Aircraft EMERGENCY EQUIPMENTS (CABIN EQUIPMENTS)

LIFE RAFTS

Inflatable life rafts are subject to general deterioration due to aging. Experience has indicated that such equipment may be in need of replacement at the end of 5 years due^. to porosity of the rubber-coated material. Wear of such equipment is acceler­ated when stowed on board aircraft because of vibration which causes chafing of the rubber­ized fabric. This ultimately results in localized leakage.  Leakage is also likely to occur where the fabric is folded because sharp corners are formed. When these corners are in contact with the carrying cases or with adjacent parts of the rubberized fabric, they tend to wear through due to vibration (Ref TSO-C70a).

When accomplishing maintenance, 

repair, and inspection of unpacked rafts, per­sonnel should not step on any part of the raft or flotation tubes. while wearing shoes. Rafts should not be thrown or dropped, since dam­age to the raft or accessories may result. Par­ticular care should be exercised at all times to prevent snagging, cutting, and contact with gasoline, acids, oils, and grease. High stan­dards of performance for proper maintenance, inspection, and repair cannot be overempha­sized, since the lives of passengers could be involved.

Inspection and inflation tests,

when applicable, will be accomplished during stor­age and after installation in an aircraft in ac­cordance with the manufacturer's specifica­tions and/or FAA-approved procedures. Ac­cessory items will be installed during these in­spections. A raft knife will be attached by a 24-inch nylon lanyard to the mooring eye lo­cated above the CO_Z cylinder case to enable rapid cutting of the mooring line.

Aircraft Freon Discharge Cartridges

Freon Discharge Cartridges

The service life of fire extinguisher discharge cartridges is calculated from the manufacturer's date stamp, which is usually placed on the face of the cartridge. The manufacturer's service life is usually recommended in terms of hours below a predetermined temperature limit. Many cartridges are available with a service life of approximately 5,000 hours. To determine the unexpired service life of a discharge cartridge, it is necessary to remove the electrical leads and discharge hose from the plug body, which can then be removed from the extinguisher container.

Care must be taken in the replacement of cart­ridge and discharge valves. Most new extinguisher containers are supplied with their cartridge and discharge valve disassembled. Before installation on the aircraft, the cartridge must be properly assembled into the discharge valve and the valve connected to the container, usually by means of a swivel nut that tightens against a packing ring gasket.

If a cartridge is removed from a discharge valve for any reason, it should not be used in another discharge valve assembly, since the distance the contact point protrudes may vary with each unit. Thus, continuity might not exist if a used plug which had been indented with a long contact point were installed in a discharge valve with a shorter contact point.

When actually performing maintenance, always refer to the applicable maintenance manuals and other related publications pertaining to a particular aircraft.

Freon Containers

Bromochloromethane and freon extinguishing agents are stored in steel spherical containers. There are four sizes in common use today ranging from 224 cu. in. (small) to 945 cu. in. (large). The large containers weigh about 33 tbs. The small spheres have two openings, one for the bonnet assembly (sometimes called an operating head), and the other for the fusible safety plug (figure 5.6). The larger containers are usually equipped with two firing bonnets and a two-way check valve.

The containers are charged with dry nitrogen in addition to a specified weight of the extinguish­ing agent. The nitrogen charge provides sufficient pressure for complete discharge of the agent. The bonnet assembly contains an electrically ignited power cartridge which breaks the disk, allowing the extinguishing agent to be forced out of the sphere by the nitrogen charge.

A single bonnet sphere assembly is illustrated

. The function of the parts shown, other than those described in the preceding para­graph, are as follows:

(l)         The strainer prevents pieces of the broken disk from entering the system,

(2)       The fusible safety plug melts and releases the liquid when the temperature is between 208° and 220° F., and

(3)       The gage shows the pressure in the container. In this type of design, there is no need for siphon tubes.

In some installations the safety plug is connected to a discharge indicator mounted in the fuselage skin, while others simply discharge the fluid into the fire extinguisher container storage compart­ment.

The gage on the container should be checked for an indication of the specified pressure as given in the applicable aircraft maintenance manual. In addition make certain that the indicator glass is unbroken and that the bottle is securely mounted.

Some types of extinguishing agents rapidly cor­rode aluminium alloy and other metals, especially under humid conditions. When a system that uses a corrosive agent has been discharged, the system must be purged thoroughly with clean, dry, com­pressed air as soon as possible.

Almost all types of fire extinguisher containers require re-weighing at frequent intervals to deter­mine the state of charge. In addition to the weight check, the containers must be hydrostatically tested, usually at 5-year intervals.

The circuit wiring of all electrically discharged containers should be inspected visually for condi­tion. The continuity of the entire circuit should be checked following the procedures in the ap­plicable maintenance manual. In general this con­sists of checking the wiring and the cartridge, by usⁱng a resistor in the test circuit that limits the circuit current to less than 35 mill amperes to pre­vent detonating the cartridge.

AIRCRAFT FIRE EXTINGUISHER SYSTEM - MAINTENANCE PRACTICES

 FIRE EXTINGUISHER SYSTEM - MAINTENANCE PRACTICES

Regular maintenance of fire extinguisher systems typically includes such items as the inspection and servicing of fire extinguisher bottles (containers), removal and re-installation of cartridge and dis­charge valves, testing of discharge tubing for leakage, and electrical wiring continuity tests. The fol­lowing paragraphs contain details of some of the most typical maintenance procedures, and are included to provide an understanding of the opera­tions involved.

Fire extinguisher system maintenance procedures vary widely according to the design and construc­tion of the particular unit being serviced. The de­tailed procedures outlined by the airframe or sys­tem manufacturer should always be followed when performing maintenance.

Container Pressure Check

A pressure check of fire extinguisher containers is made periodically to determine that the pressure is between the minimum and maximum limits pre­scribed by the manufacturer. Changes of pressure with ambient pressure must also fall within -pre­scribed limits. The graph shown in figure 5.5 is typical of the pressure/temperature curve graphs that provide maximum and minimum gage readings. If the pressure does not fall within the graph limits, the extinguisher container should be replaced.

AIRCRAFT FIRE DETECTION SYSTEM - TROUBLESHOOTING

FIRE DETECTION SYSTEM - TROUBLESHOOTING

The following troubleshooting procedures repre­sent the most common difficulties encountered in engine fire detection systems.

(1)       Intermittent alarms are most often caused by an intermittent short in the detector system wiring. Such shorts may be caused by a loose wire which occasionally touches a nearby terminal, a frayed wire brushing against a structure, or a sensing element rubbing long enough against a structural member to wear through the insulation. Intermittent faults can often best be located by moving wires to re­create the short.

(2)       Fire alarms and warning lights can occur when no engine fire or overheat condition exists. Such false alarms can most easily be located by disconnecting the engine sensing loop from the aircraft wiring. If the false alarm continues, a short must exist between the loop connections and the control unit. If, however, the false alarm ceases when the engine sensing loop is disconnected, the fault is in the discon­nected sensing loop, which should be ex­amined for areas which have been bent into contact with hot parts of the engine. If no bent element can be found, the shorted section can be located by isolating and disconnecting elements consecutively around the entire loop.

(3)       Kinks and sharp bends in the sensing ele­ment can cause an internal wire to short intermittently to the outer tubing. The fault can be located by checking the sen­sing element with a megger while tapping the element in the suspected areas to pro­duce the short.

(4)       Moisture in the detection system seldom causes a false fire alarm. If, however, moisture does cause an alarm, the warning will persist until the contamination is re­moved or boils away and the resistance of the loop returns to its normal value.

(5)       Failure to obtain an alarm signal when the test switch is actuated may be caused by a defective test switch or control unit, the lack of electrical power, inoperative indicator light, or an opening in the sen­sing element or connecting wiring. When the test switch fails to provide an alarm, the continuity of a two-wire sensing loop can be determined by opening the loop and measuring the resistance. In a single wire, continuous-loop system, the centre conductor should be grounded.

FIRE DETECTION AND EXTINGUISHER SYSTEM - MAINTENANCE PRAC¬TICES

FIRE DETECTION AND EXTINGUISHER SYSTEM - MAINTENANCE PRAC­TICES\

INTRODUCTION

Fire detector sensing elements are located in many high-activity areas around aircraft engines. Their location, together with their small size, in­creases the chances of damage to the sensing ele­ments during maintenance. The installation of the sensing elements inside the aircraft cowl panels pro­vides some measure of protection not afforded ele­ments attached directly to the engine. On the other hand, the removal and re-installation of cowl panels can easily cause abrasion or structural defects to the elements. A well-rounded inspection and mainte­nance program for all types of continuous-loop sys­tems should include the following visual checks. These procedures are provided as examples and should not be used to replace approved local main­tenance directives or the applicable manufacturer's instructions.

FIRE DETECTION MAINTENANCE PRACTICES

Sensing elements should be inspected for:

(1)       Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components.

(2)       Abrasion caused by rubbing of element on cowling, accessories, or structural mem­bers.

(3)       Pieces of safety wire or other metal parti­cles which may short the spot detector terminals.

(4)       Condition of rubber grommets in mount­ing clamps, which may be softened from exposure to oils, or hardened from exces­sive heat.

(5)       Dents and kinks in sensing element sec­tions. Limits on the element diameter, acceptable dents or kinks, and degree of smoothness of tubing contour are specified by manufacturers. No attempt should be made to straighten any acceptable dent or kink, since stresses may be set up that could cause tubing failure. (See illustra­tion of kinked tubing in figure 5.1).

(6)      Loose nuts or broken safety wire at the end of the sensing elements (figure 5.2). Loose nuts should be re-torqued to the value specified in the manufactur­er's instructions. Some types of sensing element connections require the use of copper crush gaskets. These gaskets should be replaced any time a connection is separated.

(7)      Broken or frayed flexible leads, if used. The flexible lead is made up of many fine metal strands woven into a protective covering surrounding the inner insulated wire. Continuous bending of the cable or rough treatment can break these fine wires, especially those near the connec­tors. Broken strands can also protrude into the insulated gasket and short the center electrode.

(8)      Proper sensing element routing and clamping (figure 5.3). Long unsup­ported sections may permit excessive vibration which can cause breakage. The distance between clamps on straight runs is usually about 8 to 10 in., and is speci­fied by each manufacturer. At end connec­tors, the first support clamp is usually lo­cated about 4 to 6 in. from the end connector fittings. In most cases, a straight run of 1 in. is maintained from all connectors before a bend is started, and an optimum bend radius of 3 in. is normally adhered to.

 (9)     Rubbing between a cowl brace and a sensing element (figure 5.3). This inter­ference, in combination with loose rivets holding the clamps to skin, may cause wear and short the sensing element.

(10) Correct grommet installation. The grom­mets are installed on the sensing element to prevent the element from chafing on the clamp. The slit end of the grommet should face the outside of the nearest bend. Clamps and grommets (figure 5.4) should fit the element snugly.

(11)    Thermocouple detector mounting brack­ets should be repaired or replaced when cracked, corroded, or damaged. When replacing a thermocouple detector, note which wire is connected to the identified plus terminal of the defective unit and connect the replacement in the same way.

(12)    Test the fire detection system for proper operation by turning on the power supply and placing the fire detection test switch in the "TEST" position. The red warn­ing light should flash on within the time period established for the system. On some aircraft an audible alarm will also sound.

In addition, the fire detection circuits are checked for specified resistance and for an open or grounded condition. Tests required after repair of replacement of units in a fire detection system or when the system is inoperative include:

(i) Checking the polarity, ground, resistance and continuity of systems that use ther­mocouple detector units, and

(ii) Resist­ance and continuity tests performed on systems with sensing elements or cable detector units.

In all situations follow the recommended practices and proce­dures of the manufacturer of the type system with which you are working.

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

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.

4.7.6 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.

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.

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.

AIRCRAFT FLOW TESTS

FLOW TESTS

  General: 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 PRESSURE TESTS

PRESSURE TESTS          

General: 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.

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.

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.