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.
Installation
Before
installing a new component, it should be inspected for any damage which may
have occurred during storage, the part number and modification state should be
checked, and it should be ensured that the storage life (paragraph 1.6) has not been exceeded. The thorough testing of
components drawn from stores is not normally required (paragraph 1.4), but it should be ascertained that external moving
parts function without binding, and operate in the correct sense. Components
which have been removed from an aircraft and are to be re-installed must be
thoroughly examined for cleanliness; pipes should be blown through with clean,
dry air.
New gaskets should be fitted to all components which require
their use, and other protective material such as may be used under straps or
clamps, should be inspected for condition before being refitted.
Some components, such as non-return valves, must be fitted
the correct way for the system to operate as intended, and are usually designed
with different types of fittings at each
connection to prevent incorrect installation. In
some cases, however, the fittings may be identical, and the direction of flow
marked on the component, should be checked.
The male threads on connections should
be sparingly lubricated before assembly, as recommended by the manufacturer,
and union nuts should be fitted by hand so as to check that the threads are not
binding and to ensure that the connections are correctly aligned. All union
nuts should be tightened to the torque values specified in the relevant
Maintenance Manual, and should be locked in the appropriate manner.
All blanks should be removed from pipes
before installation, and it should be ensured that the pipes are correctly
installed and free from acute bends and kinks or damaged protective covering,
are correctly aligned with mating connections, have adequate clearance between
adjacent pipes and structure, and have been correctly identified, locked and
supported. Flexible pipes should be checked to ensure that they are not bent,
twisted or stretched at the limits of movement of the component to which they
are attached, and are adequately supported.
After the installation of a component,
any mechanical or electrical connections should be made, and a full functioning
test should be carried out.
(a) Mechanical
controls should be connected and adjusted so that control lever movement and
valve operation are synchronized, and if stops are fitted to the valve the
control should be adjusted to ensure that these stops are contacted; full
details concerning the rigging and adjustment of the controls for a particular
system should be obtained from the relevant Maintenance Manual. Controls should
be free from binding over their full range of movement, and should have at
least the minimum specified clearance from adjacent structure. After adjustment
and checking, all linkage should be locked and lubricated as appropriate.
(b) The circuits to
electrically-operated control valves should be checked for correct installation
and functioning. Micro-switches should be adjusted carefully to ensure that
they operate positively without the plunger bottoming, and their mountings
should be checked for rigidity and security.
(c) Unless otherwise
stated, an actuator should be adjusted so that its piston does not bottom in
its cylinder at the ends of its travel, and it should be checked for smooth and
correct operation. When required by the relevant Maintenance Manual, actuators
should be filled with grease or other specified damping fluid before carrying
out a functional check.
Testing
The overhaul and testing of individual
components must be carried out in accordance with the manufacturer's Overhaul
Manual and requires the use of specially designed test rigs to ensure their
correct operation. Dismantling of components should not be undertaken unless
suitable test facilities are available, and the aircraft system should not be
considered to be an acceptable alternative.
Once tested after manufacture or
overhaul, components do not normally require further tests to be carried out
prior to installation, provided that their storage life has not been exceeded
and that there is no superficial damage. System tests should, however, be
carried out on new installations, after any part of a system has been
adjusted, dismantled, or renewed, and at the periods specified in the relevant
Maintenance Schedule. The method of carrying out a test of the pneumatic system
is detailed in the aircraft Maintenance Manual, and will normally include the
operations outlined in below.
(i) After a system has been
exhausted of air pressure, or parts of a system have been isolated from the
storage bottles to permit removal and installation of components, certain
precautions must be taken to prevent damage to the aircraft or injury to
personnel when the system is re-pressurized prior to testing. The electrical
circuits to electrically-operated controls should be reinstated by resetting
the appropriate circuit breakers or refitting the fuses, and the positions of
all controls, including emergency controls, should be checked as corresponding
to the positions of the actuators in the pneumatic services. Ground locks should be fitted to the
landing gear (unless the aircraft is on jacks), and air pressure should be
built up slowly in the relevant parts of the system, either through the
charging connection or by opening the isolation valves, as appropriate.
(ii) When a compressor has been changed,
or whenever a slow build-up in system pressure has been reported, the output of
the compressors should be checked; this check is usually carried out by running
the appropriate engine(s) on the ground. The engine power setting, initial
pressure, and maximum time permitted to build up pressure by a specific amount,
are usually quoted in tables provided in the relevant Maintenance Manual;
separate tables are often provided for checking new and in-service compressors.
(iii) When checking the
operation of the various control valves in the system, care should be taken to
ensure that the associated services are free to function and that adequate
clearance is provided between any moving part and adjacent structure, trestles,
etc. The air exhausted from some large components may be capable of causing
damage, and warning notices should be positioned before operating these
particular services.
(iv) The adjustment and
correct operation of all locks, actuators, selectors, control mechanisms and
indicators should be checked, using the appropriate test connections where
necessary, and the operating pressures of the regulators, pressure reducing
valves, pressure maintaining valves, brake valves and relief valves should be
verified. It should also be ascertained that there is no internal or external
air leakage from the valves or connections.
(v) All services should be checked for
correct operation, smoothness, and, when specified, speed of operation and
system pressure drop. These tests should be carried out using both the normal
and the emergency systems, and should be repeated a sufficient number of times
to ensure consistency.
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