ADS-79D-HDBK Downloaded from https://www.everyspec.com represents the availability of aircraft parameters used to quantify usage. For regime recognition based usage monitoring, the parameter coverage could range from a limited number such that only one or more key fatigue drivers can be recognized to a full complement which provides comprehensive regime recognition. The horizontal axis represents the extent to which available data provides coverage over time and across the fleet.
Under the traditional fatigue life management process, the Design Usage Spectrum (DUS) is utilized both in the design phase of a component and for fatigue life management of the component where a retirement time in terms of flight hours is established based on a calculated fatigue damage fraction associated with the DUS. The design usage spectrum is defined as a so- called ―composite worst case‖ for the fleet in terms of variability across the fleet and with some consideration for potential variability over the fleet lifetime (mission creep). With the advent of structural usage monitoring, additional Operating Usage Spectra (OUS) can be defined for fatigue life management.
The Maintenance Credits associated with the use of operating usage spectra are dependent on the sampling and parameter coverage. Possible examples include: A more accurate fleet wide operating usage spectrum used to establish flight hour retirement lives for application across the fleet. Based on fleet sampling or comprehensive monitoring with scheduled reviews to protect against time variability, this operating spectrum should be less severe than the DUS. A fleet wide operating usage spectrum in combination with an applied life factor with a local operating usage spectrum.
The local operating environment could be more severe (training or deployed) and the use of an applied life factor for flight time in time in the local operating environment would allow for a less severe usage spectrum to be applied to the fleet in general. With comprehensive usage monitoring across the fleet and with a full complement of data parameters being collected, it becomes possible to implement a fatigue life methodology that incorporates individual part fatigue lifing based on the actual part usage.
Individual part fatigue lifing requires both the tracking of usage corresponding to the component in-service time and the correlation and tracking of the calculated fatigue damage fraction associated with the tracked usage. Considerations of data parameter coverage, data sampling coverage, and the target usage credit should be included as part of the Integrity Assessment (IA), the establishment of the V&V criteria, and the V&V Plan. For usage spectrum monitoring applications, calendar time related considerations can be important as summarized in Figure K-11. Usage is typically tracked by aircraft tail number. Usage can vary among aircraft, among units, and among operational theaters.
Usage can also vary over time. Although usage is tracked by aircraft, the usage spectrum is applied to individually tracked components. 211