A method of operating an aircraft hybrid gas turbofan, the method comprising: providing supplemental power to a primary gas turbine engine during a mode of operation of the primary gas turbine engine, wherein providing supplemental power comprises boosting the high spool of the primary gas turbine engine using a secondary gas turbine engine via a first power linkage (42; 52) connecting the primary and secondary gas turbine engines (12, 14), and wherein the primary gas turbine engine is a propulsion engine and the secondary gas turbine engine is an auxiliary power unit; characterised in that: providing supplemental power to the primary gas turbine engine occurs during descent of the aircraft; the method further comprises reducing a fuel flow to the primary gas turbine engine when supplemental power is provided to the primary gas turbine engine by the APU; and boosting the high spool of the primary gas turbine engine comprises a first power supplementation method, wherein the first power supplementation method comprises: generating compressed air in the secondary gas turbine engine; and providing the compressed air directly into a turbine of the high spool of the primary gas turbine engine to provide high spool torque. providing supplemental power to a primary gas turbine engine during a mode of operation of the primary gas turbine engine, wherein providing supplemental power comprises boosting the high spool of the primary gas turbine engine using a secondary gas turbine engine via a first power linkage (42; 52) connecting the primary and secondary gas turbine engines (12, 14), and wherein the primary gas turbine engine is a propulsion engine and the secondary gas turbine engine is an auxiliary power unit; characterised in that: providing supplemental power to the primary gas turbine engine occurs during descent of the aircraft; the method further comprises reducing a fuel flow to the primary gas turbine engine when supplemental power is provided to the primary gas turbine engine by the APU; and boosting the high spool of the primary gas turbine engine comprises a first power supplementation method, wherein the first power supplementation method comprises: generating compressed air in the secondary gas turbine engine; and providing the compressed air directly into a turbine of the high spool of the primary gas turbine engine to provide high spool torque. providing supplemental power to the primary gas turbine engine occurs during descent of the aircraft; the method further comprises reducing a fuel flow to the primary gas turbine engine when supplemental power is provided to the primary gas turbine engine by the APU; and boosting the high spool of the primary gas turbine engine comprises a first power supplementation method, wherein the first power supplementation method comprises: generating compressed air in the secondary gas turbine engine; and providing the compressed air directly into a turbine of the high spool of the primary gas turbine engine to provide high spool torque. generating compressed air in the secondary gas turbine engine; and providing the compressed air directly into a turbine of the high spool of the primary gas turbine engine to provide high spool torque.