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Aviation History
1963
1963 - 0086.PDF
7S FLIGHT International, 17 January 1%] 38,000,000 HOURS. . . Rolls-Royce experience has shown that, as engine overhaul life goes up, the unscheduled removal rates and in-flight shut-down rates have diminished. They have found that in-flight shut-downs fall into three main groups:— 1. Those which could have been confined to a simple failure had the engine been shut down immediately (certain bearing and blade defects come into this category). These failures can be detected by means of early-warning devices, the most useful of which is the vibration pickup—though this sort of device must, in Rolls-Royce's view, be more reliable than the engine itself, a requirement that has not been found easy to obtain in practice. 2. Those which take a long time to develop, but which normal ground inspection fails to find—such as spinning bearings on shafts. and fretting splines. Some of these defects can be detected by means of magnetic plugs located at strategic points in the oil system; the tine metal particles collected in this way can, with careful recording, be used to predict failure. Many Conways during early (1959-60) front compressor-bearing troubles were kept flying safely using this technique. 3. Those outside the control of the manufacturer, such as contamin ated fuel and bird strike. Rolls-Royce have had the unique experience of having two engines—the Dart and the Avon—in both medium-range and short range operation. Darts in the Viscount and Friendship on the one hand and Avons in the Comst 4 and Caravelle have produced evidence of a direct relationship between overhaul cost and stage length. Overhaul costs increase when short stage lengths are flown, and the number of take-offs per thousand flying hours increases. Just as undercarriages, tyres and sometimes wings are often lifed in accordance with the number of take-offs, so turbine engines are lifed primarily by the numbsr of temperature and stress cycles from start to take-off. Thus, operators who make an effort to reduce the extended use of take-off rating or high-r.p.m. ground running will achieve economies in overhaul costs. Ground running for system maintenance reasons may in the long run prove more expansive than the use of special ground rigs. The turbine engine may be cheaper to operate than the piston engine when it is young in years, but it can become expensive when costly long-life parts reach their limit. The need to budget for this is a major factor, differentiating between the piston and the turbine, that some operators tend to forget. The effect on economics of high take-off frequency can be just as marked as the prolonged use of high cruise power. The turbine blade is usually the part to suffer, (Continued on puge HO) Examples of Rolls-Royce Long-life Parts MAGNESIUM ASSEMBLIES Conway: life achieved to date, 5,500hr; I83setsseenat3,000hr Dart: life achieved to date. I6,000hr; sets seen at I0,000hr 2,150 Avon: life achieved to date, 8,000hr; 30 sets seen at 5,000hr COMPRESSORS Conway (low-pressure rotor): life achieved to date, S.SOOhr; 183 sets seen at 3,000hr Conway (high-pressure rotor): life achieved to date, S.SOOhr; 183 sets seen at 3,000hr Avon: life achieved sets seen at 5,000hr to date, 8,l50hr; 35 FLAME TUBES Conway: life achieved to date, 5,500hr; 197 sets seen at 3,000hr and 131 sets at 4,000hr Avon: life achieved to date, 7,000hr; 300 sets seen at3,0Q0hr, I50at4,000hrand45at6,000hr Dart: life achieved to date, I0,000hr; 1,325 sets seen at 4,000hr and 500 sets at 6,000hr
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