An air turbine starter for starting an engine, comprising: a housing defining an inlet, an outlet, and a flow path extending between the inlet and the outlet for communicating a flow of gas therethrough; a turbine member journaled within the housing and disposed within the flow path for rotatably extracting mechanical power from the flow of gas; a gear train drivingly coupled with the turbine member; a clutch having a drive shaft that is operably coupled with the gear train; an output shaft configured to be operably coupled to and rotate with the engine; and a decoupler assembly for decoupling the output shaft during backdrive; characterised in that the output shaft has a threaded portion and the decoupler assembly comprises: a threaded shaft having a first end with a stop and a second end operably coupled to the output shaft and where the threaded shaft is selectively receivable and axially moveable with respect to the threaded portion of the output shaft; and a retention mechanism selectively operably coupling the output shaft to the first end of the threaded shaft; wherein when driving torque is transmitted to the output shaft the retention mechanism is not loaded, when overrunning torque is transmitted below a certain level the output shaft, retention mechanism, and threaded shaft are loaded and when the overrunning torque reaches a certain level the retention mechanism uncouples the output shaft and the first end of the threaded shaft and the output shaft moves in a direction away from the first end of the threaded shaft to define a decoupled and separated position where the output shaft is disengaged from the engine; and wherein the retention mechanism is a shear pin extending axially through a portion of the output shaft and a portion of the threaded shaft and wherein the pin shears when the overrunning torque reaches the certain level. a housing defining an inlet, an outlet, and a flow path extending between the inlet and the outlet for communicating a flow of gas therethrough; a turbine member journaled within the housing and disposed within the flow path for rotatably extracting mechanical power from the flow of gas; a gear train drivingly coupled with the turbine member; a clutch having a drive shaft that is operably coupled with the gear train; an output shaft configured to be operably coupled to and rotate with the engine; and a decoupler assembly for decoupling the output shaft during backdrive; characterised in that the output shaft has a threaded portion and the decoupler assembly comprises: a threaded shaft having a first end with a stop and a second end operably coupled to the output shaft and where the threaded shaft is selectively receivable and axially moveable with respect to the threaded portion of the output shaft; and a retention mechanism selectively operably coupling the output shaft to the first end of the threaded shaft; wherein when driving torque is transmitted to the output shaft the retention mechanism is not loaded, when overrunning torque is transmitted below a certain level the output shaft, retention mechanism, and threaded shaft are loaded and when the overrunning torque reaches a certain level the retention mechanism uncouples the output shaft and the first end of the threaded shaft and the output shaft moves in a direction away from the first end of the threaded shaft to define a decoupled and separated position where the output shaft is disengaged from the engine; and wherein the retention mechanism is a shear pin extending axially through a portion of the output shaft and a portion of the threaded shaft and wherein the pin shears when the overrunning torque reaches the certain level. a threaded shaft having a first end with a stop and a second end operably coupled to the output shaft and where the threaded shaft is selectively receivable and axially moveable with respect to the threaded portion of the output shaft; and a retention mechanism selectively operably coupling the output shaft to the first end of the threaded shaft; wherein when driving torque is transmitted to the output shaft the retention mechanism is not loaded, when overrunning torque is transmitted below a certain level the output shaft, retention mechanism, and threaded shaft are loaded and when the overrunning torque reaches a certain level the retention mechanism uncouples the output shaft and the first end of the threaded shaft and the output shaft moves in a direction away from the first end of the threaded shaft to define a decoupled and separated position where the output shaft is disengaged from the engine; and wherein the retention mechanism is a shear pin extending axially through a portion of the output shaft and a portion of the threaded shaft and wherein the pin shears when the overrunning torque reaches the certain level.