AIRCRAFT CABIN PRESSURISATION SYSTEM MAINTENANCES

INTRODUCTION

This week discusses activities during removal/installation of components, tests to be performed on pressurisation system and other allied maintenances.

3.2 COMPONENT INSTALLATION

Before installing any component of a cabin pressure control system, an inspection should be made to ensure that no deterioration of rubber components, corrosion of metal parts or other damage has occurred during storage or transit. The security of pipe connections, electrical connections, actuators, etc., should also be checked and, where specified, pre-installation functioning tests carried out.

Some aircraft pressure controllers' discharge valves and safety valves operate as preset units and must only be used in specific combinations. When renewing or replacing such units it is therefore essential to ensure that their part numbers are correct for the installation.

Pressure Controllers

It is recommended that where facilities are available, controllers should be bench tested before installation to ensure that the controlling mechanism has not been disturbed during storage or transit. Details of tests peculiar to particular types of controller should be obtained from the relevant Aircraft Maintenance Manual.

When assembling static pressure and reference pressure sensing pipelines, care is necessary to ensure that no obstruction or leakage can occur. Particular care is necessary to ensure that all pipes are correctly assembled. Cabin pressure must not leak into low pressure pipes or chambers as this would unbalance the control characteristics of the system. Where vents to cabin pressure form part of the system, they should not be obstructed by loose trimming or soundproofing materials.

After the installation is completed, functioning tests should be made to prove the correct connection or adjustment of electrical circuits, micro-switches, altitude selectors, rate of change control mechanisms and the pipeline system. Details of tests applicable to particular aircraft installations should be obtained from the relevant Aircraft Maintenance Manual. General guidance on the testing of pressurization systems is provided in this discussion.

Discharge Valves and Ground Automatic Relief Valves

Before installation the valve faces and seating surfaces of discharge valves should be cleaned with a soft lint-free cloth. High pressure air blasts should not be used on discharge valves of the diaphragm-controlled type as damage may be caused to the diaphragms.

To ensure that the joint between discharge valves and appropriate adaptors on the pressure bulkhead or fuselage skin is pressure tight, new mounting gaskets or 0-rings should be fitted. Where specified, mounting surfaces should be lightly smeared with the appropriate lubricant as specified in the relevant Aircraft Maintenance Manual.

Pipelines and valve unions should be checked to ensure that they are in correct alignment. Plug and socket connections to electrically actuated discharge valves should be secure.

If ditching cocks are fitted, the ditching control run in the aircraft should be checked before making the connections and locking the cocks to the appropriate discharge valve. Some discharge valves may be closed for ditching purposes by a directly mounted hand control wheel. In such cases the opening and closing of a valve should be checked by rotating the wheel through the requisite number of turns.

After installation of a discharge valve, functioning tests should be carried out to prove the correct connection of pressure sensing pipelines and, where appropriate, the connection of electrical cables to valve actuators. Details of the tests applicable to particular aircraft installations should be obtained from the relevant Aircraft Maintenance Manual.

Safety Valves and Inward Relief Valves

Safety Valves and Inward Relief Valves

Valves should be cleaned and installed in a similar manner to discharge valves. Where facilities allow, bellows-type safety valves should be subjected to a leak test before installation in the manner described in the Aircraft or Component Manufacturer’s Maintenance Manuals.

In some installations, safety valves are provided with electrical and manual override controls, in such cases functioning checks on the controls must be made after installation (e.g. limit switches of electrical actuators which may need to be adjusted to cut off the power supply when the valve is at the fully open and fully closed positions). Manual controls should be correctly rigged and checked to ensure that the controls and valve move in phase and operate freely over their full range of travel.

NOTES:

(1)    Limit Switch adjustment is normally carried out when the component is workshop tested.

(2)    After carrying out override control system checks, the appropriate controls should be reset to their normal positions.

Some valves incorporate an electrically-operated position transmitter which signals the valve position to an indicator in the crew compartment; checks should therefore also be made to ensure that the indicated positions correspond.

 Filters and Air Driers

Before installing a cartridge type of filter unit the cartridge and casing should be inspected for signs of damage and also to ensure freedom from dirt and other foreign matter.

In some unit designs the case is made of rubber and for this reason it may have been stored in French Chalk. Before fitting the cartridge into this type of case, all traces of chalk must be removed by lightly brushing the rubber with a suitable cleaning agent as recommended in the relevant Aircraft Maintenance Manual.

In some installations, air driers are of the type which utilises Silica Gel Crystals as drying media. Before installation, the units should be detached and charged with a fresh quantity of Silica Gel Crystals. After installation, a check should be made to ensure that the inspection windows, where provided, are readily visible.

When installing baffle-type air driers, it must be ensured that the sealant necessary for fixing is of the correct type and is correctly applied to the appropriate area of the fuselage skin and air drier casing.

After installation, the connections between the filters, air driers and all appropriate pressure sensing lines must be securely made and leak-tested in the manner described in the relevant Aircraft Maintenance Manual.

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.