One often overlooked tool to reduce costs is to build in optimum lot sizes or the Economical Build Quantity (EBQ). Of course, you should diligently try to minimize set-up costs, aim for single piece flow etc., but with many planes being built in quantities of one plane a month (or less), there will never be enough volume to justify the expense necessary to reduce setups to zero. Producing in correctly-sized lots will be the most economical option.