When a person gets ill, say from a cold like the one I have right now, we tend to slow down, maybe have trouble focusing, or otherwise performing certain tasks. Clearly we function, but not as well as we would like, and perhaps not as well as usual. This is a degraded state, clearly. But that doesn’t mean we failed, as in the usual binary state sense of either success or failure, functioning or failed, etc.
Without our systems to tell us to slow down because we aren’t feeling too well, we could run the risk of pushing ourselves beyond our limits, perhaps to the point if actual physical failure (say hospitalized, or bed ridden), and perhaps to a state which we cannot recover (death). But we have those feedback systems to tell us when to reduce the stress, and take time to heal, or enter a repair state.
Prognostics and Health Management (PHM) is the engineering field that is working to develop these feedback systems. In some cases, repair requires replacement, so the indicators suggest a replacement instead of repair. But these systems can be indicators of an engineering system being ill in some sense, in a degraded state. While Telecommunications and IT systems have had this advantage for a very long time, in many ways and on many parts of these complex systems, not all engineering system could be built cost-effectively with these systems. But with ubiquitous communications and nanotechnologies leading to inexpensive early warning systems, more engineering system can take advantage of such solutions, leading to more reliable engineering systems.
When we get ill, and we are functioning in a degraded state, we may fail at certain tasks. From a mission point of view, we can fail due to being in the degraded state. What we are usually able to do, we can’t while we’re ill. I might not make it to work if my cold gets bad enough. I’m degraded, but the tasks I intended to do don’t get done, so the mission of work will fail. That is, unless someone can take my place. As PHM develops more fully, we can clearly determine when systems or parts of a system are ill, and replace them with like parts. The missions of those systems will therefore fail less often, with less cost for maintenance, and fewer mission failures.