In this paper, we address the problem of Oscillatory Failure Case (OFC) detection in A380 aircraft. The importance of OFC detection is well known in the literature because of natural mode excitation and the structural loads they can provoke. A novel nonlinear observer based appraoech, which has two different gains is proposed in this paper to solve the problem. The main advantage of the proposed residual generator has a kind selestive (frequency dependent) attenuation property: disturbances and unmodelled dynamics effects on the residual are reduced while the effect of the fault is slightly amplified in a specific fault frequency range. Further, if the gains are chosen to be zero, the particular case of the technique implemented on A380 Airbus is obtained. A test based on real measurements of an A380 airplane is presented to show the efficiency of the proposed residual over the existing techniques.