should be ap-
plied to the joint before the adhesive becomes
too thick to flow and is accomplished by
means of clamps, presses, or other mechanical
devices.
b. Nonuniform clamping pressure
commonly results in weak and strong areas in
the same joint. The amount of pressure re-
quired to produce strong joints in aircraft as-
sembly operations varies with the type of ad-
hesive used and the type of wood to be
bonded. Typical pressures when using resor-
cinol may vary from 125 to 150 pounds per
square inch for softwoods and 150 to
200 pounds per square inch for hardwoods.
Insufficient pressure or poorly machined wood
surfaces usually result in thick bond lines,
which indicate a weak joint, and should be
carefully guarded against. Some epoxy adhe-
sives require much less clamping pressure to
produce acceptable joint strength. Be sure to
read and follow the manufacturer’s instruc-
tions in all cases.
1-11. METHOD OF APPLYING PRES-
SURE. The methods of applying pressure to
joints in aircraft bonding operations range
from the use of brads, nails, small screws, and
clamps; to the use of hydraulic and electrical
power presses. The selection of appropriate
clamping means is important to achieving
sound bond joints.
a. Hand nailing is used rather extensively
in the bonding of ribs and in the application of
plywood skins to the wing, control surfaces,
and fuselage frames. Small brass screws may
also be used advantageously when the par-
ticular parts to be bonded are relatively small
and do not allow application of pressure by
means of clamps. Both nails and screws pro-
duce adverse after effects. There is consider-
able risk of splitting small parts when install-
ing nails or screws. Metal fasteners also pro-
vide vulnerable points for moisture to enter
during service.
b. On small joints using thin plywood for
gussets or where plywood is used as an outer
skin, the pressure is usually applied by nailing
or stapling. Thin plywood nailing strips are
often used to spread the nailing pressure over a
larger area and to facilitate removal of the nails
after the adhesive has cured.
c. The size of the nails must vary with
the size of the members. If multiple rows of
nails are required, the nails should be 1 inch
apart in rows spaced 1/2 inch apart. The nails
in adjacent rows should be staggered. In no
case should the nails in adjacent rows be more
than 3/4 inch from the nearest nail. The length
of the nails should be such that they penetrate
the wood below the joint at least 3/8 inch. In
the case of small members, the end of the nail
should not protrude through the member below
the joint. Hit the nails with several light
strokes, just seating the head into the surface
of the gusset. Be careful not to crush the wood
with a heavy hammer blow.
d. In some cases the nails are removed
after adhesive cure, while in others the nails
are left in place. The nails are employed for
clamping pressure during adhesive cure and
must not be expected to hold members to-
gether in service. In deciding whether to re
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