CNDs and RTOKs: are we addressing these problems correctly?

High rates of Cannot Duplicates (CNDs) and Retest OKs (RTOKs) in aircraft avionics has led to excessive line and shop maintenance times and maintenance loading. This, in turn, results in decreased sortie rates, long logistic tails, and higher life-cycle cost. Grumman has been addressing these problems, via the development of smart BIT (built-in test) technology. This paper discusses this work and provides strong evidence that intermittent faults may be a major cause of many of the CNDs and RTOKs referred to above. The unique environmental testing that led to this conclusion will be described. Data from the environmental testing led to a rethinking of some of the original smart BIT techniques, resulting in an improved smart BIT implementation. The new integrated approach has two key components: A temporal monitor (TM) that replaces conventional BIT filtering and properly classifies fault behavior; A system level fault correlator (FC) that correlates fault behavior to environmental stress when applicable.<
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