Figure 3-31.—Bullet sealing action.
by leaking fuel. It keeps the aircraft’s fuel intact so the
aircraft may continue operating and return to its base.
The most commonly used types of self-sealing fuel
cells are the standard construction type and the type
that uses a bladder along with the self-sealing cell. Of
the two, the standard construction cell is used the
most. It is a semiflexible cell, made up of numerous
plies of material.
The combination bladder and self-sealing cell is
made up of two parts. One part is a bladder-type cell,
and the other part is identical to the standard
construction cell. It is designed to self-seal holes or
damage in the bottom and the lower portions of the
side areas. The bladder part of the cell (nonself-sealing)
is usually restricted to the upper portion. This type of
cell is also semi flexible.
SELF-SEALING
CELL
(STANDARD
CONSTRUCTION).— There are four primary layers
of materials used in the construction of a self-sealing
cell. These layers are the inner liner, nylon fuel
barrier, sealant, and retainer. All self-sealing fuel cells
now in service contain these four primary layers of
materials. If additional plies are used in the
construction of the cell, they will be related to one of
the primary plies.
The inner liner material is the material used inside
the cell. It is constructed of Buna N synthetic rubber.
Its purpose is to contain the fuel and prevent it from
coming in contact with the sealant. This will prevent
premature swelling or deterioration of the sealant.
Buna rubber is an artificial substitute for crude or
natural rubber. It is produced from butadiene and
sodium, and is made in two types, Buna S and Buna N.
The Buna S is the most common type of synthetic
rubber. It is unsuitable for use as inner liner material
in fuel cells. It causes the petroleum fuels used in
aircraft to swell and eventually dissolve. The Buna N is
not affected by petroleum fuels, making it ideal for this
application. However, the Buna N is slightly porous,
making it necessary to use a nylon barrier to prevent
the fuel from contacting the sealant.
The nylon fuel barrier is an unbroken film of nylon.
The purpose of the nylon fuel barrier is to prevent the
fuel from diffusing farther into the cell. The nylon is
brushed, swabbed, or sprayed in three or four hot coats
to the outer surface of the inner liner during
construction.
The sealant material is the next material used in
fuel cell construction. It remains dormant in the fuel
cell until the cell is ruptured or penetrated by a
projectile. It is the function of the sealant to seal the
ruptured area. This will keep the fuel from flowing
through to the exterior of the fuel cell (fig. 3-31.)
The mechanical reaction results because rubber,
both natural and synthetic, will “give” under the shock
of impact. This will limit damage to a small hole in the
fuel cell. The fuel cell materials will allow the projectile
to enter or leave the cell, and then the materials will
return to their original position. This mechanical
reaction is almost instantaneous.
The chemical reaction takes place as soon as fuel
vapors penetrate through the inner liner material and
reach the sealant. The sealant, upon contact with fuel
vapors, will extend or swell to several times its normal
size. This effectively closes the rupture and prevents
the fuel from escaping. The sealant is made from
natural gum rubber.
3-45
