Page 274 - The Jet Engine

Overhaul

Fig. 25-1 Example of growth of time between overhaul (T.B.O.).

obtained may be two, three, or even four times the 7. In addition to scheduled overhauls, there are
original period permitted. The development of the problems that arise from damage and defects. A
T.B.O, from the introduction of an engine into service, proportion of these, which are uneconomic or
through several years of operation, is shown as an impractical to rectify in the aircraft, necessitate
example in fig, 25-1. unscheduled removals and require the engine to be
returned to an engineering base or an overhaul shop
5. Among the main factors affecting the overhaul for partial or complete overhaul.
period for an engine is the use to which it is put in
service. For example, a military engine will generally 8. The purpose of overhaul is to restore an engine
have a much lower T.B.O. than its civil counterpart, enabling it to complete a further life by complying
as performance capability is the operating criterion with new engine performance acceptance limitations
rather than economics. Due to the effect of rapid and maintaining the same reliability. This is achieved
temperature changes in the hot parts of the engine, by dismantling the engine in order that parts can be
the most arduous treatment is the frequent changing inspected for condition and to determine the need for
of power output to which short-haul transports and
fighter aircraft are subjected. renewal or repair of those parts whose deterioration
would reduce the performance, or would not remain
6. When aircraft are based in areas with exception- in a serviceable condition until the next overhaul.
ally high humidity or salt content in the atmosphere,
there exists the added danger of corrosion, resulting 9. The design of the modular constructed engine
in the need for more frequent overhauls. In peace (Part 22, fig. 22-1) makes it particularly suited to a
time, some military aircraft have a very low utilization, different technique of overhaul/repair. This technique
this introduces the additional problem of certain is based on 'on-condition' monitoring (Part 24). This
materials used in its construction deteriorating before means that a life is not declared for the total engine
the engine has otherwise reached a condition which but only certain parts of the engine. On reaching their
would normally require an overhaul. Elapsed time, as life limit, these parts are replaced and the engine
well as flying hours, would then influence the returned to service, the remainder of the engine
overhaul period. being overhauled 'on-condition'.

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