In the development of complex systems the requirements for the system will almost always remain uncertain late into the software development. In gas turbine engine control systems at Rolls-Royce, typically 50% of requirements will change between Critical Design Review and Entry into Service. Ignoring or not planning for requirements uncertainty will cause scrap and rework that will manifest later in the project. This paper evaluates the impact of not managing these uncertainties and describes how Rolls-Royce uses Requirements Uncertainty Analysis to reduce this impact. The paper summarises the findings from an extensive Six Sigma study into requirements uncertainty and provides an overview of the technique now used to identify and monitor uncertainty through a project life. The return on investment of this technique has been between 100:1 and 500:1.