Downloaded from https://www.everyspec.com ADS-79D-HDBK FIGURE C-3. Illustration of damage progression and effect of inspections During repair or replacement, the damage in the structural component will be reduced or removed. The process essentially removes a significant portion of right tail of the crack size distribution obtained prior to the repair which results in the left shift of statistical distribution of damage population. This effect is illustrated in Figure C-3. C.5.3 Load monitoring.
For certain critical components and structural elements for which traditional safe-life fatigue methodologies or scheduled application of NDI methods are not able to provide cost-effective retirement intervals, calibrated strain-gage bridges and other load sensors may be considered as an alternative to component redesign. Use of actual service loads measured in the field removes pilot-induced loads/usage uncertainties from traditional application of composite worst case design usage and top of scatter flight test loads. It is recommended that load monitoring be considered for non-rotating components and airframe structural elements.
Although it is possible to incorporate load monitoring for rotating components, there are a number of technical challenges which are likely to increase the cost of implementing a robust load monitoring system. Diagnostics tools for use with load monitoring should incorporate historical trending of maximum, minimum, vibratory, and steady loads for certain common steady-state flight conditions in consistent density altitude ranges. Typically, flight conditions of primary interest may include hover, forward flight, climbs, and steady turns. Sensitivity analysis can be used to identify additional flight conditions that may have a significant impact on fatigue reliability.
Automated checking of trends against predetermined thresholds could provide notices to maintainers. Automatic zeroing procedures should be considered, as well as a generous number of backup gages. 69