We support a large airline to establish user experience as a key differentiating factor of a large software package of safety-critical applications such as flight planning for worldwide clients. Recommendations for prototype redesign were derived from empirical and expert evaluations. Together with findings from paper prototyping they contributed substantially to an improved interaction design of the system. Extending an initial generic style guide all interaction design solutions have been documented through design patterns and a style guide that will be the basis for future extensions and newly developed systems. Our immediate client and its clients we worked with in the testing sessions were highly satisfied with the proceeding and the results. We are continuing our cooperation.