MAIN TENANCES
General: 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.
Charging. 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.
1.5.3 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 .
(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.
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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