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
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.
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 disconti-nued owing to lack of demand for turbo-props at this time.
25: Overhaul
Contents Page
Introduction 263 Overhaul/Repair 265 Disassembly Cleaning Inspection Repair Balancing Moment weighing of blades Assembling Testing Preparing for storage/despatch
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.
INTRODUCTION
3. The length of time between overhauls varies con-1. It is most important that the cost of maintaining siderably between different engine types, being an engine in service is considered at the design established as a result of discussions between the stage. All aspects of engine repairability are also operator, the airworthiness authority and the manu-considered, both to reduce the requirement for facturer, when such considerations as the total overhaul or repair and to avoid, where possible, experience gained with the particular engine series, designs which make repairs difficult to effect. Engine the type of operation, the utilization, and sometimesconstruction must allow the operator to complete the climatic conditions, are taken into account. In overhaul or repair work as quickly and cheaply as improving the overhaul period the airworthinesspossible.
authority may take into consideration the background of the operator, his servicing facilities and the 2. In service, the engine is inspected at routine experience of his maintenance personnel.
periods based on manufacturers' recommendations and agreed between the operator and the relevant 4. When a new type of engine enters service,airworthiness authority. The engine is removed from sampling (i.e. engine removal, dismantling andthe aircraft when it fails, during these inspections, to inspection) may be called for at a modest life. The meet the specified standards. This concept is a form sampling will be continued until the life at which theof 'on-condition' monitoring, reference para. 9, engine should be overhauled is indicated by the however, regardless of condition, some engines are condition of the sample engines or by its reliability removed when a stipulated number of engine flying record in service. In some instances, the ultimate life
Overhaul
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 exception-ally 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.
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