Aircraft fuel tanks and fuel cells repair

Welded or riveted fuel tanks that are made of commercially pure aluminum, 3003, 5052, or similar alloys. may be repaired by welding. Tanks made from heat-treatable aluminum alloys are generally assembled by riveting. In case it is necessary to rivet a new piece in a tank, use the same material as used in the tank undergoing repair, and seal the seams with a compound that is insoluble in gasoline. Special sealing compounds are available and should be used in the repair of tanks.

Inspect fuel tanks and cells for general condition, security of attachment. and evidence of leakage. Examine the fuel tank or fuel cell vent lines, fuel lines and sump drain attachment fittings closely.

Purge de-fueled tanks of explosive fuel/air mixtures in accordance with the manufacturer's service instructions. In the absence of such instructions, utilize an inert gas such as CO2, as a purgative to assure the total deletion of fuel/air mixtures.

a.  Integral Tanks. Examine the interior surfaces and seams for sealant deterioration and corrosion (especially in the sump area). Follow the manufacturer's instructions for repair and cleaning procedures.

b. Internal Metal Tanks. Check the exterior for corrosion and chafing. Dents or other distortion, such as a partially-collapsed tank caused by an obstructed fuel tank vent, can adversely affect fuel quantity gauge accuracy and tank capacity. Check the interior surfaces for corrosion. Pay particular attention to the sump area, especially for those of which sumps are made of cast material.    

c. Removal of Flux After Welding. It is especially important, after repair by welding to completely remove all flux in order to avoid possible corrosion. Promptly upon completion of welding, wash the inside and outside of tank with liberal quantities of hot water then drain. Next, immerse the tank in either 5 percent nitric or 5 percent sulfuric acid solution.   If the tank cannot be immersed, fill the tank with either solution, and wash the outside with the same solution. Permit the acid to main in contact with the weld for about one hour and then rinse thoroughly with clean water. Test the efficiency of the cleaning operation applying some acidified 5 percent silver nitrate solution to a small quantity of the rinse water used last to wash the tank.If a heavy white precipitate is formed, the cleaning is insufficient and the washing should be repeated.

d. Flexible aircraft Fuel Cells. These are sometimes referred to as fuel bladders. To try and reduce and remove Volatile Organic Compounds (VOCs) from fuel tank manufacturing, engineers at GKN Aerospace in Portsmouth, UK, have developed the first single-skin flexible fuel bladder material that is able to offer crash resistance as well as puncture tolerance. This new material is manufactured from a thermoplastic polymer rather than traditional nitrile rubber and incorporates the crash resistant textile within it. The material also contains an integrated fuel barrier to provide fuel tolerance equal to that of traditional tanks. This means the crash resistant layer is not applied using adhesives (weight saving), and no final lacquer coating is required, significantly reducing VOC solvent use.

Inspect the interior for checking, cracking, porosity, or other signs of deterioration.           Make sure the cell retaining fasteners are properly positioned. If repair or further inspection is required, follow the manufacturer's instructions for cell removal, repair, and installation. Do not allow flexible fuel cells to dry out.

Inspect the fuel tank caps to determine they are the correct type or size for the installation, and that "O" rings are in good condition.

Unvented caps, substituted for vented caps, will cause fuel starvation and possible collapse of the fuel tank or cell. Malfunctioning of this type occurs when the pressure within the tank decreases as the fuel is drawn. Eventually, a point is reached the fuel will no longer flow, and/or the atmospheric pressure collapses the tank.