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

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.

PNEUMATIC SYSTEM MAINTENANCES

Like every aircraft system, pneumatic system requires some essential maintenance including testing. This week highlights some common maintenances to perform in the aircraft pneumatic system including activities in component removal/installation, testing, storage etc.

Pressure Demand Systems

Pressure Demand Systems

These systems are used primarily for high al­titude military aircraft and these will be discussed in later week along with the components (regulators) that form the basis of these classifications.

DILUTER DEMAND SYSTEM FOR CREW, WITH CONTINUOUS FLOW SYSTEM FOR PASSENGERS

Diluter Demand System For Crew, With Continuous Flow System For Passengers

Pressurized aircraft do not normally have oxygen available for passengers all of the time, but FAR Part 91 requires that under certain flight conditions, the pilot operating the controls wear and use an oxygen mask. Because of this require­ment, most executive aircraft that operate at high altitude are equipped with diluter demand or pres­sure demand oxygen regulators for the flight crew and a continuous flow system for the occupants of the cabin. A schematic of this type of system 

Installed Oxygen Systems

Installed Oxygen Systems

Installed oxygen system may be any of the three systems, in general:

(a) Continuous Flow System

(b) Diluter Demand System

(a) Continuous Flow System

(c) Pressure Demand Systems

Subsequent sections will discuss these systems along with their typical system layouts.

Continuous Flow System

a typical continuous flow oxygen system is illustrated which is installed in a single engine general aviation type of aircraft. The exter­nal filler valve is installed in a location that is convenient to service and is usually covered with an inspection door. It has an orifice that limits the filling rate and is protected with a cap to prevent contamination when the charging line is not con­nected. The storage cylinder is of an approved type and is installed in the aircraft in such a location that is most appropriate for weight and balance considerations. The shutoff valve on the cylinder is of the slow-opening type and requires several turns of the knob to open or close it to prevent too rapid a change in the flow rate which could place too much strain on the system or could generate too much heat. Some installations use a pressure reducing valve on the cylinder, and when the reducer is placed here, the pressure gauge must be mounted on the cylinder side of the reducer to determine the amount of oxygen in the cylinder.

Forms Of Oxygen

Oxygen may be on an aircraft in three basic forms:

·         As a gas

·         As a liquid

·         As a chemical (solid)

The oxygen may also be generated on board the aircraft by means of a special system. This is called On-Board Oxygen Generators (OBOGS).

Oxygen supply systems of any of the three types may be either permanently installed in the aircraft or may be portable in which there is no attachment to the airplane.

Gaseous Oxygen: Most of the aircraft in the general aviation fleet use gaseous oxygen usually stored in steel cylinders, under a pressure of between 1,800 and 2,400 psi. The main reason for using gaseous oxygen is its ease of handling and the fact that it is available at most of the airports used by these aircraft. It does have the disadvantage of all of the dangers associated with any high-pressure gas, and also there is a weight penalty because of the heavy storage cylinders.

CHARACTERISTICS OF OXYGEN

Characteristics of Oxygen

Oxygen is one of the most abundant elements on the earth. As an uncombined gas it makes up more than one-fifth of the air we breathe. Nearly 90% of the weight of water is oxygen, and oxygen is found in most of the soil and rock that makes up the earth's crust.

As a gas, oxygen is colourless, odourless and taste­less, and it is extremely active chemically and will combine with almost all other elements and with many compounds. When any fuel burns, it unites with oxygen to produce heat, and in the human body, our tissues are continually being oxidized which causes the heat our bodies produce. This is the reason an ample supply of oxygen must be available at all times to support our life.

Oxygen is produced commercially by liquefying air, and when the nitrogen is allowed to boil off, relatively pure oxygen is left. Gaseous oxygen may also be produced by the electrolysis of water. When electrical current is passed through water (H₂O), it will break down into its two elements, hydrogen and oxygen.

Oxygen will not burn, but it does support com­bustion so well that special care must be taken when handling it that it is not used where there is any fire, hot material or any petroleum products. If pure oxygen is allowed to come in contact with oil, grease or any such product, it will combine violently and generate enough heat to ignite the material, and it will burn with a very hot flame.

Iron and steel may be cut by heating it red hot with an oxyacetylene flame and then directing a jet of pure oxygen onto the hot metal. The oxygen will combine with the hot metal and produce a flame hot enough to burn through it, cutting it as though with a knife.

Commercial oxygen is used in great quantities for welding and cutting and for medical use in hospitals and ambulances. Aviators breathing oxygen is similar to that used for commercial purposes, except that it is additionally processed to remove almost all of the water that could freeze and stop the flow of oxygen when it is so vitally needed. Because of the additional purity required, you must never service an aircraft oxygen system with any oxygen that does not meet the specifica­tions for aviators breathing oxygen. This is usually military specification MIL-O-21749 or MIL-0­27210. These specifications require the oxygen to have no more than two millilitres of water per litre of gas.

AIRCRAFT OXYGEN SYSTEM

Man is an earthbound creature adapted to the lower levels of the atmosphere for required amount of oxygen to enter into his blood. This required of oxygen is essential for normal activity of human brain.

The atmosphere is a mixture of gases composed of approximately 78% nitrogen and 21% oxygen. The remaining 1% is inert gases and other gaseous element including water vapour. The percentage of these components is relatively constant towards altitude, but the pressure exerted by the air decreases as altitude increases. At lower pressure, needed amount of oxygen does not enter into the blood stream. So, there become shortages of oxygen as altitude increases resulting in anoxia/hypoxia to the passengers.

Pressurization of cabin gave the solution to the above problem. In a pressurized aircraft, there is no need of extra oxygen for the passenger. But, oxygen must be carried onboard to meet the emergency need in case of failure of pressurization and also for patients.

Oxygen is carried and stored in the aircraft in specially manufactured cylindrical bottles.

DEMISTING OF WINDSCREENS AND WINDOWS

demisting of windscreens and windows

Some aircraft uses a demisting process of cockpit windows, including the windshield and the cabin windows where warm air from aircraft pneumatic system is distributed to pass through a space inside those panels. This helps removal of fog accumulated on those panels ensuring clear vision of cockpit and cabin crew as well as the passengers.