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of the operator, his servicing facilities and the
experience of his maintenance personnel.
4. When a new type of engine enters service,
sampling (i.e. engine removal, dismantling and
inspection) may be called for at a modest life. The
sampling will be continued until the life at which the
engine should be overhauled is indicated by the
condition of the sample engines or by its reliability
record in service. In some instances, the ultimate life
Contents Page
Introduction 263
Overhaul/Repair 265
Disassembly
Cleaning
Inspection
Repair
Balancing
Moment weighing of blades
Assembling
Testing
Preparing for storage/despatch
263
obtained may be two, three, or even four times the
original period permitted. The development of the
T.B.O, from the introduction of an engine into service,
through several years of operation, is shown as an
example in fig, 25-1.
5. Among the main factors affecting the overhaul
period for an engine is the use to which it is put in
service. For example, a military engine will generally
have a much lower T.B.O. than its civil counterpart,
as performance capability is the operating criterion
rather than economics. Due to the effect of rapid
temperature changes in the hot parts of the engine,
the most arduous treatment is the frequent changing
of power output to which short-haul transports and
fighter aircraft are subjected.
6. When aircraft are based in areas with exceptionally
high humidity or salt content in the atmosphere,
there exists the added danger of corrosion, resulting
in the need for more frequent overhauls. In peace
time, some military aircraft have a very low utilization,
this introduces the additional problem of certain
materials used in its construction deteriorating before
the engine has otherwise reached a condition which
would normally require an overhaul. Elapsed time, as
well as flying hours, would then influence the
overhaul period.
7. In addition to scheduled overhauls, there are
problems that arise from damage and defects. A
proportion of these, which are uneconomic or
impractical to rectify in the aircraft, necessitate
unscheduled removals and require the engine to be
returned to an engineering base or an overhaul shop
for partial or complete overhaul.
8. The purpose of overhaul is to restore an engine
enabling it to complete a further life by complying
with new engine performance acceptance limitations
and maintaining the same reliability. This is achieved
by dismantling the engine in order that parts can be
inspected for condition and to determine the need for
renewal or repair of those parts whose deterioration
would reduce the performance, or would not remain
in a serviceable condition until the next overhaul.
9. The design of the modular constructed engine
(Part 22, fig. 22-1) makes it particularly suited to a
different technique of overhaul/repair. This technique
is based on ’on-condition’ monitoring (Part 24). This
means that a life is not declared for the total engine
but only certain parts of the engine. On reaching their
life limit, these parts are replaced and the engine
returned to service, the remainder of the engine
being overhauled ’on-condition’.
Overhaul
264
Fig. 25-1 Example of growth of time between overhaul (T.B.O.).
10. Modular construction, together with associated
tooling, enables the engine to be disassembled into
a number of major assemblies (modules). Modules
which contain life limited parts can be replaced by
similar assemblies and the engine returned to
service with minimum delay. The removed modules
are disassembled into mini-modules for life limited
part replacement, repair or complete overhaul as
required.
OVERHAUL/REPAIR
11. The high cost of new engines has a considerable
influence on the overhaul/repair arrangements,
as the number of spare engines normally bought by
the operator is kept to an absolute minimum. This
means that an unserviceable engine must be quickly
restored to serviceability by changing a module, or a
part if the modular construction will permit it, or by
careful scheduling of planned removals for overhauls
at time expiry. This scheduling, through the
workshop, of engines or modules that require the use
of specialized equipment for repair is important, both
to keep the flow of work even and to stagger
removals to avoid aircraft being grounded by
shortage of serviceable engines or modules.
12. Because the work that is to be implemented
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