Flying drones (multirotors) are expected to bring tremendous benefit to our industry and society. However, there is a potential risk to civil safety if multirotors crash. Therefore, it is important to establish multirotor flight controls in the case of complete motor failures. In this paper we discuss recent research activities on multirotor motor failure problems, focusing on the problems of multirotor flight states to avoid a crash in the case of complete motor failure. In addition, by the Euler angle state variable approach, research results on the relationship the between operating points (equilibrium points) of multirotors and the flight states to avoid a crash in the case of complete motor failure, are also reviewed.