A PROBABILISTIC METHOD OF RISK ASSESSMENT IN TERMS OF MEETING DESIGN GOALS

Any affordability assessment of a conceptual design must consider the probability of achieving the design goals and the consequences of failure to meet them. Historically, it can be shown that new helicopters seldom meet the desired weight, cost, and performance goals. This poor track record is due to several contributing factors, including requirement changes during design, unjustified optimism in engineering estimates, and, in some cases, shortfalls in technological prediction tools. Undue optimism in the predesign or conceptual design phase can be the result of competitive pressure; however, some of this problem resides in the precision and correlation tolerance of the methods employed. On a component basis, most weight methodologies can establish a weight within a few percent. Performance estimation methods also have tolerance bands. To establish the conceptual or predesign configuration, both methodologies must be used in series, resulting in the probability of being correct to be the product of the individual probabilities. This effect results in a low probability of success and, to some extent, explains the historical trend. The most obvious means of correcting these deficiencies is to apply power and weight margins to the design; however, if this is done, weight growth will become a self-fulfilling prophecy. To achieve some confidence of success, a careful process of application of design margins must be used in conjunction with a knowledge of the costs and penalties associated with reworking design elements.