First let us look at the history of our engines’ reliability. Fig. 1 shows the reliability in terms of unscheduled removals per 1000 hours and in-flight shut-downs for four Rolls-Royce civil engines since the time of their introduction into service.

It is significant that the general trend is similar and particularly, that after the initial years settling down period, the level of reliability defined in this way has stayed almost constant, despite considerable increases in overhaul life and, for the Dart, great increases in power output as later types of engine were produced.