A nonself-sealing fuel cell is commonly called a bladder cell. It is a fuel container that does not self-seal holes or punctures. The advantage of using a bladder fuel cell results from the saving in weight. Some of the other advantages are the simplicity of repair techniques and the reduced procurement costs over self-sealing fuel cells.

Bladder-type cells are usually made of very thin material to give minimum possible weight. They require 100-percent support from a smooth cavity. The cell is made slightly larger than the cavity of the aircraft for better weight and distribution throughout the aircrafts fuel cavity structure.

The thinner wall construction increases the fuel capacity over the self-sealing cells, thus increasing the range of the aircraft. Many of our aircraft that were formerly equipped with self-sealing cells have been changed to bladder-type cells.

There are two types of bladder fuel cellsrubber type and nylon type.

RUBBER-TYPE BLADDER CELLS.The rubber-type bladder cells are made in the same manner as self-sealing cells. They have a liner, nylon barrier, and a retainer ply. The sealant layers are omitted. All three plies are placed on the building form as one material in the following order: liner, barrier, and retainer. Figure 3-33 shows this type construction. The inner liner may consist of Buns N rubber, Buna N coated square-woven fabric (cotton or nylon), or Buna N coated cord fabric. The purpose of the inner liner is to contain the fuel and provide protection for the nylon barrier. 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 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.

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.

RUNNING LEAK. A running leak is a fuel leak that flows steadily. 

Most aircraft structural repair manuals do not classify a slow seep or seep in an open area (the surfaces of the aircraft exposed to the airstream) as a flight hazard. A slow seep or seep in an open area need not be repaired before flight if structural integrity exists and there is no danger of an increase in leak intensity during flight. Slow seeps and seeps considered acceptable for flight should be frequently inspected to ensure the leak intensity does not increase prior to flight.

Heavy seeps and running leaks are classified as flight hazards, regardless of their location in the aircraft. Any leak classified as a flight hazard must be corrected before flight.