This work presents an application-oriented use of the man-machine interactive system concept. The paper specifically addresses itself to a description of a procedure developed by Allison Div. of General Motors to effectively perform the preliminary design of a gas turbine engine. The tools used in this approach are a set of complex programs written for design and analysis of engine components, an integrating system used for interprogram communication to guarantee data compatibility, and a graphic display terminal to allow on-line action and reaction. With these tools the heuristic problem of preliminary design can be solved in units of time that can be orders of magnitude less than previous more conventional efforts. Also discussed is the logical extension of this type of approach into areas other than preliminary design.