Gas turbine discs are classified as Critical Parts since, in the event of their primary failure, high energy debris can be released potentially resulting in hazardous consequences to the aircraft. Critical Parts are monitored during the life cycle of a gas turbine engine to ensure that integrity is established and maintained. The predicted safe cyclic life for an engine disc must be calculated as part of this monitoring process. For calculating the life of a turbine disc, a thorough understanding of material properties, operating conditions, metal temperatures and the resultant stress field is required. These inputs are obtained variously by component or whole engine testing or by predictive methods. These methods evolve over time, and materials may need to be changed, so for legacy engine designs, it’s important that the monitoring process recognises this and reacts appropriately. This paper describes the application of probabilistic methods to determine the uncertainty of turbine disc cyclic life for a two shaft low by-pass ratio gas turbine engine designed originally in the 1950s but predicted to be in service to beyond 2030. For the subject gas turbine the original material used to manufacture the turbine discs was declared technically and commercially obsolete. A new material was selected, requiring a new cyclic life to be determined. Rather than run an engine test to measure temperatures of the new discs, an analytical approach was adopted involving air system and thermal modelling and robust design techniques. These included Monte-Carlo analyses and the linking of thermal modelling and cyclic lifing codes using optimisation tools. It is shown how a probabilistic approach to air system and thermal modelling has enabled: (i) a quantitative judgment on the value of an air system survey (ii) the uncertainty of thermal predictions and the resultant variation in life to be quantified. These methods and results have then been used to release a safe cyclic life of a turbine disc for operation in an aircraft without the use of a dedicated thermocouple test.

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