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Abstract
Usage credits may be used to extend retirement lives for structural components. However, any credit substantiation must account for the contribution of conservative usage assumptions to the current level of safety. Structural reliability methods have been proposed as a means to achieve this end. Herein a new, relative method to determine a practically equivalent reliability (and safety) for aircraft fleets is developed using system reliability theory. Simple mathematical examples are used to illustrate the basic principles. A more realistic example based on the AHS Fatigue and Damage Tolerance subcommittee Round Robin problem is presented. These examples show that, even if only a few aircraft in a fleet operate in a severe manner, these aircraft drive the overall fleet reliability. This means that many aircraft may be able to receive credit without having any appreciable change on fleet reliability. A generalized procedure to apply the method to real world problems is developed. Use of the method as part of a certification methodology is presented along with a justification of what change in reliability would be practically equivalent. Application of this method may allow for safe extension of component lives based on usage.

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  Equivalent Level of Safety Methodology for Structural Usage Credit

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