A wet wing is an aircraft wing that doubles as a fuel tank. Instead of holding a separate container inside the wing, the wing’s own structural skin, ribs, and spars form the walls of the tank. The joints are sealed with a fuel-resistant compound so that fuel sits directly against the wing’s internal surfaces. Nearly every commercial airliner flying today, from the Boeing 737 to the Airbus A380, uses this design.
How a Wet Wing Differs From a Bladder Tank
Aircraft need to carry large volumes of fuel, and the wing is the most practical place to put it. The space between the front and rear spars (the main structural beams running the length of the wing) creates a natural cavity. Engineers have two options for turning that cavity into a fuel reservoir.
A wet wing, also called an integral fuel tank, seals the existing structure so fuel can be stored directly inside. The wing skins, ribs, and spars are the tank. Every fastener hole, every joint between panels, and every rivet line is coated with a special sealant to prevent leaks.
The alternative is a bladder tank: a flexible rubber or polymer bag fitted inside the wing cavity. Bladder tanks are common on smaller general aviation aircraft and military helicopters because they’re easier to replace and offer some crash resistance. But they add weight, reduce usable volume, and eventually wear out. Wet wings eliminate that extra layer entirely, giving the designer more fuel capacity for the same amount of space.
What Keeps the Fuel From Leaking
The critical ingredient in a wet wing is the sealant. Most integral fuel tanks use a two-part polysulfide compound that cures into a flexible, fuel-resistant rubber. Technicians apply it to every seam, fastener, and faying surface (where two metal parts press together) inside the wing. The sealant needs to stay flexible over decades because the wing constantly flexes during flight, and a rigid seal would crack.
Polysulfide sealants can withstand repeated contact with jet fuel and aviation gasoline without breaking down. Some formulations are rated for intermittent temperatures up to 360°F. The compounds come in different consistencies: thin versions that can be brushed into tight seams, thicker pastes for filling gaps, and even sprayable grades for covering large interior surfaces.
Internal Structure and Fuel Management
A wet wing isn’t just a hollow box filled with fuel. Inside, the ribs and spars that give the wing its strength also divide the fuel space into compartments. These internal walls serve a second purpose: they act as baffles that prevent fuel from sloshing freely during flight.
Uncontrolled fuel movement is a real problem. When an aircraft banks or hits turbulence, fuel rushing to one side of the wing shifts the center of gravity and can destabilize the plane. Internal partitions break the fuel volume into smaller sections, reducing the mass of liquid that can move at once and increasing the frequency of any sloshing oscillations, which makes them easier to dampen. Some designs use ring-shaped baffles along the tank walls or cruciform (cross-shaped) partitions to counteract different types of fuel motion. Engineers only need these baffles to extend about one-quarter of the tank’s diameter below the fuel surface, since deeper fuel doesn’t participate much in surface oscillations.
As fuel burns off during a flight, the liquid level drops. Fixed baffles distributed throughout the depth of the tank ensure damping remains effective even as the tank empties. Small holes in the ribs allow fuel to flow between compartments so it can reach the fuel pumps, but the holes are sized to slow bulk sloshing while still permitting normal fuel feed.
Venting and Thermal Expansion
Fuel expands as it warms up, and air pressure drops as an aircraft climbs. A sealed wing with no way to relieve pressure would eventually rupture. Federal aviation regulations require every fuel tank to have a vent system at the top of the tank, and most tanks must include an expansion space equal to at least 2% of total tank capacity. This empty headroom gives fuel room to expand without forcing liquid out of the vents.
Vent outlets are positioned so they won’t get blocked by ice, won’t siphon fuel during normal flight, and won’t discharge fumes near the cabin or anywhere that could create a fire hazard. If multiple tanks share interconnected fuel outlets, their airspaces must also be interconnected so pressure stays balanced. The vents are designed to prevent fuel loss when the aircraft is parked on a sloped ramp, though small amounts may discharge during extreme thermal expansion on a hot day.
Lightning Strike Protection
Storing fuel inside the wing skin raises an obvious concern: lightning. Commercial aircraft are struck by lightning roughly once or twice a year on average, and the wing is a common entry or exit point. For traditional aluminum wings, this is manageable because aluminum conducts electricity well, allowing current to flow across the surface without generating dangerous heat in any one spot.
Composite wings, made from carbon fiber or fiberglass, are less conductive. To protect wet wings built from composites, manufacturers embed a conductive mesh into the outermost layer of the wing skin. Expanded copper or aluminum foil is cocured with the laminate during manufacturing, creating a continuous conductive path across the surface. This mesh lets lightning current spread quickly and dissipate without concentrating enough energy to ignite fuel vapors inside. Copper mesh avoids corrosion issues that can occur when aluminum contacts carbon fiber, though it weighs about three times as much.
Which Aircraft Use Wet Wings
Virtually all modern jet transports use integral fuel tanks. The Boeing 737 family, every Airbus narrow-body from the A220 to the A321neo, wide-bodies like the 787 and A350, and large aircraft like the 747 and A380 all store fuel directly in their wing structures. Regional jets such as the Embraer E175 and CRJ700 use the same approach. Even some regional turboprops, including the ATR 72 and Dash 8, incorporate wet wing designs.
In general aviation, the picture is more mixed. Many Cessna singles from the late 1960s onward use wet wings, while other light aircraft stick with bladder tanks. The choice often comes down to manufacturing complexity: sealing an integral tank requires precision and quality control at every rivet, while dropping a bladder into a wing cavity is simpler on the production line.
Maintenance and Leak Detection
The biggest maintenance headache with wet wings is resealing. Over years of flight cycles, the constant flexing of the wing structure can cause sealant to crack or peel away from fasteners, leading to slow fuel leaks. These leaks often show up as fuel stains on the underside of the wing.
Finding the exact source of a leak requires some ingenuity. A common technique on smaller aircraft involves draining the tank, removing an access panel on top of the wing, and connecting a vacuum source (often through the tank’s vent line) to create slight negative pressure inside. The mechanic then sprays soapy water on the exterior skin and watches for bubbles being drawn inward, which pinpoint the leak location. On larger aircraft, technicians may enter the wing through access panels and inspect the interior directly, looking for areas where sealant has separated or fuel residue has accumulated.
Resealing is labor-intensive. The old sealant must be stripped from the affected area, the metal surfaces cleaned and prepared, and fresh polysulfide compound applied. Cure times vary, but the wing typically needs to sit unused for 24 to 72 hours while the sealant sets. For general aviation owners, a full reseal of both wings can cost several thousand dollars and take a shop one to two weeks, making it one of the more significant maintenance events in the life of a light aircraft.