The nylon barrier consists of three to four coats of
nylon applied hot by brush, swab, or spray. The purpose
of the nylon barrier is to keep fuel from diffusing
through the cell wall.
The retainer consists of Buna N coated square-
woven fabric (cotton or nylon) or cord fabric. The
purpose of the retainer ply or plies is to lend strength to
the fuel cell and provide protection for the nylon fuel
barrier.
NYLON-TYPE BLADDER CELLS (PLIO-
CEL).—Nylon bladder cells differ in construction and
material from the Buna N rubber cells. This type of cell
may be identified by the trade name “Pliocel” stenciled
on the outside of the cell. The Pliocel construction
consists of two layers of nylon woven fabric laminated
with three layers of transparent nylon film.
The repair of this type of cell must be accomplished
by entirely different methods and with different
materials. The adhesive and Buna N rubber used to
repair the rubber-type bladder cell cannot be used on the
nylon-type cell.
INTEGRAL FUEL CELLS REPAIR
Integral fuel cells are usually contained in the wing
structure; however, in some aircraft integral fuel cells
are built into the fuselage. An integral cell is a part of
the aircraft structure that has been built in such a reamer
that after the seams, structural fasteners, and access
doors have been properly sealed, it will hold fuel without
leaking. This type of construction is usually referred to
as a “wet wing.”
Usually, the cell area is located between two spars,
and is capped on the ends by sealed end ribs. The skin
covering may be standard riveted sheet or may be milled
from a solid plate of aluminum alloy. The milled skins
are usually bolted in place instead of being riveted.
The wing mating surfaces are built to extremely
close tolerances to allow for proper sealing. The sealing
of these mating surfaces is attained by using gaskets or
sealants, or a combination of both. In most cases, the
perimeter of the cell is sealed by using a nonhardening
sealant that is injected into a groove machined in one
structural member along the mating surface. The
attachment screws and bolts are sealed by placing
O-ring seals under the heads. Protruding bolt heads are
sealed by special seals that consist of an O-ring
embedded in a metal washer. Figure 3-34 shows the
sealing of integral fuel cell screws and bolts.
Figure 3-34.—Sealing integral fuel cell screws and bolts.
Inspection
The inspection of integral fuel cells consists mainly
of a check for external leakage around skin joints, rivets,
screws, and bolts on every preflight inspection. The fuel
cell fittings and connections should also be inspected for
evidence of leakage. Fuel cell leaks are classified in the
following categories: slow seep, seep, heavy seep, and
running leak.
• SLOW SEEP.—The least severe leak classifi-
cation is the slow seep. This is a very slow fuel seepage
that wets a small area. Over a period of hours, the wetted
area may become larger. A slow seep, when wiped dry,
will not reappear in a short period of time.
• SEEP.—A seep is a fuel leak that reappears in less
than an hour (approximately) after it has been wiped dry.
• HEAVY SEEP.—A heavy seep is a fuel leak that
reappears immediately after it has been wiped dry.
3-47
