Downloaded from https://www.everyspec.com ADS-79D-HDBK Time of Inspection Service Life (Flight Hours) FIGURE C-1. Current approach of determining inspection interval Although the outlined approach is well received in the industry for the purpose of inspection planning, there are several folders of drawbacks. The approach is often based on average behavior of fatigue crack growth, which is unconservative. While a damage tolerance inspection interval factor is employed to make further adjustment, its value may not be fully justified but is often selected to add conservatism. In addition, there is no justification to use a single characteristics value representing the capability of an NDI .
Therefore, the lack of rigorous statistics to address uncertainties in damage progression and inspection capability limits the applicability and creditability of the current approach. In general, the inspection plan obtained from the aforementioned approach results in many unnecessary inspections. Damage tolerance inspection interval factors to be employed in practice depend on allowable probability of structural element failure and the NDI POD. For example, a 90% POD would require six inspections to maintain six nines reliability in critical structural elements (designed for slow crack growth).
It should also be understood that the allowable probability of structural element failure depends on criticality of the failure mode. For example, six nines reliability may be retained in redundant fail-safe structure with less reliability for each element failure mode. Damage tolerance inspection interval factors employed for fail safe failure modes typically range from 3 to 4, but substantiation via reliability analysis should consider changes in load path which would occur as a result of the various element failure modes.
Due to the aforementioned shortcomings for the current approach to determine inspection interval, a technical approach that addresses the inherent scatter of damage progression and incorporates the reliability model of specific inspection is highly desirable. As depicted in Figure C-2, there are two major aspects of uncertainty involved in risk assessment of damage inspection, namely the scatter of damage progression (as a function of time) and the variability associated with inspection capability.
To establish an efficient inspection plan, the overlap between the distributions representing these two 67 Initial Defect Size POD(a) Critical Defect Size Design Life a NDE Tmin T Damage Growth Life Tmax lead to many Inspections D Tmin T Tmin Inspection Interval T = T/SF 0 SF = 3 ~ 4 Tmin amage Growth Life Tmax Tmax