AIRCRAFT FUEL SYSTEM MAINTENANCE

  FUEL SYSTEM MAINTENANCE

INTRODUCTION

Maintain, service, and adjust aircraft fuel systems and fuel system components in accordance with the applicable manufacturer's maintenance instructions. Certain general fuel system maintenance prin­ciples are outlined in the following para­graphs.

FUEL LINES AND FITTINGS.

When fuel system lines are to be replaced or repaired, consider the following fundamentals in addition to the applicable airworthiness re­quirements.

a.              Compatibility of Fittings. All fittings are to be compatible with their mating parts. Although various types of fittings appear to be interchangeable in many cases they have dif­ferent thread pitch or minor design differences which prevent proper mating and may cause the joint to leak or fail.

b.               Routing. Make sure that the line does not chafe against control cables, airframe structure, etc., or come in contact with electri­cal wiring or conduit. Where physical separa­tion of the fuel lines from electrical wiring or conduit is impracticable, locate the fuel line below the wiring and clamp it securely to the airframe structure. In no case should wiring be supported by the fuel line.

c.    Alignment. Locate bends accurately so that the tubing is aligned with all support clamps and end fittings and is not drawn, pulled, or otherwise forced into place by them. Never install a straight length of tubing be­tween two rigidly-mounted fittings. Always incorporate at least one bend between such fit­tings to absorb strain caused by vibration and temperature changes.

d.               Bonding. Bond metallic fuel lines at each point where they are clamped to the structure. Integrally bonded and cushioned line support clamps are preferred to other clamping and bonding methods.

e.               Support of Line Units. To prevent possible failure, all fittings heavy enough to cause the line to sag should be supported by means other than the tubing.

f.                Support clamps.

(1)             Place support clamps or brackets for metallic lines as follows.

(2)                   Locate clamps or brackets as close to bends as possible to reduce 

FUEL TANKS AND CELLS. Welded or riveted fuel tanks that are made of commercially pure aluminium, 3003, 5052, or similar alloys, may be repaired by welding. Tanks made from heat-treatable aluminium 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 fuel tank or cell vent line, fuel line, and sump drain attachment fittings closely.

CAUTION: Purge de-fuelled 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 CO, 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.

      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 the tank with liberal quantities. of hot water and then drain. Next, immerse the tank in either a 5 percent nitric or 5 percent sulphuric 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 re­main in contact with the weld about one hour and then rinse thoroughly with clean water. Test the efficiency of the cleaning operation by applying some acidified 5 percent silver nitrate solution to 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 Fuel Cells. 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. Preserve them in accordance with the manufacturer's instruc­tions.