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opened to allow fuel to pass to the fuel spray
nozzles, this point varying with aircraft and engine
type; on some installations the shut-off valve may be
opened before the starting cycle is initiated. During
the engine light-up period and subsequent acceleration
to idling speed, the engine exhaust gas
temperature must be carefully monitored to ensure
Maintenance
259
Fig. 24-8 A pre-start inspection sequence.
that the maximum temperature limitation is not
exceeded. If the temperature limitation appears likely
to be exceeded, the shut-off valve must be closed
and the starting cycle cancelled; the cause and
possible effect of the high temperature must then be
investigated before the engine is again started.
38. When a turbo-propeller engine is being started,
the propeller must be set to the correct starting pitch,
as recommended by the engine manufacturer. To
provide the minimum resistance to turning and thus
prevent an excessive exhaust gas temperature
occurring during the starting cycle, some propellers
have a special fine pitch setting.
39. Throttle movements should be kept to a
minimum and be smooth and progressive to avoid
thermal stresses associated with rapid changes in
temperature. Rapid throttle movements to check the
acceleration and deceleration capabilities of the
engine should be made only after all other major
checks have proved satisfactory and after some
slower accelerations and decelerations have proved
successful.
40. Before an engine is stopped, it should normally
be allowed to run for a short period at idling speed to
ensure gradual cooling of the turbine assembly. The
only action required to stop the engine is the closing
Maintenance
260
Fig. 24-9 Overheated turbine blades.
of the shut-off valve. The shut-off valve must not be
re-opened during engine rundown, as the resulting
supply of fuel can spontaneously ignite with
consequent severe overheating of the turbine
assembly. An example of turbine blades which have
been subjected to overheating is shown in fig. 24-9.
41. The time taken for the engine to come to rest
after the shut-off valve is closed is known as the
’rundown time’ and this can give an indication of any
rubbing inside the engine. However it should be
borne in mind that variations in wind velocity and
direction may affect the run-down time of an engine.
Maintenance
261
Rolls-Royce Tyne
Bristol BE 25 Orion
The Orion was a two-spool turbo-prop
designed to operate at its full rated power to
20,000ft, achieved by throttling its sea level
power to a maximum of 5150 ehp. Flight
testing commenced in August 1956 with the
engine installed in the port outer nacelle of the
Bristol Britannia. Development was discontinued
owing to lack of demand for turbo-props
at this time.
25: Overhaul
INTRODUCTION
1. It is most important that the cost of maintaining
an engine in service is considered at the design
stage. All aspects of engine repairability are also
considered, both to reduce the requirement for
overhaul or repair and to avoid, where possible,
designs which make repairs difficult to effect. Engine
construction must allow the operator to complete the
overhaul or repair work as quickly and cheaply as
possible.
2. In service, the engine is inspected at routine
periods based on manufacturers’ recommendations
and agreed between the operator and the relevant
airworthiness authority. The engine is removed from
the aircraft when it fails, during these inspections, to
meet the specified standards. This concept is a form
of ’on-condition’ monitoring, reference para. 9,
however, regardless of condition, some engines are
removed when a stipulated number of engine flying
hours have been achieved, this concept is known as
time between overhauls (T.B.O.). Operators will often
remove engines in order to acquire ’fleet stagger’
thus preventing a situation when an unacceptable
number of engines require removal at the same
period of time.
3. The length of time between overhauls varies considerably
between different engine types, being
established as a result of discussions between the
operator, the airworthiness authority and the manufacturer,
when such considerations as the total
experience gained with the particular engine series,
the type of operation, the utilization, and sometimes
climatic conditions, are taken into account. In
improving the overhaul period the airworthiness
authority may take into consideration the background
　
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