Aircraft fuel tanks are designed to safely store fuel and supply it to the aircraft fuel system under all operating conditions. Different fuel tank designs are used depending on the aircraft type, structural requirements, performance needs, and maintenance considerations. The three basic types of aircraft fuel tanks are rigid removable tanks, bladder tanks, and integral fuel tanks.
Rigid Removable Fuel Tanks
Many aircraft, especially older aircraft, utilize rigid removable fuel tanks. Rigid tanks are manufactured from various materials and secured within the airframe structure. The tanks are often riveted or welded together and can include baffles, as well as the other fuel tank features described above. They are typically made from 3003 or 5052 aluminum alloy or stainless steel and are riveted and seam welded to prevent leaks.
Many early tanks were made of a thin sheet steel coated with a lead/tin alloy called terneplate. The terneplate tanks have folded and soldered seams. Figure 1 shows the parts of a typical rigid removable fuel tank.
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| Figure 1. A typical rigid removable aircraft fuel tank and its parts |
Regardless of the actual construction of removable metal tanks, they must be supported by the airframe structure and secured in place with some sort of padded strap arrangement to resist shifting in flight. The wings are the most popular location for fuel tanks. Figure 2 shows a fuel tank bay in a wing root with the tank straps.
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| Figure 2. A fuel tank bay in the root of a light aircraft wing on a stand in a paint booth |
Some tanks are formed to be part of the leading edge of the wing. These are assembled using electric resistance welding and are sealed with a compound that is poured into the tank and allowed to cure. Many fuselage tanks also exist. [Figure 3]
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| Figure 3. A fuselage tank for a light aircraft |
In all cases, the structural integrity of the airframe does not rely on the tank(s) being installed, so the tanks are not considered integral.
As new materials are tested and used in aircraft, fuel tanks are being constructed out of materials other than aluminum, steel, and stainless steel. Figure 4 shows a rigid removable fuel tank from an ultralight category aircraft that is constructed from Vipel® isophthalic polyester UL 1316/UL 1746 resin composite.
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| Figure 4. A composite tank from a Challenger ultra light aircraft |
Its seamless, lightweight construction may lead to the use of this type of tank in other aircraft categories in the future.
Being able to remove and repair, or replace, a fuel tank can be a great convenience if a leak or malfunction exists. Repairs to fuel tanks must be done in accordance with manufacturers’ specifications. It is especially critical to follow all safety procedures when welding repairs are performed.
Fuel vapors must be removed from the tank to prevent explosion. This typically involves washing out the tank with water and detergent, followed by a steam or water flush, as determined by the manufacturer. Once repaired, fuel tanks need to be pressure checked, usually while installed in the airframe, to prevent distortion while under pressure.
Bladder Fuel Tanks
A fuel tank made out of a reinforced flexible material called a bladder tank can be used instead of a rigid tank. A bladder tank contains most of the features and components of a rigid tank but does not require as large an opening in the aircraft skin to install.
The tank, or fuel cell as it is sometimes called, can be rolled up and put into a specially prepared structural bay or cavity through a small opening, such as an inspection opening. Once inside, it can be unfurled to its full size.
Bladder tanks must be attached to the structure with clips or other fastening devices. They should lie smooth and unwrinkled in the bay. It is especially important that no wrinkles exist on the bottom surface so that fuel contaminants are not blocked from settling into the tank sump. [Figure 5]
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| Figure 5. A bladder fuel tank for a light aircraft |
Bladder fuel tanks are used on aircraft of all sizes. They are strong and have a long life with seams only around installed features, such as the tank vents, sump drain, filler spout, etc. When a bladder tank develops a leak, the technician can patch it following manufacturer’s instructions. The cell can also be removed and sent to a fuel tank repair station familiar with and equipped to perform such repairs.
The soft, flexible nature of bladder fuel tanks requires that they remain wet. Should it become necessary to store a bladder tank without fuel in it for an extended period of time, it is common to wipe the inside of the tank with a coating of clean engine oil. Follow the manufacturer’s instructions for the dry storage procedures for fuel cells.
Integral Fuel Tanks
On many aircraft, especially transport category and high-performance aircraft, portions of the wing or fuselage structure are sealed with a fuel-resistant two-part sealant to form a fuel tank. The sealed skin and structural members provide the highest volume of space available with the lowest weight. This type of tank is called an integral fuel tank since it forms a tank as a unit within the airframe structure.
Integral fuel tanks in the otherwise unused space inside the wings are most common. Aircraft with integral fuel tanks in the wings are said to have wet wings. For fuel management purposes, sometimes a wing is sealed into separate tanks and may include a surge tank or an overflow tank, which is normally empty but sealed to hold fuel when needed.
When an aircraft maneuvers, the long horizontal nature of an integral wing tank requires baffling to keep the fuel from sloshing. The wing ribs and box beam structural members serve as baffles and others may be added specifically for that purpose.
Baffle check valves are commonly used. These valves allow fuel to move to the low, inboard sections of the tank but prevent it from moving outboard. They ensure that the fuel boost pumps located in the bottom of the tanks at the lowest points above the sumps always have fuel to pump regardless of aircraft attitude. [Figure 6]
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| Figure 6. Integral fuel tank |
Integral fuel tanks must have access panels for inspection and repairs of the tanks and other fuel system components. On large aircraft, technicians physically enter the tank for maintenance. Transport category aircraft often have more than a dozen oval access panels or tank plates on the bottom surface of the wing for this purpose. [Figure 7A]
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| Figure 7. Fuel tank access panel locations on a Boeing 737 (A), and typical fuel tank access panel seals (B) |
These aluminum panels are each sealed into place with an O-ring and an aluminum gasket for electrostatic bonding. An outer clamp ring is tightened to the inner panel with screws, as shown in Figure 7B.
When entering and performing maintenance on an integral fuel tank, all fuel must be emptied from the tank and strict safety procedures must be followed. Fuel vapors must be purged from the tank and respiratory equipment must be used by the technician. A full-time safety observer must remain positioned outside the tank to assist if needed.
Aircraft using integral fuel tanks normally have sophisticated fuel systems that include in-tank boost pumps. There are usually at least two pumps in each tank that deliver fuel to the engine(s) under positive pressure. On various aircraft, these in-tank boost pumps are also used to transfer fuel to other tanks, jettison fuel, and defuel the aircraft.