Vertical autorotation is proposed and investigated as an emergency landing technique for a multirotor drone suffering a total loss of one motor. A quadrotor with collective pitch control mechanisms is assumed. The blades have no twist because the rotors operate in windmill and propeller modes. The vertical autorotation consists of two phases. In phase 1, all rotors operate in windmill mode and the collective pitch for no attitude control is determined so as to minimize the terminal descending velocity. The quadrotor attitude in phase 1 is adjusted by varying the collective pitch. Flight control in this phase will be a research theme in future. In phase 2, two rotors, including a damaged one, operate in windmill mode while the other two operate in propeller mode. The collective pitch is determined so that the terminal descending velocity at landing is close to 0. Each multirotor has a particular critical altitude value, which relates to the specifics of this vertical autorotation. Critical altitude is the altitude for the case in which phases 1 and 2 are switched just when the descending velocity converges to the terminal velocity during phase 1. The detailed autorotation process depends on the magnitude relationship between the quadrotorʼs critical and initial altitudes.