Page 262 - The Jet Engine

Maintenance

5. Because the T.B.O. is actually determined by the required may also result from malfunction, trouble
life of one or two assemblies within the engine, shooting, scheduled maintenance, and occasionally,
during overhaul, it is generally found that the other manufacturers' specific recommendations. This type
assemblies are mechanically sound and fit to of maintenance usually involves rectification
continue in service for a much longer period. adjustment or replacement.
Therefore, with the introduction of modular engines
and the improved inspection and monitoring CONDITION MONITORING
techniques available, the T.B.O. method on limiting 11. Condition monitoring devices must give
the engine's life on-wing has been replaced by the
'on-condition' method. indication of any engine deterioration at the earliest
possible stage and also enable the area or module in
6. Basically this means that a life is not declared for the total which deterioration is occurring to be identified. This
engine but only for certain parts of the engine. On reaching facilitates quick diagnosis, which can be followed by
their life limit, these parts are replaced and the engine scheduled monitoring and subsequent programmed
continues in service, the remainder of the engine being rectification at major bases, thereby avoiding in-flight
overhauled 'on condition', Modular constructed engines are shut-down, with resultant aircraft delay, and
particularly suited to this method, as the module containing minimizing secondary damage. Monitoring devices
a life limited part can be replaced by a similar module and and facilities can be broadly categorized as flight deck
the engine returned to service with minimum delay, The indicators, in-flight recorders and ground indicators.
module is then disassembled for life limited part
replacement, repair or complete overhaul as required. Flight deck indicators
12. Flight deck indicators are used to monitor
ON-WING MAINTENANCE engine parameters such as thrust or power, r.p.m.,
turbine gas temperature, oil pressure and vibration.
7. On-wing maintenance falls into two basic Most of the indicators used are described in Part 12.
categories: scheduled maintenance and Other devices, however, may be used and these
unscheduled maintenance. include:
Accelerometers for more reliable and precise
Scheduled maintenance vibration monitoring.
8. Scheduled maintenance embraces the periodic Radiation pyrometers for direct measurement of
and recurring checks that have to be effected in turbine blade temperature.
accordance with the engine section of the Return oil temperature indicators.
appropriate aircraft maintenance schedule. These Remote indicators for oil tank content.
checks range from transit items, which do not Engine surge or stall detectors.
normally entail opening cowls, to more elaborate Rub indicators to sense eccentric running of
checks within specified time limits, usually calculated rotating assemblies.
in aircraft flying hours and phased with the aircraft
check cycle. In-flight recorders
13. Selected engine parameters are recorded, either
9. Continuous 'not-exceed-limit' maintenance, manually or automatically, during flight. The
whereby checks are carried out progressively and as recordings are processed and analyzed for significant
convenient within given time limits rather than at trends indicative of the commencement of failure. An
specific aircraft check periods, has been widely in-flight recording device that may be used is the
adopted to supersede the check cycle. With the time/temperature cycle recorder. The purpose of this
progressive introduction of condition monitoring device is to accurately record the engine time spent
devices (para. 11) of increased efficiency and operating at critical high turbine gas temperatures,
reliability, a number of traditionally accepted thus providing a more realistic measure of 'hot-end'
scheduled checks may become unnecessary. life than that provided by total engine running hours.
Extracts from a typical maintenance schedule are
shown in fig. 24-1. 14. Automatic systems (Part 12) known as aircraft
integrated data systems (A.I.D.S.) are able to record
Unscheduled maintenance parameters additional to those normally displayed
10. Unscheduled maintenance covers work neces- e.g. certain pressures, temperatures and flows.
sitated by occurrences that are not normally related
to time limits, e.g. bird ingestion, a strike by lightning, 15. Many of the electronic components used in
a crash or heavy landing, Unscheduled work modern control systems have the ability to monitor

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