FREE AVIATION STUDY: AIRCRAFT FLOW TESTS

FREE AVIATION STUDY: AIRCRAFT FLOW TESTS: FLOW TESTS Flow tests should be carried out in accordance with the relevant Maintenance Manual, as and when required by the approved ...

FREE AVIATION STUDY: FUEL SYSTEM MAINTENANCE

FREE AVIATION STUDY: FUEL SYSTEM MAINTENANCE: FUEL SYSTEM MAINTENANCE INTRODUCTION Maintain, service, and adjust aircraft fuel systems and fuel system components in accordan...

FREE AVIATION STUDY: FUEL TANK CAPS, VENTS, AND OVERFLOW LINES.

FREE AVIATION STUDY: FUEL TANK CAPS, VENTS, AND OVERFLOW LINES.: FUEL TANK CAPS , VENTS, AND OVERFLOW LINES. Inspect the fuel tank caps to determine they are the correct type and size for the inst...

FREE AVIATION STUDY: FUEL CROSS-FEED, FIREWALLSHUTOFF, AND TANK SELECTO...

FREE AVIATION STUDY: FUEL CROSS-FEED, FIREWALLSHUTOFF, AND TANK SELECTO...: FUEL CROSS-FEED, FIREWALL SHUTOFF, AND TANK SELECTOR VALVES.  Inspect these valves for leakage and proper operation as follows: ...

FUEL CROSS-FEED, FIREWALL SHUTOFF, AND TANK SELECTOR VALVES.

 Inspect these 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.

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

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.

INDICATOR SYSTEMS. Inspect, service, and adjust the fuel indicator systems according to the manufacturer's instructions. Determine that the required placards and in­strument markings are complete and legible.

4.7.9 FUEL SYSTEM PRECAUTIONS. In servicing fuel systems, remember that fuel is flammable and that the danger of fire or ex­plosion always exists. The following precau­tions should be taken:

a.                           Aircraft being serviced or having the fuel system repaired must be properly grounded.

b.               Spilled fuel must be neutralized or re­moved as quickly as possible.

c.               Open fuel lines must be capped.

d.               Fire-extinguishing equipment must always be available.

e.                           Metal fuel tanks must not be welded or soldered unless they have been adequately purged of fuel fumes. Keeping a tank or cell filled with carbon dioxide will prevent explo­sion of fuel fumes.

f.                            Do not use Teflon tape on any fuel lines to avoid getting the tape between the flare and fitting, which can cause fluid leaks. 

FUEL TANK CAPS, VENTS, AND OVERFLOW LINES.

FUEL TANK 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, the 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 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 

AIRCRAFT FLOW TESTS

FLOW TESTS

Flow tests should be carried out in accordance with the relevant Maintenance Manual, as and when required by the approved Maintenance Schedule, or when necessitated by repairs, replacements or modifications. The tests are designed to ensure that the system will provide a fuel flow to each engine which is in excess of the requirements of the engine when it is operating at maximum power, and at a pressure suitable for proper operation of the carburettor or engine-driven pump, as appropriate. For all tests the aircraft should be levelled laterally and longitudinally, and the fuel tanks should contain the minimum quantity of fuel (i.e. unusable fuel plus sufficient for the test only); tank vents should be clear, and over wing filler caps should be fitted. All equipment used should be bonded and electrically earthed.

Full Flow Test: A full flow test is normally only required after initial installation or major breakdown of the system. Fuel flow test rigs are required for the test, and should be located adjacent to each engine, with the test rig pump at the same level as the engine-driven pump. The rig inlet hose is usually connected to a self-sealing coupling on the engine bulkhead, and the outlet directed to a suitable container.  An external electrical supply should be connected to the aircraft, in order to operate the fuel system valves and to check operation of the associated warning lamps and indicators. The test includes suction feed operation (using the test rig pump), pressure feed operation (using the aircraft booster pumps), and all possible combinations of cross-feeding, to ensure that fuel flow is satisfactory under all flight conditions. The schedule of test operations, and the flow rates and pressures which should be achieved, are detailed in the relevant Maintenance Manual.

For the suction test, the test rig pump is used to draw fuel from the tanks. Valve selections should be made according to the test schedule, and the flow rates and pressures obtained at each stage of the test should be recorded. These results should be within the limitations prescribed for the suction test.

For the pressure test, the aircraft booster pumps should be used to pump fuel from the tank. The test rig pump is switched off, and its by-pass opened. Selections of pumps and valves should be made in accordance with the test schedule, and the flow rates and pressures obtained at each stage of the test should be recorded. These results should be within the limitations prescribed for the pressure test.

Limited Flow Test: A limited flow test is often considered as a satisfactory method of checking a fuel system after a component has been changed; only that part of the system affected by the component change needs to be tested. The fuel feed pipe is dis­connected at the engine, or, in some instances, a drain pipe is connected to a special drain valve at the engine, and a suitable container is positioned to catch the drained fuel.

The appropriate low pressure cock should be turned on, and the flow rates should be checked with the associated pumps operating separately and together. For each part of the test, when the fuel flow is free from bubbles, it should be directed into a calibrated container, and the time taken to pump a given quantity of fuel should be recorded. These figures should be converted to flow rates, which should not be less than the minimum flow rates specified in the relevant Maintenance Manual.

Gravity Feed Test: To check a gravity feed system such as is fitted to some light aircraft, the feed pipe should be disconnected at the carburettor, and a suitable container should be positioned below the engine. With the fuel outlet positioned at the same height as the carburettor, and the fuel valve turned on, the fuel should be checked for freedom from bubbles and for full-bore flow, and then directed into a calibrated container. The time taken to drain a given quantity of fuel should be recorded, and the equivalent flow rate should not be less than the minimum flow rate specified in the relevant Main­tenance Manual.

AIRCRAFT FUEL LEAKAGE

Fuel Leakage

When leakage or spillage of fuel has occurred, care must be taken to ensure that all traces of fuel and vapour are removed.    Where lagging has become contaminated with fuel in areas adjacent to passenger cabins and crew com­partments, the lagging should be removed and cleaned, and any residual fuel should be mopped up. Where fuel has leaked into a compartment which is vented and drained, the venting and drainage arrangements should be checked to ensure that they are functioning correctly and that there is a flow of air through the compartment. It is sometimes specified that a check of the venting system of such a compartment should be carried out with the cabin pressurised. In the event of a gross leakage, consideration should be given to the effects that fuel may have on other materials and components, such as cable insulation, seals, transparencies and bearings.

4.5 PRESSURE TESTS   

Pressure tests are normally required at regular intervals, after repairs, modifications, and replacement of components, and whenever leakage is suspected. In those vent systems which utilise part of the wing structure (e.g. top hat sections) to form the vent duct, vent pressure tests may also be required after structural repairs.

The tests required will be specified in the relevant Maintenance Manual, and should be carefully carried out. Test rigs, capable of supplying fuel or air under pressure, are required, and should include an accurate pressure gauge, a relief valve, and, in the case of a fuel pumping rig, a flow meter.     All test rigs should be clearly identified with the certification (or re­certification) date. In addition, special blanks, plugs, cover plates, and dummy com­ponents may be required. The vent, feed, and transfer systems are usually tested separately since different test pressures are generally prescribed.

Vent System Pressure Test: For this test, the vent system on each side of the aircraft should normally be tested separately. All vent^- openings should be blanked, and it will often be necessary to gag float-operated valves, or to replace them with dummy components. Alternative means of venting the tanks during the tests should be provided. Air pressure should be applied to the system either through a water drain valve or through an adaptor fitted to one of the blanks, and the pressure should be slowly raised to the test pressure quoted in the relevant Maintenance Manual. When the air pressure supply cock is turned off, any decrease in pressure will indicate leakage, and the drop in pressure over a prescribed time should be noted. The source of any leakage in excess of that permitted should be traced and rectification action should be taken.

4.5.3 Feed System Pressure Test: The feed system from a tank to its associated engine should be tested individually. Cross-feed and inter-engine valves should be closed, and the low-pressure cock should be opened. On some aircraft the feed systems are pres­surized by switching on both pumps in the tank concerned, whilst on others the booster pumps are replaced by dummy components, and fuel pressure is applied by means of an external test rig. In some systems there will be flow through the bleed hole in the suction valve, and this must be within prescribed limits. Rates of flow indicated on the test rig flow meter, which are in excess of these limits, will be indicative of either an internal or external fuel leak. All pipes, connections, and valves should be checked visually for signs of leakage under pressure; no leakage is normally permitted.

NOTE: In systems in which drip shields or heat shields are fitted to some couplings, the test pressure must be applied for a sufficient length of time to enable any leakage to collect and flow through the drain. Alternatively, a separate pressure test of the drip shield may be specified, or the shield may be required to be removed for the test.

Transfer System Pressure Test: The pipes and couplings in the fuel transfer system may be pressure tested in a similar manner to the feed system. Pipes should be dis­connected and blanked at the positions specified in the relevant Maintenance Manual, and fuel pressure should be applied by means of the transfer pump, or by use of an external test rig, supplying fuel through a dummy pump. No leaks should be evident, and no fuel flow should be recorded on the test rig flow meter.

4.5.5 Additional Pressure Tests: A number of other pressure tests may be specified, in order to ensure that there is no leakage which could prove hazardous, or prevent proper operation of the fuel system. One example is the pressure testing of conduits which pass through the fuel tanks, and house electrical cables. These conduits are usually sealed by means of a pressure bung or pressure seal, and are tested by applying air pressure to the inside, through a drain pipe, or special adaptor. When the air supply is shut off, there should be no drop in pressure over a prescribed period of time. If leakage is evident at the pressure bung, it is usually permissible to apply sealant to seal the bung and the holes through which the cables pass.

Types of Aircraft contaminants

Types of contaminants:

a.   Solid particles: Sand blown into the storage tanks or into the aircraft tank during the fuelling operation or rust from unclean storage tanks are solid particles which can clog strainers and restrict the flow of fuel.

b.                                    Surficant:                These are partially soluble compounds which are by-products of the fuel processing, or they may come from fuel additives. They have the tendency to adhere to other contaminants and cause them to drop out of the fuel and settle to the bottom of the fuel tank as sludge.

c. Water:                                 Although it has always been present in avia­tion fuel, water is now considered to be a major source of contamination since aircraft fly at alti­tudes; where the temperature is low enough to cause water entrained, or “dissolved,” in the fuel to precipitate out and form free water. This free, water can freeze and clog the fuel screens.

d.                                    Micro organisms: Tiny micro organisms grow from airborne bacteria and gather in the fuel. They lie dormant until they can come into contact with free water, but then they grow at a prodigious rate as they live in the water and feed on the hydrocarbon fuel, and on some of the surficant contaminants. The scum which they form holds water against the walls of the fuel tanks and causes corrosion.

Detection of contaminants:

Draining a sample of fuel from the main strainers of an aircraft has in the past been con­sidered an acceptable method of assuring that the fuel in the system is clean. But the fuel cleanli­ness requirements for modern aircraft engines re­quire more extensive testing than this.

One test recommended by the FAA for check­ing for fuel contaminants is to drain about a quart of fuel into a spotlessly clean ten-quart white enamel bucket. Stir the fuel with a clean paddle and swirl it in the form of a vortex cone. All of the contaminants will gather at the centre of the vortex and can be easily seen. A few drops of food colouring will dye any water that is in the fuel so it will be readily visible, but the food colouring will not colour the fuel

.

A commercial water test kit is used far more often than the white-bucket test. This kit con­tains a small glass jar and a supply of capsules containing a grayish-white powder, and a 100-cc sample of fuel is taken from the truck and put in­to the jar. Then one of the test capsules is dumped into this fuel, the lid is screwed on, and the contents are shaken for about ten seconds. If the powder changes its colour from gray-white to pink or purple, the fuel has more than 30 parts per million of water, and it is not considered safe for use. It may be pumped through the water traps and filters in the truck and another sample taken. This test is fail-safe, because any error in performing the test will cause it to give an unsafe indication.

If the fuel sample is taken from the aircraft, the fuel should be drawn from the main strainer and some fuel should be allowed to flow from every one of the tanks. Drawing fuel from the main strainer when the tank selector is in the both position will not necessarily get fuel from all of the tanks.

Fire Hazards: when Fuelling and De fuelling Aviation fuels are both highly flammable and volatile, and special care must be exercised when transferring them into or out of an aircraft. Be sure that the proper type of fire extinguisher is available at the aircraft and that it has been pro­perly serviced and has not been used, even par­tially, since it was last serviced.

Never service an aircraft with fuel inside a hangar or in any other closed area. If fuel is stored in containers other than the fuel service truck or the aircraft fuel tanks, be sure that the containers are closed, not only to prevent the en­try of contaminants, but also to prevent the re­lease of fuel vapours.

If any fuel is spilled, wipe it up immediately; or, if too much has been spilled to wipe up, wash it away with water or cover it with sand.

Be sure that there are no open fires, gasoline motor exhausts, or electrical equipment opera­ting in the vicinity of fuelling or defuelling opera­tions, and be sure that all electrical power on the aircraft except that required for the fuelling opera­tion is turned off.

Wear only a type of shoes that will not cause sparks and clothing that will not produce sparks from static electricity. And, be sure that the air­craft and fuel truck or hydrant are properly bond­ed electrically.

Static electricity builds up when fuel flows through the fuel lines and its discharge must be guarded against.) About fifteen minutes is needed for a tank full of fuel, especially turbine fuel, to relax itself of the charges of static electricity after the tank has been filled.

When fuelling or refuelling an aircraft, first connect the fuelling vehicle or fuel cabinet to the aircraft with a static ground wire, and then con­nect the nozzle to the grounding point at the tank filler opening. Only after this has been done should the filler cap be removed.