1. A fault reasoning system comprising: a model-based reasoner to take in a plurality of fault signals, and predict a likely root cause for the plurality of fault signals based upon predictions developed utilizing a model of a particular piece of equipment; a case database storing a plurality of prior fault signals, and associating a particular combination of fault signals with a particular root cause that has been identified in actual equipment in the past; and a case-based reasoner having access to said case database, said case-based reasoner taking in the plurality of fault signals, and searching the case database for a prior case having a similar set of fault signals, with said case-based reasoner predicting a likely root cause if there is a prior case having a number of similar fault signals, and said model-based reasoner being utilized to predict a root cause when said case-based reasoner does not have such a previously stored case.