Recent events within the U.S. Air Force (USAF) have highlighted the reality that cracks large enough to be readily detected by non-destructive inspection (NDI) can be missed during in-service NDI actions. These so-called "missed crack" or "NDI miss" events may pose serious risks to the safety and integrity of aircraft structures. This paper will review USAF actions: 1) to understand the severity and ramifications of the NDI miss problem, 2) to minimize the probability that cracks will be missed, and 3) to mitigate the effects of undetected cracks. Under the current USAF inspection philosophy, missed cracks that are larger than a crack that can grow to a critical size before the next scheduled inspection pose the greatest risk to structural safety. Significant challenges exist in understanding what causes cracks to be missed, in quantifying how many and how frequently cracks are missed based upon available maintenance data, in calculating the risk to flight safety of missed cracks, and, finally, in developing technologies that will minimize the future probability that cracks greater than a specified size will be missed. The probability that a structural component will be inspected as required and that a detectable crack will, indeed, be found, are both key factors affecting NDI misses. In this paper, the probability of detection (POD) concept will be described from the standpoint of the variables that govern the detectable crack size associated with a given POD. Special attention will be given to the utility of the "90/95" crack size (the size of a crack that can be found 90% of the time with 95% confidence). The discussion of the POD idea will be augmented to include the concept of a probability of miss (POM). POM, though computed simply by subtracting POD from unity, focuses attention on the size of a crack that can go undetected and, therefore threaten safety and structural integrity. Examples will be presented that illustrate the ramifications missed cracks have on aircraft structural safety. A technique for estimating the probability distribution of missed cracks based on NDI findings (i.e. "found cracks") and on the knowledge of a specific NDI technique's POD capability will be described. In addition, a brief review of recent advances in NDI technologies and techniques designed to minimize NDI misses will be presented.
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