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the engine returned to service with minimum delay, The
module is then disassembled for life limited part
replacement, repair or complete overhaul as required.
ON-WING MAINTENANCE
7. On-wing maintenance falls into two basic
categories: scheduled maintenance and
unscheduled maintenance.
Scheduled maintenance
8. Scheduled maintenance embraces the periodic
and recurring checks that have to be effected in
accordance with the engine section of the
appropriate aircraft maintenance schedule. These
checks range from transit items, which do not
normally entail opening cowls, to more elaborate
checks within specified time limits, usually calculated
in aircraft flying hours and phased with the aircraft
check cycle.
9. Continuous ’not-exceed-limit’ maintenance,
whereby checks are carried out progressively and as
convenient within given time limits rather than at
specific aircraft check periods, has been widely
adopted to supersede the check cycle. With the
progressive introduction of condition monitoring
devices (para. 11) of increased efficiency and
reliability, a number of traditionally accepted
scheduled checks may become unnecessary.
Extracts from a typical maintenance schedule are
shown in fig. 24-1.
Unscheduled maintenance
10. Unscheduled maintenance covers work necessitated
by occurrences that are not normally related
to time limits, e.g. bird ingestion, a strike by lightning,
a crash or heavy landing, Unscheduled work
required may also result from malfunction, trouble
shooting, scheduled maintenance, and occasionally,
manufacturers’ specific recommendations. This type
of maintenance usually involves rectification
adjustment or replacement.
CONDITION MONITORING
11. Condition monitoring devices must give
indication of any engine deterioration at the earliest
possible stage and also enable the area or module in
which deterioration is occurring to be identified. This
facilitates quick diagnosis, which can be followed by
scheduled monitoring and subsequent programmed
rectification at major bases, thereby avoiding in-flight
shut-down, with resultant aircraft delay, and
minimizing secondary damage. Monitoring devices
and facilities can be broadly categorized as flight deck
indicators, in-flight recorders and ground indicators.
Flight deck indicators
12. Flight deck indicators are used to monitor
engine parameters such as thrust or power, r.p.m.,
turbine gas temperature, oil pressure and vibration.
Most of the indicators used are described in Part 12.
Other devices, however, may be used and these
include:
Accelerometers for more reliable and precise
vibration monitoring.
Radiation pyrometers for direct measurement of
turbine blade temperature.
Return oil temperature indicators.
Remote indicators for oil tank content.
Engine surge or stall detectors.
Rub indicators to sense eccentric running of
rotating assemblies.
In-flight recorders
13. Selected engine parameters are recorded, either
manually or automatically, during flight. The
recordings are processed and analyzed for significant
trends indicative of the commencement of failure. An
in-flight recording device that may be used is the
time/temperature cycle recorder. The purpose of this
device is to accurately record the engine time spent
operating at critical high turbine gas temperatures,
thus providing a more realistic measure of ’hot-end’
life than that provided by total engine running hours.
14. Automatic systems (Part 12) known as aircraft
integrated data systems (A.I.D.S.) are able to record
parameters additional to those normally displayed
e.g. certain pressures, temperatures and flows.
15. Many of the electronic components used in
modern control systems have the ability to monitor
Maintenance
252
their own and associated component operation. Any
fault detected is recorded in its built-in memory for
subsequent retrieval and rectification by the ground
crew. On aircraft that feature electronic engine
parameter flight deck displays (Part 12) certain faults
are also automatically brought to the flight crew’s
attention.
Ground indicators
16. The devices used or checked on the ground, as
distinct from those used or checked in flight, may
conveniently be referred to as ground indicators; this
title is also taken to embrace instruments used for
engine internal inspection.
17. Internal viewing instruments can be either
flexible or rigid, designed either for end or angled
　
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