Failure is a concern that is always present in the operation of a drone. Given the continued increase in acquisition and use of these devices by the general public, safety is of paramount importance. Even the smaller devices of about 2 kg of total weight, in case of total failure at altitude, will fall like a brick and cause damage to whatever is below.
Most configurations such as quadcopters, don't offer any intrinsic redundancy (in spite of having 4 motors). The failure of a single motor will prevent the quadcopter of maintaining level and yaw-locked flight. Some enhanced control algorithms are being developed to provide level and controlled descent in case of single motor failure. Naturally, yaw control is lost, but the on-board inertial sensors and magnetometer are capable of keeping track of the orientation, therefore knowing how to distribute throttle by the surviving motors.