A gas turbine engine (20;100) comprising: an engine static structure (36;130); at least one shaft rotatable relative to the engine static structure (36;130) about an engine axis of rotation; a turbine section configured to drive the at least one shaft; a geared architecture driven by the at least one shaft; a fan driven by the geared architecture and coupled to the at least one shaft for rotation about the engine axis of rotation; an actuator mounted to the engine static structure (36;130), wherein the actuator is activated to prevent the fan from rotation by preventing rotation of the at least one shaft and is inactivated to allow the fan to rotate wherein the actuator includes a first feature and the at least one shaft includes a second feature that locks with the first feature when the actuator is activated; a control that activates the actuator under a first predetermined condition and inactivates the actuator under a second predetermined condition, wherein the first predetermined condition comprises a non-operational engine condition and the second predetermined condition comprises an operational engine condition, wherein the non-operational condition comprises a windmilling condition while the gas turbine engine is powered off, and the operational engine condition occurs when the gas turbine engine is powered on; and at least one sensor used to identify the non-operational engine condition, wherein sensor inputs from the at least one sensor to the control allow the control to determine: if an aircraft is on the ground with weight on wheels; if engine shut-down has been completed; and if the aircraft has stopped moving, characterised in that: the actuator is aft of the turbine section; one of the first and second features (134,136) comprises a dog extension; and the other of the first and second features (134,136) comprises a recess that receives the dog extension to prevent rotation of the at least one shaft. an engine static structure (36;130); at least one shaft rotatable relative to the engine static structure (36;130) about an engine axis of rotation; a turbine section configured to drive the at least one shaft; a geared architecture driven by the at least one shaft; a fan driven by the geared architecture and coupled to the at least one shaft for rotation about the engine axis of rotation; an actuator mounted to the engine static structure (36;130), wherein the actuator is activated to prevent the fan from rotation by preventing rotation of the at least one shaft and is inactivated to allow the fan to rotate wherein the actuator includes a first feature and the at least one shaft includes a second feature that locks with the first feature when the actuator is activated; a control that activates the actuator under a first predetermined condition and inactivates the actuator under a second predetermined condition, wherein the first predetermined condition comprises a non-operational engine condition and the second predetermined condition comprises an operational engine condition, wherein the non-operational condition comprises a windmilling condition while the gas turbine engine is powered off, and the operational engine condition occurs when the gas turbine engine is powered on; and at least one sensor used to identify the non-operational engine condition, wherein sensor inputs from the at least one sensor to the control allow the control to determine: if an aircraft is on the ground with weight on wheels; if engine shut-down has been completed; and if the aircraft has stopped moving, characterised in that: the actuator is aft of the turbine section; one of the first and second features (134,136) comprises a dog extension; and the other of the first and second features (134,136) comprises a recess that receives the dog extension to prevent rotation of the at least one shaft. if an aircraft is on the ground with weight on wheels; if engine shut-down has been completed; and if the aircraft has stopped moving, the actuator is aft of the turbine section; one of the first and second features (134,136) comprises a dog extension; and the other of the first and second features (134,136) comprises a recess that receives the dog extension to prevent rotation of the at least one shaft.