A novel non-linear observer for detection of oscillatory faults in commercial airplanes is proposed in this work. Oscillatory Failure detection in airplanes is an important problem because of natural mode excitation and the structural loads they can provoke. A simple and novel non-linear observer with two correction terms is developed. Formal proof of stability and convergence of the observer in the fault free case is provided. The proposed approach satisfies technical requirements of no false alarm, small detection time, detectability of small faults while keeping a simple structure. A test with real measurements is presented to show the efficiency of the proposed observer over the existing techniques.