A gas turbine engine (10; 310; 510) comprising: an engine spool with a turbine section; a propeller; an electric motor; and a variable drive gearbox (28; 328; 528) configured to mechanically couple the engine spool and the motor to the propeller, the variable drive gearbox (28; 328; 528) comprising: a first gear connection (38; 338; 538) rotationally coupled to the propeller; a second gear connection (42; 342; 542) rotationally coupled to the engine spool; and a first clutch (40; 340; 540) rotationally coupled to the first gear connection (38; 338; 538) and a power control unit disposed to control and route power to the motor; and a first generator disposed to provide electrical power to the power control unit, wherein the variable drive gearbox (28; 328; 528) further comprises a third gear connection disposed to mechanically couple the engine spool to the first generator via engagement of the third gear connection with the second gear connection (42; 342; 542); and characterised in that: the first clutch (40; 340; 540) is mono-directionally rotationally coupled to the motor, such that the first clutch (40; 340; 540) is configured to passively decouple the motor from the first gear connection (38; 338; 538) when a torque input from the engine spool exceeds a torque input from the motor, and is configured to passively couple the motor to the first gear connection (38; 338; 538) when the torque input from motor exceeds a torque input from the engine spool; the variable drive gearbox (328; 528) is an epicyclic gear assembly; the first gear connection (338; 538) comprises a ring gear; the second gear connection (342; 542) comprises a sun gear; the third gear connection comprises a second clutch (344; 544) rotationally biased opposite to the first clutch (340; 540); and in that the first and second clutches (340, 344; 540, 544) are disposed as a set of planet gears engaged with and between the first and second gear connections (338, 342; 538, 542). an engine spool with a turbine section; a propeller; an electric motor; and a variable drive gearbox (28; 328; 528) configured to mechanically couple the engine spool and the motor to the propeller, the variable drive gearbox (28; 328; 528) comprising: a first gear connection (38; 338; 538) rotationally coupled to the propeller; a second gear connection (42; 342; 542) rotationally coupled to the engine spool; and a first clutch (40; 340; 540) rotationally coupled to the first gear connection (38; 338; 538) and a first gear connection (38; 338; 538) rotationally coupled to the propeller; a second gear connection (42; 342; 542) rotationally coupled to the engine spool; and a first clutch (40; 340; 540) rotationally coupled to the first gear connection (38; 338; 538) and a power control unit disposed to control and route power to the motor; and a first generator disposed to provide electrical power to the power control unit, wherein the variable drive gearbox (28; 328; 528) further comprises a third gear connection disposed to mechanically couple the engine spool to the first generator via engagement of the third gear connection with the second gear connection (42; 342; 542); and characterised in that: the first clutch (40; 340; 540) is mono-directionally rotationally coupled to the motor, such that the first clutch (40; 340; 540) is configured to passively decouple the motor from the first gear connection (38; 338; 538) when a torque input from the engine spool exceeds a torque input from the motor, and is configured to passively couple the motor to the first gear connection (38; 338; 538) when the torque input from motor exceeds a torque input from the engine spool; the variable drive gearbox (328; 528) is an epicyclic gear assembly; the first gear connection (338; 538) comprises a ring gear; the second gear connection (342; 542) comprises a sun gear; the third gear connection comprises a second clutch (344; 544) rotationally biased opposite to the first clutch (340; 540); and in that the first and second clutches (340, 344; 540, 544) are disposed as a set of planet gears engaged with and between the first and second gear connections (338, 342; 538, 542). the first clutch (40; 340; 540) is mono-directionally rotationally coupled to the motor, such that the first clutch (40; 340; 540) is configured to passively decouple the motor from the first gear connection (38; 338; 538) when a torque input from the engine spool exceeds a torque input from the motor, and is configured to passively couple the motor to the first gear connection (38; 338; 538) when the torque input from motor exceeds a torque input from the engine spool; the variable drive gearbox (328; 528) is an epicyclic gear assembly; the first gear connection (338; 538) comprises a ring gear; the second gear connection (342; 542) comprises a sun gear; the third gear connection comprises a second clutch (344; 544) rotationally biased opposite to the first clutch (340; 540); and in that the first and second clutches (340, 344; 540, 544) are disposed as a set of planet gears engaged with and between the first and second gear connections (338, 342; 538, 542).