For those in the avionics repair and maintenance business, the acronyms NFF (No Fault Found), NTF (No Trouble Found), and CND (Cannot Duplicate) are, unfortunately, all too familiar terms. After several decades of frustration with this illusive phenomenon, it continues to consume an enormous amount of test and diagnostic effort and is the source of considerable cost and discomfort within the multi-level avionics repair model.
There are undoubtedly many causes of NFF and all of them should be addressed. The question is: Where do you start and which solution will be the most beneficial?
Our particular efforts have focused on the literal or statistical analysis of NFF, recognizing that if the system’s MTBF (Mean Time Between Failure) has decreased, or if the device's NFF rate has increased with age and deterioration, a physical fault is most likely present. However, if it isn’t found during conventional testing then it probably only fails intermittently. Similarly, having an intermittent failure mode, it in all probability cannot be detected or diagnosed at testing time because of known and demonstrated limitations in the conventional measurement equipment used to perform the tests.
In this paper we will outline the problem of intermittence and its testing difficulties. More importantly, we will describe the unique equipment and process which has produced overwhelming success in Intermittence / NFF resolution and MTBF extension. Our team-developed overhaul system called IFDIS 2.0 (Intermittent Fault Detection and Isolation System 2.0) incorporates all the necessary testing procedures and technological capabilities that are proving to be critical to the resolution of the chronic intermittent / NFF problem.