1. A system for use in diagnosing a malfunction in an aircraft engine from engine sensor signals including signals corresponding to engine faults, said system comprising: a diagnostic supervisory signal generator means for generating system command signals; an input/output device for receiving signals from and sending signals to a technician; a knowledge memory means having stored therein signals corresponding to a database of engine sensor signal values and facts and rules relating each of said engine sensor signal values and facts with selected ones thereof, each of said rules including a logical premise and conclusion; an inference engine means for receiving said knowledge memory means signals and, in response to said system command signals, for performing an inference engine test to establish the cause of the engine faults and generating signals indicative thereof, said inference engine test including the steps of: determining from said engine sensor signals a first series of rules to be tested; searching said knowledge memory means for signals corresponding to one of said engine sensor signal values and facts that establishes the validity of a first rule conclusion; generating signals for said input/output means for querying said technician for input signals that establishes the validity of said first rule conclusion; receiving signals from said input/output means indicative of technician input signals; inferring the value of a first rule premise signal corresponding to a first rule premise associated with said first rule conclusion in dependence on the establishment of the validity of said first rule conclusion, determining a second series of rules to be tested in dependence on said first rule premise signal value; and a retroceding means for receiving a signal indicative of an erroneous input signal and, in response thereto, determining all of said first rule conclusion and premise signal values established as a result of said erroneous input signal; said retroceding means further for removing all the erroneously established signal values from said knowledge database.