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viewing and, in some instances adaptable for still or
video photography which may be linked to closed
circuit television. These instruments are used for
examining and assessing the condition of the
compressor and turbine assemblies, nozzle guide
vanes (fig. 24-2) and combustion system, and can be
inserted through access ports located at strategic
points in the engine main casings.
18. The engine condition indicators include
magnetic chip detectors, oil filters and certain fuel
filters. These indicators are frequently used to substantiate
indications of failures shown by flight deck
monitoring and in-flight recordings. For instance,
inspection of the oil filters and chip detectors can
reveal deposits from which experienced personnel
can recognize early signs of failure. Some
maintenance organizations progressively log oil filter
Maintenance
253
Fig. 24-1 A typical maintenance schedule (extracts).
and magnetic chip detector history and catalogue the
yield of particles. Fuel filters may incorporate a silver
strip indicator that detects any abnormal concentration
of sulphur in the fuel.
MAINTENANCE PRECAUTIONS
19. During engine maintenance, it is necessary to
observe certain precautions. The ignition system is
potentially lethal and, therefore, before any work is
done on the high energy ignition units, igniter plugs
or harness, the low tension supply to the units must
be disconnected and at least one minute allowed to
elapse before disconnecting the high tension lead.
Similarly, before carrying out work on units
connected to the electrical system, the system must
be made safe, either by switching off power or by
tripping and tagging appropriate circuit breakers.
With some installations, the isolation of certain
associated systems may be required.
20. When the oil system is being replenished, care
must be taken that no oil is spilt. If any oil is accidentally
spilt, it should be cleaned off immediately as it is
injurious to paintwork and to certain rubber
compounds such as could be found in the electrical
harnesses, Oil can also be toxic through absorption
if allowed to come into contact with the human skin
for prolonged periods. Care should be taken not to
overfill the oil system; this may easily occur if the
aircraft is not on level ground or if the engine has
been stationary for a long period before the oil level
is checked.
21. Before an inspection of the air intake or exhaust
system is made it must be ascertained that there Is
no possibility of the starter system being operated or
the ignition system being energized.
22. A final inspection of the engine, air intake and
exhaust system, must always be made after any
repair, adjustment or component change, to ensure
that no loose items, no matter how small, have been
left inside. Unless specific local instructions rule
otherwise, air intake and exhaust blanks or covers
should be fitted when engines are not running.
TROUBLE SHOOTING
23. The procedure for locating a fault is commonly
referred to as trouble shooting, and the requirement
under this procedure is for quick and accurate
diagnosis with the minimum associated work and the
prevention of unnecessary unit or engine removals.
24. The basic principle of effective trouble shooting
is to clearly define and interpret the reported
symptom and then proceed to a logical and
systematic method of diagnosis (fig. 24-3).
Maintenance
254
Fig. 24-2 Inspection of H.P. nozzle guide
vanes.
Fig. 24-3 Trouble shooting - logical
sequence.
25. The reported symptom will frequently originate
from flight deck instrument readings and, unless it is
apparent from supporting information that the
readings are genuine, instrumentation should be
checked before proceeding further. Similarly, quick
elimination checks should normally be undertaken
before more involved tasks. The manufacturers’
maintenance manual contains trouble shooting
information, usually in chart form and fig. 24-4 shows
a typical example.
Maintenance
255
Fig. 24-4 A typical trouble shooting chart.
26. The progressive introduction of improved and
more reliable condition monitoring devices (para. 11)
will have considerable influence on accepted trouble
shooting practice, since to a large extent these
devices are designed to pin-point, at an early stage,
the specific system or assembly at fault. The
development of suitable test sets could eventually
eliminate the need for engine ground testing after
trouble shooting
　
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