detecting and indicating the failure of the primary drive train of a flap actuator system that includes primary and secondary drive trains driven by a common power supply is disclosed . when the primary drive trains fails , a free motion zone formed between the common power supply and the secondary drive train is crossed . crossing of the free motion zone causes the position of an electro - mechanical sensor to change . the change in position of the electro - mechanical sensor creates or changes the value of an electrical signal or signals to provide an indication of the failure of the primary drive train .