Aircraft fuel systems maintenance

Maintenance and service of aircraft fuel systems and fuel system components should be in accordance with the applicable manufacturer's maintenance instructions. Certain general aircraft fuel systems maintenance principles are as follows.

Fuel system lines.

 When fuel system lines and aircraft tubing 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 structures, etc., or come in contact with electrical wiring or conduit. Where physical separation of the aircraft 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 aircraft fuel lines.

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 aircraft tubing between two rigidly-mounted fittings. Always incorporate at least one bend between such fittings to absorb strain caused by vibration and temperature changes.

 Locate clamps or brackets as close to bends as possible to reduce overhang.

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.

Fuel tanks, caps, vents and overflow lines.

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

a. 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 Malfunctioning of this type occurs when the pressure within the tank decreases as the fuel is withdrawn. Eventually, a point is reached where the fuel will no longer flow, and/or the outside atmospheric pressure collapses the tank. Thus, effects will occur sooner with a full fuel tank than with one partially filled.

b. Check tank vents and overflow lines thoroughly for condition, obstructions, correct installation, and proper operation of any check valves and ice protection units. Pay particular attention to the location of the tank vents when such information is provided in the manufacturer's service instructions. Inspect for cracked or deteriorated filler opening recess drains, which may allow spilled fuel to accumulate within the wing or fuselage. One method of inspection is to plug the fuel line at the outlet and observe fuel placed in the filler opening recess. If drainage takes place, investigate condition of the line and purge any excess fuel from the wing.

c. Assure that filler opening markings are affixed to, or near, the filler opening; marked according to the applicable airworthiness requirements; and are complete and legible.

Fuel valves, fuel cross-feed, firewall shutoff and tank selector valves.

Inspect these aircraft valves for leakage and proper operation as follows.

a. Internal leakage can be checked by placing the appropriate valve in the "off' position, draining the fuel strainer bowl, and observing if fuel continues to flow into it. Check all valves located downstream of boost pumps with the pump(s) operating. Do not operate the pump(s) longer than necessary.

b. External leakage from these units can result in a severe fire hazard, especially if the unit is located under the cabin floor or within a similarly-confined area. Correct the cause of any fuel stains associated with fuel leakage.

c. Selector Handles. Check the operation of each handle or control to see that it indicates the actual position of the selector valve to the placard location. Movement of the selector handle should be smooth and free of binding. Assure that stops and detents have positive action and smooth operational feel. Worn or missing detents and stops can cause unreliable positioning of the fuel selector valve.

d. Worn Linkage. Inaccurate positioning of fuel selector valves can also be caused by worn mechanical linkage between the selector handle and the valve unit. An improper fuel valve position setting can seriously reduce engine power by restricting the available fuel flow. Check universal joints, pins, gears, splines, cams, levers, etc., for wear and excessive clearance which prevent the valve from positioning accurately or from obtaining fully "off' and "on" positions.

e. Assure that required placards are complete and legible. Replace those that are missing or cannot be read easily.

Aircraft fuel pumps repair.

Inspect, repair, and overhaul boost pumps, emergency pumps, auxiliary pumps, and engine-driven pumps in accordance with the appropriate manufacturer's instructions.

Aircraft fuel filters strainers and drains.

Check each strainer and filter element for contamination. Determine and correct the source of any contaminants found. Replace throw-away filter elements with the recommended type. Examine fuel strainer bowls to see that they are properly installed according to the direction of the fuel flow. Check the operation of all drain devices to see that they operate properly and have positive shutoff action.