Wednesday, June 10, 2015

MAINTENANCE

MAINTENANCE

 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.

 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.

 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 1.5.4.

(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.

1.5.4 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

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