Jet engine, having an engine nacelle, a core engine arranged within the engine nacelle with a core engine casing surrounding the core engine, wherein a core flow is conductible or conducted through the core engine, a bypass channel running between the core engine casing and the engine nacelle, wherein a bypass flow is conductible or conducted through the bypass channel, a thrust reverser, which is designed such that it only diverts the bypass flow of the bypass channel in the activated operating state, a variable bypass nozzle which is designed to provide a variable exit area of the bypass nozzle and has variable control areas, a single common actuator system for the thrust reverser and the variable bypass nozzle, which is provided in such a way that the thrust reverser and the variable bypass nozzle can be mechanically coupled together and decoupled from each other, the actuator system has a actuating drive unit, which is connected to the variable bypass nozzle, that the actuating drive unit is designed to set the variable control areas of the bypass nozzle, and that the actuator system is designed so that the thrust reverser is selectively engaged and moved along, the engine nacelle has, in the flow direction of the flow medium passing through the jet engine, a front first engine nacelle part (24, 100), which has an inlet on the input side, and that the first engine nacelle part (24, 100) is particularly static, the engine nacelle has, in the flow direction of the flow medium passing through the jet engine, a second engine nacelle part (26, 200) adjoining the first engine nacelle part (24, 100), which has the thrust reverser, and that in particular the second engine nacelle part (26, 200) is particularly mounted to be movable in the axial direction relative to the first engine nacelle part (24, 100), the actuator system has a subsystem, referred to as third subsystem, with actuation components and translation components that are axially shiftable and that are directly connected to the variable bypass nozzle, and that the actuation components and translation components have a first actuator ring, which cooperates with a securing device to limit the axial movement of the first actuator ring, and the actuator system has a further subsystem, referred to as fourth subsystem, with at least one securing device provided to secure the thrust reverser against unintentional actuation, and wherein at least one securing device has at least one hook closure device, which is designed for the releasable connection of the first engine nacelle part (24, 100) with the second engine nacelle part (26, 200), and that the actuation of the hook closure device is particularly performed via the actuator system, characterized in that, at least one securing device is designed as at least one ring closure device, and that the ring closure device is designed to limit the movement path of the first actuator ring of the third subsystem of the actuator system in the axial direction in the activated operating state, and that at least one securing device is designed as a closure-coupling device provided in such a way that in a first function it allows the variation of the variable bypass nozzle and prevents a translation of the second engine nacelle part (26, 200) with the thrust reverser, and that in a second function it allows a translation of the second engine nacelle part (26, 200) with the thrust reverser.