We developed a heuristic for assessing the usability of mobile user-interfaces in life-threating, time-critical and unstable situations in a qualitative way. The major advantages of our approach as opposed to standardized quantitative questionnaires is the independence from a baseline, the possibility to make absolute statements and the potential for adaptations. When creating a qualitative semi structured interview we adhere to the common modus operandi of the qualitative social research. On the basis of 17 common quantitative questionnaires on usability we identified the five major categories Utility, Intuitiveness, Memorability, Learnability and Personal Effect. We selected all questions from the questionnaires which are useful for assessing the usability of user-interfaces in emergencies. Furthermore, we rephrased the closed-ended questions to open-ended ones. The quantification of research results is possible by weighting the qualitative results in dependence to the research question.