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.