An aircraft engine assembly comprising: an engine having an engine shaft; an output shaft; a clutch in driving engagement between the engine shaft and the output shaft, the clutch having a first component in driving engagement with the engine shaft and a second component, the clutch operable between a first configuration and a second configuration, in the first configuration the first component is rotatable relative to the second component and the engine shaft is rotatable relative to the output shaft, in the second configuration the first and second components (34f, 30b) are engaged with one another such that rotation of the first component relative to the second component is limited and the engine shaft rotates with the output shaft, the clutch configured to selectively engage the engine shaft to the output shaft via the engagement between the first component and the second component; and a gear train including a driver sun gear secured to the engine shaft, a driven sun gear (34b; 134b), planet gears meshed with both of the driver sun gear and the driven sun gear (34b; 134b), and a planet carrier (34d; 134d) rotatably supporting the planet gears, the output shaft drivingly engaged to one of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d), the other of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d) rotating upon rotation of the engine shaft in the first configuration of the clutch (30; 230), rotation of the other of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d) limited by the clutch (30; 230) in the second configuration of the clutch (30; 230), characterised in that: the engine is an intermittent internal combustion engine; and a mechanical lock operable between a first position and a second position, in the first position the mechanical lock is disengaged from the first component of the clutch, and in the second position the first component and the second component of the clutch are secured for joint rotation one relative to the other, wherein the mechanical lock includes a rod and an actuator engaged to the rod, the actuator operable to move the rod between the first and second positions of the mechanical lock. an engine having an engine shaft; an output shaft; a clutch in driving engagement between the engine shaft and the output shaft, the clutch having a first component in driving engagement with the engine shaft and a second component, the clutch operable between a first configuration and a second configuration, in the first configuration the first component is rotatable relative to the second component and the engine shaft is rotatable relative to the output shaft, in the second configuration the first and second components (34f, 30b) are engaged with one another such that rotation of the first component relative to the second component is limited and the engine shaft rotates with the output shaft, the clutch configured to selectively engage the engine shaft to the output shaft via the engagement between the first component and the second component; and a gear train including a driver sun gear secured to the engine shaft, a driven sun gear (34b; 134b), planet gears meshed with both of the driver sun gear and the driven sun gear (34b; 134b), and a planet carrier (34d; 134d) rotatably supporting the planet gears, the output shaft drivingly engaged to one of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d), the other of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d) rotating upon rotation of the engine shaft in the first configuration of the clutch (30; 230), rotation of the other of the driven sun gear (34b; 134b) and the planet carrier (34d; 134d) limited by the clutch (30; 230) in the second configuration of the clutch (30; 230), characterised in that: the engine is an intermittent internal combustion engine; and a mechanical lock operable between a first position and a second position, in the first position the mechanical lock is disengaged from the first component of the clutch, and in the second position the first component and the second component of the clutch are secured for joint rotation one relative to the other, wherein the mechanical lock includes a rod and an actuator engaged to the rod, the actuator operable to move the rod between the first and second positions of the mechanical lock. the engine is an intermittent internal combustion engine; and a mechanical lock operable between a first position and a second position, in the first position the mechanical lock is disengaged from the first component of the clutch, and in the second position the first component and the second component of the clutch are secured for joint rotation one relative to the other, wherein the mechanical lock includes a rod and an actuator engaged to the rod, the actuator operable to move the rod between the first and second positions of the mechanical lock.