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In a constant speed, variable pitch propeller an over‐ride device is automatically brought into operation to move the blades to the feathering position when the power unit stops, and means are provided by which the over‐ride is automatically put out of action when an attempt is made to start the power unit by ‘wind‐milling’, and by which the over‐ride is automatically restored when the speed rises to that at which the unit should be self‐operating. When the power unit stops a governor 23 allows a spring 22 to force a valve 19 to the left, thereby allowing motive fluid from a pipe 12 to flow into the left‐hand end of a cylinder 16 to operate a plunger 15 which moves the propeller blades 13 to the minimum pitch position. The resultant change of torque acting on a torque meter 39 closes a switch 37 to complete an electrical circuit for actuating a solenoid 35 controlling a valve 34 which allows motive fluid to be applied from pipe 31 to the spring‐loaded plunger 28 of the over‐ride device. The plunger moves the valve 19 to the right, allowing fluid into the right‐hand end of cylinder 16, thereby moving the blades to the feathering poistion. The circuit may also be completed by the pilot closing the fuel isolator valve 36. To start the power unti by wind‐milling, ignition switch 46 of the power unit is closed, the govenor controlled switch 47 being already closed. A solenoid 45 then opens a switch 44 and breaks the circuit to the solenoid 35 so that the valve 34 closes to cut‐off the supply of fluid to the over‐ride plunger. The spring 22 is thus able to move the valve 19 to the left with the result that the blades are moved from the feathered position to cause ‘wind‐milling’. If at a predetermined speed the unit does not then become self‐operating, its governor will open the switch 47 causing switch 44 to close to complete the circuit to the solenoid 35. The valve 34 will be raised to bring the over‐ride into operation, causing plunger 28 to move valve 19 so that the propeller blades are moved to the feathered position.