This paper demonstrates how the robust performance analysis framework can be used to determine worst case loads of flexible aircraft due to 1-cos gusts. This framework is a part of control theory and is used to characterize a system by input-output norms: e.g. the induced L2-L∞ norm represents the worst case energy-to-peak gain. Two very efficient ways to compute this norm are presented. Based on the physical interpretation of this norm, an aircraft model is weighted such that the induced L2-L∞ norm of the weighted system represents a tight but guaranteed upper bound for 1-cos gusts and similar excitations. The analysis results are compared to simulations. Since the energy-to-peak gain can be computed in a very efficient way for LTI models, the norm is used to quickly obtain an upper bound for loads and to identify critical flight conditions. In a second example, the analysis is additionally performed on an uncertain model.