Aircraft Distribution Systems

Distribution Systems

The methods of installing ducting and other components of distribution systems depend on the type of air conditioning system and reference must, therefore, always be made to the relevant Aircraft Maintenance Manual and the procedures specified carried out.

The following summary serves as a guide to some important aspects common to installation procedures:­

(a)       Ducting should be inspected externally and internally for cleanliness, signs of damage and security of end fittings.

(b)       Lagging, where fitted, should be inspected to ensure freedom from tears, damage and evidence of deterioration.

(c)       When fitting ring clamps, the sealing rings must be correctly positioned between duct and fittings and the fittings should abut each other squarely before the clamps are tightened.

(d)       Ring clamps should be torque-tightened to the loads specified; the loadings should be rechecked after the engine run following installation.

(e)       Ducts made from fibreglass, plastic and reinforced plastic should not be subjected to any weight or load during installation, and the straps or clamps attaching the ducts to support brackets should not be over tightened.

(f)        After replacement of a duct, the disturbed joints should be checked for leakage.

(g)       Where specified, ducts must carry identification labels.

(h)       When assembled on ducts, rubber sleeves should be in a free condition, i.e. they should not be twisted, stepped or collapsed.

(i)         Bedding tape or metal clips must be fitted between rubber sleeves and adjustable clamps to prevent damage to the sleeves when tightening the clamps. Expansion bellows, sliding clamps or gimbal mountings where installed, should be checked for full and free movement.

(j)         Electrical bonding leads must be properly secured.

MAINTENANCE

General: The information given in the following paragraphs on maintenance, periodic inspection and testing, is of a general nature and should be read in conjunction with the Maintenance manuals and Schedules for the components and aircraft concerned.

 Compressors and Blowers

(a)       Units should be inspected for damage and for security of mounting attachment to engine drives and accessory gearboxes, and also duct attachments.

(b)       Oil transfer pipes should be examined for security of attachment, signs of chafing and other damage, and for leaks. At the periods specified in the Maintenance Manual, oil filters should be removed for examination and cleaning or renewed as appropriate. If it is suspected that dirt is present in the lubrication system, all pipes and oil passages should be cleaned in the manner prescribed in the Maintenance Manual for the relevant unit. In units having an integral lubricating system, the oil level in the sump should be checked and replenished as necessary taking care that the equipment for dispensing the oil is scrupulously clean.

(c)       Where magnetic chip detectors are fitted to the lubrication system they should be removed and inspected for metal particles. If no particles are found, the chip detector, together with a new sealing ring, should be refitted and wire locked. If metal particles are present the unit should be replaced with a serviceable item.

NOTE: When refitting bayonet type chip detectors extreme care should be taken to ensure positive engagement.

2.3.3 Combustion Heaters

(a)       Heaters should be examined for security of attachment and signs of malfunctioning, the fuel system should be carefully checked for signs of leakage and drain pipes should be checked to ensure freedom from obstruction. At the specified inspection periods, igniter plugs should be cleaned, and heaters should be subjected to a pressure test in accordance with the procedure laid down by the manufacturer.

(b)       Electrical wiring and associated components should be checked for security of attachment, loose connections, chafing of insulation, etc. The sheath of the igniter plug cable should be examined for any possible indications of arcing, which would be evidenced by ' burning or discolouration of the sheath.

(c)       Filters, air and fuel regulating devices, safety devices (e.g. overheat switches, fuel cut-off valves, etc.), and all controls should be inspected, adjusted and tested as required by the Approved Maintenance Schedule.

(d)       System operation should be checked in accordance with the procedure laid down in the relevant Aircraft Maintenance Manual.

NOTE: In order to reduce the risk of the cabin air supply becoming contaminated by high concentrations of carbon monoxide from the exhaust system, it is imperative that the procedures for inspection, servicing and overhaul of combustion heaters and their associated exhaust systems are maintained to a high level.

Engine Exhaust Heating: Careful examination of heater muffs is necessary to ensure that no leakage of exhaust gases into the air delivered to the cabin can occur. Unless damage can be rectified within the scope of an approved repair scheme, exhaust pipes or muffs which show signs of cracking, corrosion or excessive high temperature scaling should be renewed. All muffs should be pressure-tested when specified in the Maintenance Schedule.

Hot and cold air ducts associated with the heating system should be free from obstruction and all controllable shutters, valves, etc., should be checked for correct functioning. The operation of the complete system should be checked during engine running.

Heat Exchangers

(a)       Heat exchangers should be inspected for security of attachment to the aircraft structure, security of air duct connections and freedom from damage.

(b)       The external surfaces of a heat exchanger matrix must be clean and the cooling air passages free from obstruction. If dirt or other forms of contamination are found the surface and air passages should be cleaned by means of a clean dry air blast.

NOTE: Instructions laid down in specific Aircraft Maintenance Manuals regarding the closing of cooling air flaps to ground blower units and ground test connections must be observed.

(c)       If a matrix has not been satisfactorily cleaned due to the contamination being excessive or hardened on to the surfaces, or if internal contamination or leakage from the charge air passages is suspected, the heat exchanger should be removed for cleaning and repair and replaced by a serviceable unit.

(d)       Cooling air shutters or flaps, linkages and actuators should be examined for freedom of movement and should be lubricated when necessary. Linkages and hinges of shutters or flaps should be checked for excessive play and lost motion.

(e)       During functional testing of a complete air conditioning system, a check should be made at all joints for air leakage.

Cold Air Units

(a)       Cold air units should be inspected for security of mountings and external locking devices, cleanliness, freedom from damage, oil leaks, and leakage of air from duct connections. In some units a magnetic chip detector is fitted to the oil sump drain plug; this should be removed and inspected for metal particles. If particles are present, the cold air unit should be replaced by a serviceable unit. If no particles are present, the chip detector together with a new sealing ring should be refitted and wire-locked.

(b)       The oil level must be checked and replenished if necessary taking care that the oil is to the specification approved for the unit that the equipment for dispensing the oil is scrupulously clean, and that the system is not overfilled.

Refrigeration Systems: 

Refrigeration packs and associated components should be checked for security of mountings, security of pipe line connections between components, and level of refrigerant. If the level is low the system should be checked for leaks and, after rectification, recharged with the refrigerant specified for the system taking care that all precautions are observed.

Temperature Control Systems

(a)       All components should be inspected for security of attachment and electrical connection, signs of damage, deterioration of electrical cables etc.

(b)       The operation of individual components should be checked during specified ground tests to ensure that they respond correctly whenever different heating and cooling conditions are selected, and also that, in combination, they maintain cabin temperature conditions within a comfortable range. It should be borne in mind that, apart from considerations of comfort, cabin temperature control limits the misting and icing of windscreens and windows and therefore affects the safe operation of aircraft. The operation of components, systems and circuits, designed specifically for emergency operating conditions, must also be checked during ground test procedures. 10.8.3            The test procedures vary and the extent to which a system can be tested may be limited, particularly in relation to ram air methods of cooling. On the other hand, full-range temperature control of a system in some aircraft may be checked on the ground. Reference must therefore always be made to the relevant Aircraft Maintenance Manual and Maintenance Schedule for the procedure to be adopted and precautions to be observed.

Valves

(a)       The maintenance of valves associated with air temperature control is usually confined to; inspection for cleanliness, security of attachment ducting attachments and, where applicable, security of electrical connections, functioning tests and light lubrication specified by the manufacturer of the component.

(b)       Sliding or rotating parts of valve assemblies should be free from scores, damage or excess static friction. The maximum effort required to move a valve should be checked when necessary and should not exceed the figure recommended by the manufacturer. However some electro-mechanically operated valves are not designed to operate without the application of an electrical supply. Therefore reference should be made to the specific Maintenance Manual for test instructions, before manual operation.

(c)       Lubricants should be of the type specified for the component and should be applied sparingly taking care to prevent oil entering air supply ducts.

(d)       Valve seats and valves faces should be kept free of dust or traces of lubricant.

(e)       Checks on the operation of valves should normally be carried out during ground testing of temperature control systems since their functions are integrated.

  Distribution Systems

(a)       All ducting and associated air distribution components should be inspected for security and general condition, particular attention being given to joints between duct sections and components.

(b)       Lagging should always be properly secured and free from oil, hydraulic fluids etc. It should be remembered that duct sections in some parts of a system often become heated to a degree sufficient to make oil-soaked lagging flammable.

(c)       When specified, ducts should be proof-tested at the pressure recommended by the manufacturer; normally a workshop function. Pressure tests are however, more often made with the object of detecting leaks, in which case the test pressure is not critical provided it does not exceed a value which might damage the duct.

(d)       It is usually more convenient to test a complete distribution system by dividing it into sections and applying a recommended pressure separately and in sequence. The sections should be selected so that all critical joints are subjected to the test pressure; advantage being taken of shut-off valves, non-return valves, etc., where these provide convenient boundaries between sections.

(e)       Leaks can be detected by sound or feel, although these are sometimes revealed by discolouration and holes blown in the lagging. If there is difficulty in locating leaks, the soap and water method can be used.

NOTE: Because of the high operating temperatures and pressures involved, it is recommended that care should be taken when carrying out a physical check for air leaks.

Refrigeration Systems

Refrigeration Systems: 

Refrigeration packs and associated components should be checked for security of mountings, security of pipe line connections between components, and level of refrigerant. If the level is low the system should be checked for leaks and, after rectification, recharged with the refrigerant specified for the system taking care that all precautions are observed.

Temperature Control Systems

(a)       All components should be inspected for security of attachment and electrical connection, signs of damage, deterioration of electrical cables etc.

(b)       The operation of individual components should be checked during specified ground tests to ensure that they respond correctly whenever different heating and cooling conditions are selected, and also that, in combination, they maintain cabin temperature conditions within a comfortable range. It should be borne in mind that, apart from considerations of comfort, cabin temperature control limits the misting and icing of windscreens and windows and therefore affects the safe operation of aircraft. The operation of components, systems and circuits, designed specifically for emergency operating conditions, must also be checked during ground test procedures. 10.8.3            The test procedures vary and the extent to which a system can be tested may be limited, particularly in relation to ram air methods of cooling. On the other hand, full-range temperature control of a system in some aircraft may be checked on the ground. Reference must therefore always be made to the relevant Aircraft Maintenance Manual and Maintenance Schedule for the procedure to be adopted and precautions to be observed.

Valves

(a)       The maintenance of valves associated with air temperature control is usually confined to; inspection for cleanliness, security of attachment ducting attachments and, where applicable, security of electrical connections, functioning tests and light lubrication specified by the manufacturer of the component.

(b)       Sliding or rotating parts of valve assemblies should be free from scores, damage or excess static friction. The maximum effort required to move a valve should be checked when necessary and should not exceed the figure recommended by the manufacturer. However some electro-mechanically operated valves are not designed to operate without the application of an electrical supply. Therefore reference should be made to the specific Maintenance Manual for test instructions, before manual operation.

(c)       Lubricants should be of the type specified for the component and should be applied sparingly taking care to prevent oil entering air supply ducts.

(d)       Valve seats and valves faces should be kept free of dust or traces of lubricant.

(e)       Checks on the operation of valves should normally be carried out during ground testing of temperature control systems since their functions are integrated.

  Distribution Systems

(a)       All ducting and associated air distribution components should be inspected for security and general condition, particular attention being given to joints between duct sections and components.

(b)       Lagging should always be properly secured and free from oil, hydraulic fluids etc. It should be remembered that duct sections in some parts of a system often become heated to a degree sufficient to make oil-soaked lagging flammable.

(c)       When specified, ducts should be proof-tested at the pressure recommended by the manufacturer; normally a workshop function. Pressure tests are however, more often made with the object of detecting leaks, in which case the test pressure is not critical provided it does not exceed a value which might damage the duct.

(d)       It is usually more convenient to test a complete distribution system by dividing it into sections and applying a recommended pressure separately and in sequence. The sections should be selected so that all critical joints are subjected to the test pressure; advantage being taken of shut-off valves, non-return valves, etc., where these provide convenient boundaries between sections.

(e)       Leaks can be detected by sound or feel, although these are sometimes revealed by discolouration and holes blown in the lagging. If there is difficulty in locating leaks, the soap and water method can be used.

NOTE: Because of the high operating temperatures and pressures involved, it is recommended that care should be taken when carrying out a physical check for air leaks.

Aircraft Temperature Control System Components

Aircraft Temperature Control System Components

(a)       The temperature control of complex air conditioning systems is usually accomplished either electrically or electronically. Consequently the following precautions are normally adopted when installing such equipment.

(b)       As temperature-sensing elements are positioned so that they will be directly affected by the changes in duct and cabin air temperatures. Therefore, care should be taken to ensure that elements sensing cabin air temperature are not shielded by loose upholstery, and are protected if paint spraying or similar operations are performed in their vicinity.

(c)       The damping effect of shock absorbers and anti-vibration mountings which may provide support for electronic amplifiers and similar sensitive equipment, should be checked by hand after installation.

(d)       Cables interconnecting components must be of the rating specified by the manufacturer and all connections must be clean and securely made.

(e)       When installing control units, care should be taken that such controls as pre-set potentiometers and fine adjustment resistors are not disturbed.

(f)        On completion of the installation of a component, sensitivity tests and final balance adjustments should be carried out in accordance with the procedure laid down for the specific aircraft system. Tests of the overall controlling function should also be made by selecting various temperature settings and noting that the actuators controlling such components as heat exchanger cooling air flaps, by-pass valves, etc., move in the appropriate directions.

Valves

(a)       Mechanically and electrically-operated valves are employed in the various types of heating, ventilating and air conditioning systems and therefore Maintenance Manuals should always be referred to for the appropriate installation procedures. The details given in the following paragraphs are of a general nature.

(b)       All valves should be inspected before installation for cleanliness, signs of damage and freedom of movement. Functional checks should be made on electrically-operated valves, e.g. spill valves, by-pass valves and choke valves to ensure that limit switches are correctly adjusted at the extremes of valve travel.

(c)       Valves are often marked with arrows to indicate the direction of flow and particular care is necessary to ensure that the valve is installed in correct relation to flow.

(d)       The attachment of valves to their respective mountings and duct sections must be secure and torque loadings strictly observed.

(f)        Electrical connections to actuators and to position indicators where fitted, should be checked against the relevant wiring diagrams, and plugs, sockets and terminal screws checked for security.

(f)        On completion of the installation of a valve, an in-situ functional test should be carried out in accordance with the procedure specified in the relevant Component and Aircraft Maintenance Manual.

AIRCRAFT AIR-CONDITIONING PLANT INSTALLATION AND MAINTENANCES

INTRODUCTION

This week describes installation and maintenance activities that are generally performed on aircraft air-conditioning plant.

  INSTALLATION PROCEDURES

General: The information given in the following paragraphs is of a general nature, and is intended as a guide to the procedures associated with the installation of the principal components of air conditioning systems. Full details are contained in the Aircraft Maintenance Manuals for specific aircraft types. Therefore, reference must be made to these publications.

Compressors and Blowers

(a)      Before installation a check should be made to ensure that units are free from damage and that ducts, air inlets and outlets, and mating surfaces are free from oil, dust and other foreign matter. Rotors should also be checked for freedom of rotation observing any special precautions and procedures specified for the appropriate types of unit.

(b)      Pipes, metering units and filters of bearing lubricating oil systems should also be inspected for cleanliness and signs of cracks or other damage. Priming of the lubricating oil system should be carried out as specified in the Component and Aircraft Maintenance Manuals.

(c)      Units must be adequately supported during installation to ensure that their weight is not allowed to bear on r arts of the main drive; for example, a quill shaft which drives a displacement blower. In some aircraft employing compressors a special hoist is provided for installation and removal of units and this should be used in the prescribed manner.

(d)      After a compressor or blower has been lowered on to the engine or gearbox mounting pad, its securing nuts or bolts, as appropriate, should be torque-tightened to the values specified in the Aircraft Maintenance Manual. In some compressor installations the units must also be secured by bolting them to the casing of their respective engines via link assemblies.

(e)      Inlet and outlet duct attachment flanges should be clean and free from damage. In displacement blower systems, manifolds normally provide for the attachment of duct sections to the blower casing. The bolts securing each manifold to the blower, are, in some cases, of different lengths. Therefore to avoid distortion of the inner face of the blower casing they must be refitted in their correct position before tightening. New sealing rings should be fitted between duct sections and corresponding attachment points on compressors and blowers. The sections should fit squarely and not be subjected to undue strain or load, or be in contact with other components which may abrade duct surfaces.

Combustion Heaters

(a)       Before installation, combustion heaters should be inspected, and when necessary, pressure tested in the manner prescribed in the Aircraft Maintenance Manual to ensure that no fuel or combustion products leak into the cabin air supply

(b)       Heaters should be installed in the manner specified in the Aircraft Maintenance Manual concerned, taking care that air and fuel leakages do not occur at duct joints or connections. There should be no connection between the combustion air and cabin air supplies and no leakage of air or exhaust gas into the aircraft.

(c)       Equipment associated with the heating system such as flow valves, air regulators, thermostatic devices and ducts should be correctly interconnected, and mechanical movements, flows and temperature settings checked and adjusted.

(d)       After the installation of a heater the system should be ground tested in the manner specified in the relevant Aircraft Maintenance Manual.

NOTE: Unburnt fuel or fuel vapour should not be allowed to accumulate within the combustion system or aircraft particularly during component functioning tests

2.2.4 Engine Exhaust Heaters

(a)       When installing heater muffs around piston engine exhaust systems it must be ensured that they are in such isolation that exhaust gases cannot enter the muff and subsequently be discharged into the heating and ventilating system.

(b)       Cooling air intakes and hot air ducting should be installed so that no obstruction or leakage of the air supply can occur. All joints should be correctly aligned and clamps securely fixed.

Heat Exchangers

(a)       Before installation, heat exchangers should be inspected to ensure that no foreign matter has entered the various connections, that there are no evident cracks or other damage and that ram air passages are free from obstruction.

(b)       Heat exchangers are heavy units and they must therefore be adequately supported during installation to prevent them fouling ducting, other system components and parts of the aircraft structure.

(c)       The fore-and-aft and transverse clearances for mounting flanges and bolts should be checked to ensure that they are within the limits specified in the relevant Maintenance Manuals. Mounting bolts should be tightened to the required torque values.

(d)       New seals and 0-rings should be fitted to the joints between system ducts, cooling air inlet and outlet flanges, and charge-air connections. Nuts, bolts and clamps should not be over tightened as connection flanges may distort and cause damage to adjacent brazed joints. After installation the joints should be leak tested in accordance with the procedure laid down in the relevant Aircraft Maintenance Manual.

(e)       If disturbed during installation of a heat exchanger, cooling air shutters or flaps should be tested and adjusted as necessary. Movable parts should operate freely, and the limit switches of electrical actuators should isolate the power supply when the shutter or flap has moved through its full travel.

 Cold Air Units

(a)       When installing cold air units care is necessary to exclude dirt and oil from the air ducts and casings. Dirt and other foreign matter may damage the rotating parts and oil may introduce unpleasant or flammable vapours into the cabin air supply. Duct attachment flanges, unit mounting flanges, and casings, should be examined for signs of burns, cracks, distortion or other damage.

(b)       Units with integral wet sump lubrication should be primed with oil to the approved specification to ensure that all bearing surfaces have been lubricated. Reference should be made to the Maintenance Manuals of relevant units for details of the lubricants required. The unit should be supported on a bench in the normal operating attitude while the quantity of oil specified for the unit is poured in. To ensure that oil is distributed to the bearings, the rotating assembly of the unit should be spun over by hand at the same time checking that the rotation is free and without noise or vibration (see Note). The unit should then be drained and installed in the aircraft and after securing it to its appropriate mounting, refilled with oil to the level marked on the sump dipstick.

NOTE: In some cold air units, air bearings are used to support the main rotating assembly, which do not allow free rotation from the idle state. Therefore reference should be made to the specific Maintenance Manual for further details.

(c)       The lubricants recommended for Cold Air Units are various and possibly incompatible with each other. Therefore, when priming or servicing these units, care should be taken to ensure that the oil is of the correct type and specification and the containers used are clean and free from contamination of any kind.

(d)       New seals should be fitted between the air distribution ducts and attachment flanges on the cold air unit, and when securing the ducts it should be ensured that they fit squarely and are not subjected to undue strain or load. Leak checks on units should be carried out during functional testing of the air conditioning system.

Refrigeration Systems: The individual components of a refrigeration system can usually be removed and installed separately. However the Maintenance Manuals appropriate to the system and aircraft should always be referred to before attempting such work. Some of the general precautions applicable to closed circuit Vapour Cycle Systems are as follows:­

(a)       Gloves and goggles should be worn when handling liquid refrigerants which can be harmful to the skin and eyes.

(b)       Before charging a newly installed system, or recharging a system which has been partly disconnected, all air should be evacuated in the manner prescribed in the relevant Maintenance Manual.

(c)       While refilling is in progress, care should be taken to ensure that refrigerant used is of the specified type, and quantity, and that all precautions recommended by the manufacturer are observed.

NOTE: The Refrigerant used in Vapour Cycle Cooling Systems, usually contains a specific amount of oil to lubricate the compressor bearings. Therefore, in order to maintain the correct ratio of constituent parts, reference should be made to the relevant Maintenance Manual for the correct volume of oil to be added.

MAIN TENANCES

 MAIN TENANCES

 General: Maintenance of the pneumatic system should be carried out in accordance with the relevant Maintenance Manual and Schedule, and should include replenishment from an external source as necessary, routine inspections for condition, cleaning of filters, replacement of desiccants and checking for leaks.

Charging. A pneumatic system is fitted with one or more charging valves, by means of which the system may be fully pressurized from an external source. These valves also act as, or include, a non-return valve, and are fitted with a dust cap which must be removed when connecting an external supply. Any external supply, whether from high-pressure storage bottles or a mobile compressor, must be fitted with oil-and-water traps, and, preferably, a dehydrator, to ensure that the air supplied is clean and dry. The supply hose should be capped when not in use, and should be blown through with compressed air before being connected to the charging valve, to prevent the introduction of moisture or dirt into the aircraft system. Care should be taken to turn off the external supply and to release air pressure from the supply hose before disconnecting it from the aircraft.

1.5.3 Routine Inspection. The scheduled routine servicing of the pneumatic system should include the following operations:­

(i)         Filters. Wire-gauze air and oil filters such as may be fitted to a compressor, should be removed for cleaning and inspection at frequent intervals; cleaning in solvent is usually recommended, and the filters should be dried thoroughly before being refitted. The main air filter usually has a paper or felt element, and this should be renewed at the specified periods. This filter should also be drained periodically in order to check for the presence of water or oil, and this is best carried out by un­screwing the drain plug a quarter turn and releasing the trapped air; if moisture is found, the filter housing should be thoroughly dried and the element renewed, and if oil is found the compressor and the oil-and-water trap should be examined.     A porous metal filter may also be fitted in some systems, and this is usually cleaned by reverse­ flushing with methylated spirits; the filter must be thoroughly dried before replacing it in the system.

(ii)        Physical Condition. All components and pipelines in the system should be examined periodically, for corrosion, cracks, dents and other superficial damage. Minor damage may often be removed and the area re-protected, but some components (e.g. storage bottles) must be considered unserviceable if the damage extends beyond the protective treatments. The components should also be checked for security and locking, and the pipelines for satisfactory clamping, protection and identification. Any leaks found should be treated as outlined in paragraph .

(iii)     Storage Bottles. Storage bottles should be drained periodically to remove any sediment or moisture which may have accumulated. Draining is best carried out with pressure in the system, but the drain plug should not be unscrewed more than a quarter turn; without pressure in the bottle the drain plug may be completely removed, and it may be necessary to use a thin rod to clear any congealed sediment.     After draining, the drain plug should be tightened to the specified torque and re-locked. The pressure testing of storage bottles should be carried out in accordance with, and at the times specified in, the relevant manuals.

(iv)      Oil and-Water Trap. The oil-and-water trap should be drained daily, or after each flight if freezing conditions exist, to prevent the freezing of water in the pipe from the compressor. Draining should be carried out as soon as possible after flight, and the procedures outlined in paragraph 1.6 for storage bottles should be used.

(v)      Dehydrator. The periods at which the alumina charge or other desiccant should be changed, depend on the weather conditions in general, and may vary considerably; the actual periods should be determined by experience, and should be such that the dehydrating agent never becomes saturated with moisture. In many cases it will be necessary to remove the dehydrator in order to recharge it, and the following pro­cedure should be used:­

(a)    Remove residual pressure from the container by means of the drain plug on the oil-and-water trap.

(b)    Disconnect the pipe connection on the container, release the securing strap, and remove the container from the aircraft.

(c)    Unscrew the end cap from the container and remove the dehydrating agent.

(d)    Remove any moisture from the container by passing warm, dry air through it, and clean the outlet filter in methylated spirit. Check the container for corrosion.

(e)    Examine any seals for damage or deterioration, and renew as necessary.

(f)     Fill the container with a fresh charge of dehydrating agent, then refit and lock the end cap.

(g)    Refit the container in the aircraft, and tighten and lock the connections and securing strap.

         NOTE: The dehydrating agent is normally delivered in air-tight tins, but if permitted by the manu­facturer the old charge may be re-activated, in emergency, by heating to 250^°C to 300^°C for 4 to 5 hours. 6.2.6 Lubrication. Any linkage associated with the control levers and valves in the pneumatic system, should be lubricated in accordance with the relevant Maintenance Manual, at the periods specified in the Maintenance Schedule. Engine oil is generally satisfactory for use on the threads of fasteners and components, but silicone grease may be recommended for use on some components (e.g. the dehydrator end cap), where it may come into contact with rubber seals.

(vi)      System Operation. The operation of the complete system should be checked at the intervals specified in the Maintenance Schedule, whenever components are changed, and whenever faulty operation is reported.     The method of testing a system is specified in the relevant Maintenance Manual, and the operations which are usually included are outlined in paragraph 1.4.

Leakage. In high-pressure pneumatic systems some leakage will inevitably occur, and manufacturers usually lay down a maximum permissible leakage rate for a particular aircraft system.                             Leakage will sometimes become apparent through the slow or incorrect operation of a service, or failure to maintain system pressure, but a small leakage may only be noticed by a drop in system pressure when the aircraft is out of use for a short period (e.g. overnight).The leakage rate is checked by fully pressurizing the system, then re-checking the pressure after a period of 12 hours (or other specified time). The initial and final pressures should be recorded, taking into account the ambient tempera­ture at the time; if this drop exceeds the maximum permitted, a check for leaks should be carried out.

(i)         Checking for Leaks. Large external leaks can often be traced aurally or by the application of a non-corrosive soapy water solution (bubbles will appear at the position of a leak); all traces of soap solution must be removed after the test, using plenty of clean water, and the parts must be thoroughly dried. Smaller external leaks may not be detectable by these methods, but several types of electronic leak detectors are available which can be used to detect even the smallest leak. These detectors usually operate on ultrasonic principles, or by measurement of the positive ions emitted from the leak after a small quantity of carbon tetrachloride has been intro­duced into the system; operation of these detectors should be in accordance with the manufacturer's instructions. Internal leakage may be difficult to trace, and a know­ ledge of the particular system is essential. Leakage past seals and valves may often be found by checking the exhaust pipes, or by removing a connection and substituting a length of hose, the other end of which is held below the surface in a bucket of water; bubbles will indicate leakage from the component upstream of the disconnected pipe.

(ii)      Curing Leaks. Leakage may be caused by a number of faults, such as deteriora­tion of seals, loosening of nuts, splits in pipes, scoring of cylinder walls, or worn valve seats. Leakage from a pipe connection may sometimes be cured by tightening the union nut, but excessive force must not be used; if the leak persists after tightening, new parts should be fitted. Internal leakage from components will often require their removal for overhaul, but the replacement of seals and gaskets is sometimes permitted. Extreme care is necessary when refitting seals, and special tools may be  required; any damage to the seal or component caused by careless handling could result in further leaks. When re-assembling components, absolute cleanliness is essential, and the tests specified in the relevant manual should be carried out before installing them in an aircraft.

STORAGE

Pneumatic components are normally packed in sealed containers or plastic bags, and should not be unpacked until required for use.   They should be stored in conditions which are dry, and free from corrosive fumes. The storage life of assemblies is determined by the non-metallic parts, such as seals, that they contain, and upon storage conditions. The date of packing, record of tests carried out, and storage life of a component should be marked on the container, but storage life may also be checked by reference to the Maintenance Manual.

(i)         Pipes are usually blanked and wrapped for storage, but flexible pipes should always be stored in the shape in which they were manufactured or have assumed during use.

(ii)        Components removed from storage for installation on an aircraft should be examined for external damage and corrosion, and the condition of all threads should be checked. Where applicable the components should be blown through with clean, dry compressed air, and every precaution should be taken to prevent the ingress of dirt or moisture.

REMOVAL AND INSTALLATION OF PNEUMATIC COMPONENTS

 REMOVAL AND INSTALLATION OF PNEUMATIC COMPONENTS

 Aircraft pneumatic installations vary consi­derably, and reference should be made to the relevant Maintenance Manual before any work is carried out on a particular aircraft. Failure to observe any precautions detailed by the manufacturer could result in damage to the aircraft and, possibly, in physical injury. High pressures exist in parts of the system even when the aircraft engines are not running, and this pressure must be released before attempting to disconnect or remove any com­ponents or pipelines. Rapid operation of the system services is also a feature of pneumatic systems, and care must be taken during any tests to ensure that the services have complete freedom of movement and that the area is clear of personnel.

 Cleanliness: 

The cleanliness of a pneumatic system is of the utmost importance to its correct operation. The filters fitted in the system will, if serviced at the appropriate intervals, protect the system components from contamination during normal use, but whenever a connection is broken or components are removed, the open pipes should be blanked immediately to prevent the entry of dirt and moisture; blanks should be left in position until the component is re-installed or the connection is re-made. Proper blanking caps should be fitted wherever possible, and on no account should rags or masking tape be used. Any external rig which is likely to be used to charge an aircraft system must be kept to the same standards of cleanliness, and the supply line should be blown through before being connected to the aircraft charging point.

  Removal of Components: 

Before removing any components or disconnecting any pipelines, all pressure should be released from that part of the system. In some cases release of all pressure from the storage bottle will be specified by the manufacturer as being necessary; in some systems this is done in by operating the discharge valve, but in other systems it may be necessary to unscrew a connection a quarter turn to release the air. Even those parts of the system protected from storage bottle pressure by a non-return valve or isolation valve may retain sufficient residual pressure to cause damage, and pipe connections should, therefore, be unscrewed slowly, pausing after the first quarter turn of the union nut to ensure that air pressure escapes slowly.

On aircraft which have a pneumatically-operated landing gear retraction system, ground locks should be fitted before releasing air from the `down' lines in the system, and the landing gear control lever and emergency landing gear selector should be labelled to ensure that they are not operated.

On systems which have electrically-operated control valves it will usually be necessary to electrically isolate the part of the system being worked on, and this may be done by tripping the associated circuit-breakers or removing the associated fuses. Electrical isolation and placarding of controls is advisable in order to avoid any possible inadvertent selection, whether or not power is available at the time. Note should be taken of the disconnected circuits for reference when re-assembling.

Where a component, such as the compressor, has to be removed because of mechanical failure, other parts of the system may have become contaminated by metal particles. Filters downstream of the component which has failed should be checked for contamination, and if this is found, all components and pipes which may have been affected should be removed and cleaned or renewed as necessary.

Immediately after removing a component all openings should be blanked; flexible pipes should be secured to adjacent structure to prevent them from becoming damaged.

 Installation

Before installing a new component, it should be inspected for any damage which may have occurred during storage, the part number and modification state should be checked, and it should be ensured that the storage life (paragraph 1.6) has not been exceeded. The thorough testing of components drawn from stores is not normally required (paragraph 1.4), but it should be ascertained that external moving parts function without binding, and operate in the correct sense. Components which have been removed from an aircraft and are to be re-installed must be thoroughly examined for cleanliness; pipes should be blown through with clean, dry air.

New gaskets should be fitted to all components which require their use, and other protective material such as may be used under straps or clamps, should be inspected for condition before being refitted.

Some components, such as non-return valves, must be fitted the correct way for the system to operate as intended, and are usually designed with different types of fittings at each connection to prevent incorrect installation.   In some cases, however, the fittings may be identical, and the direction of flow marked on the component, should be checked.

The male threads on connections should be sparingly lubricated before assembly, as recommended by the manufacturer, and union nuts should be fitted by hand so as to check that the threads are not binding and to ensure that the connections are correctly aligned. All union nuts should be tightened to the torque values specified in the relevant Maintenance Manual, and should be locked in the appropriate manner.

All blanks should be removed from pipes before installation, and it should be ensured that the pipes are correctly installed and free from acute bends and kinks or damaged protective covering, are correctly aligned with mating connections, have adequate clearance between adjacent pipes and structure, and have been correctly identified, locked and supported. Flexible pipes should be checked to ensure that they are not bent, twisted or stretched at the limits of movement of the component to which they are attached, and are adequately supported.

After the installation of a component, any mechanical or electrical connections should be made, and a full functioning test should be carried out.

(a)       Mechanical controls should be connected and adjusted so that control lever move­ment and valve operation are synchronized, and if stops are fitted to the valve the control should be adjusted to ensure that these stops are contacted; full details concerning the rigging and adjustment of the controls for a particular system should be obtained from the relevant Maintenance Manual. Controls should be free from binding over their full range of movement, and should have at least the minimum specified clearance from adjacent structure. After adjustment and checking, all linkage should be locked and lubricated as appropriate.

(b)       The circuits to electrically-operated control valves should be checked for correct installation and functioning. Micro-switches should be adjusted carefully to ensure that they operate positively without the plunger bottoming, and their mountings should be checked for rigidity and security.

(c)       Unless otherwise stated, an actuator should be adjusted so that its piston does not bottom in its cylinder at the ends of its travel, and it should be checked for smooth and correct operation. When required by the relevant Maintenance Manual, actuators should be filled with grease or other specified damping fluid before carrying out a functional check.

 Testing

The overhaul and testing of individual components must be carried out in accordance with the manufacturer's Overhaul Manual and requires the use of specially designed test rigs to ensure their correct operation. Dismantling of components should not be undertaken unless suitable test facilities are available, and the aircraft system should not be considered to be an acceptable alternative.          

Once tested after manufacture or overhaul, components do not normally require further tests to be carried out prior to installation, provided that their storage life has not been exceeded and that there is no superficial damage. System tests should, however, be carried out on new installa­tions, after any part of a system has been adjusted, dismantled, or renewed, and at the periods specified in the relevant Maintenance Schedule. The method of carrying out a test of the pneumatic system is detailed in the aircraft Maintenance Manual, and will normally include the operations outlined in below.

(i)       After a system has been exhausted of air pressure, or parts of a system have been isolated from the storage bottles to permit removal and installation of components, certain precautions must be taken to prevent damage to the aircraft or injury to personnel when the system is re-pressurized prior to testing. The electrical circuits to electrically-operated controls should be reinstated by resetting the appropriate circuit breakers or refitting the fuses, and the positions of all controls, including emergency controls, should be checked as corresponding to the positions of the actuators in the pneumatic services.         Ground locks should be fitted to the landing gear (unless the aircraft is on jacks), and air pressure should be built up slowly in the relevant parts of the system, either through the charging connection or by opening the isolation valves, as appropriate.

(ii)        When a compressor has been changed, or whenever a slow build-up in system pressure has been reported, the output of the compressors should be checked; this check is usually carried out by running the appropriate engine(s) on the ground. The engine power setting, initial pressure, and maximum time permitted to build up pressure by a specific amount, are usually quoted in tables provided in the relevant Maintenance Manual; separate tables are often provided for checking new and in-service compressors.

(iii)     When checking the operation of the various control valves in the system, care should be taken to ensure that the associated services are free to function and that adequate clearance is provided between any moving part and adjacent structure, trestles, etc. The air exhausted from some large components may be capable of causing damage, and warning notices should be positioned before operating these particular services.

(iv)     The adjustment and correct operation of all locks, actuators, selectors, control mechanisms and indicators should be checked, using the appropriate test connections where necessary, and the operating pressures of the regulators, pressure reducing valves, pressure maintaining valves, brake valves and relief valves should be verified. It should also be ascertained that there is no internal or external air leakage from the valves or connections.

(v)       All services should be checked for correct operation, smoothness, and, when specified, speed of operation and system pressure drop. These tests should be carried out using both the normal and the emergency systems, and should be repeated a sufficient number of times to ensure consistency.

Aircraft Weight and Balance Adverse-Loaded CG Checks

Aircraft Weight and Balance Adverse-Loaded CG Checks

Many modern aircraft have multiple rows of seats and
often more than one baggage compartment. After any
repair or alteration that changes the weight and balance,
the A&P mechanic or repairman must ensure that no
legal condition of loading can move the CG outside of its
allowable limits. To determine this, adverse-loaded CG
checks must be performed and the results noted in the
weight and balance revision sheet.

Aircraft Weight and Balance Empty-Weight CG Range

Aircraft Weight and Balance Empty-Weight CG Range

The fuel tanks, seats, and baggage compartments of
some aircraft are so located that changes in the fuel or
occupant load have a very limited effect on the balance
of the aircraft. Aircraft of such a configuration show an
EWCG range in the TCDS. [Figure 5-5] If the EWCG is
located within this range, it is impossible to legally load
the aircraft so that its loaded CG will fall outside of its
allowable range.

If the TCDS list an empty-weight CG range, and after the
alteration is completed the EWCG falls within this range,
then there is no need to compute a fore and aft check for
adverse loading.

But if the TCDS lists the EWCG range as “None” (and
most of them do), a check must be made to determine
whether or not it is possible by any combination of legal
loading to cause the aircraft CG to move outside of either
its forward or aft limits