An aircraft fuel tank leak is serious. At best it is a waste of expensive aviation fuel and, at worst, a fuel leak can lead to in-flight fuel starvation to your aircraft’s engine(s). Causes of fuel tank leaks vary, ranging from the easy-to-fix to more difficult and expensive problems. Your aircraft mechanic won’t know the severity of the leak until he carefully inspects the aircraft.

Do some detective work. Observe and note everything you can about the leak. When does it appear? Does it leak only when the tank is full? Does the leak stop after the tanks are half full? Does the leak occur continuously? Where on the aircraft is the leak visible? Is the leak coming only from a rivet?

Confirm your fuel tank system. Your aircraft’s maintenance manual will provide information about your fuel tank system. There are three types of fuel tanks. The first is known as an integral or “wet wing” fuel tank. In this system, the fuel tank is actually part of the wing. The top and bottom wing skins are also the top and bottom of the fuel tank. Each rivet through the wing’s skin is a potential source for a fuel leak, since fuel is on the other side of the rivet.

The second type of fuel tank is a fuel cell, which is a rubber bladder inside the wing. The rubber in fuel cells becomes brittle with age, which can lead to cracks.

The third type of fuel tank is a built-up metal or fiberglass tank inside the wing. The built-up tank will have seams or welds that can be a source of leaks.

Inspect the aircraft’s fuel tanks. Your maintenance manual will show you the panels that can be removed to gain access to the fuel tanks. Open these panels, and inspect for the source of the leak. Look at the bottom of each panel as you remove it. Note if the panel is wet or a strong odor of fuel is present. Remember that a leak can travel a long way inside the plane before it drips out. 100 low-lead aviation fuel is blue, and leaves a blue stain after it is dry. Jet fuel is colorless, but leaves a strong odor and is slow to dry. Make sure all connections and clamps are tight. See if your observations have helped you to find the leak.

Remove the fuel tank. You have discovered that the leak is coming from the tank itself, rather than from a loose connection. Your aircraft’s maintenance manual will guide you on the removal of your fuel tank. You will first need to de-fuel the aircraft before you can remove the tank.

If you have a “wet wing” tank, it cannot be removed because it is part of the aircraft’s structure. If your tank is a fuel cell, loosen all connections and remove the fuel cell. If your tank is built-up, there will be large panels that have to be removed before the tank can be extricated. On Piper Cherokee aircraft, for example, the built-up tanks form a section of each wing. To remove the tank, remove the screws around the perimeter of the tank; then slide the tank forward and out.

Repair your “wet wing” fuel tank. Consult your aircraft’s maintenance manual for procedures and materials allowed for the fuel tank repair. On a “wet wing” tank, repair involves the resealing of the leaking seams or rivets. To accomplish this, unseal one or more access panels into the tank to gain access.

Repair your fuel cell or built-up fuel tank. A fuel cell or built-up tank can be leak checked with air pressure on a work bench. Using an air regulator and pressure gauge, plumb the fuel tank with air pressure of 3 psi. Cap the fuel tank lines, and cover the filler opening with your hand. This allows you to depressurize the tank quickly in case of a problem. Use a solution of soap and water to search for leaks, which will show up as large bubbles.

Once the leaks are found, you have two options: perform a field repair yourself or send the fuel cell or built-up tank to a FAA-approved repair station. This service facility can overhaul the tank and provide you with a warranty. In the case of fuel cells, a patch can be installed using a repair kit specified by your aircraft manufacturer. A built-up tank can be field repaired by removing the sealant around the leak, and applying a bead of new sealant specified by the manufacturer. Once the repair has been made and has properly cured, check for leaks by using air pressure and soapy water.

Reinstall the repaired fuel tank. To reinstall the fuel tank, meticulously follow the aircraft maintenance manual procedures. Torque all connections to recommended specifications. Leave access panels off the areas surrounding the tank. Fuel the aircraft, and check for obvious large leaks. Because small leaks may not appear until the next day, wait at least a day before you install the rest of the access panels.