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.