In this paper, an emergency controller is developed for AscTec Pelican quadrotor suffering a severe failure in one of its motors or rotors. With one of its motors badly damaged, it is impossible to perform the control of a quadrotor using old control strategies or conventional fault tolerant control techniques. The emergency controller designed in this paper detects online any failure or fault in the quadrotor UAV motors, and whenever a severe fault (one which the Passive Fault Tolerant Sliding Mode Controller of the quadrotor cannot hold) occurs the controller applies some weight modifications so the three remaining motors are used to control the UAV as a trirotor. The controller uses a nonlinear sliding mode observer as Fault Diagnosis and Identification (FDI) unit to detect and estimate the magnitude of the fault online. SIMULINK results show that the proposed controller is fast in fault detection and successful in controlling the damaged quadrotor until it finishes its path.