Of course, "talking the talk" and "walking the walk" are entirely different and one is left with the question of how to accomplish the transition. How does one begin with a good diagnostic approach and add prognostics without disturbing the diagnostic foundation? Our solution will focus on eXpress, specifically on two key features--its hybrid model and its test capability.

First, it is worth reiterating how good diagnostics are achieved. As is true of nearly all System Engineering processes, the process of developing a diagnostic solution is iterative. eXpress uses tests to capture diagnostic knowledge as part of the iterative process. With each iteration, the design is modified (more test points, new tests, etc) and the tests provide the mechanism by which those design changes can be assessed. Ultimately, and especially in the hands of skilled engineer, the diagnostics of the system can be improved to meet requirements (e.g. 98% detection, 95% isolation to 1 LRU).

What also happens throughout design capture, as well as test definition to a certain degree, is the addition of functions and failure modes. eXpress' ability to support both modes simultaneously through its hybrid diagnostic model provides the engineer with a powerful testing capability. Since functions and/or failure modes can be tested, the engineer can decide which approach best captures the diagnostic knowledge. Functional testing tends to be used the higher in the system one tests, or the earlier in the design process one is capturing diagnostic knowledge (that is, before failure modes are known).