A method of operating a multi-engine drive system of an aircraft includes driving a main rotor of the aircraft by a main engine of the multi-engine drive system at an operating speed of the main engine, operating a supplemental engine of the multi-engine drive system at approximately 80% of an operating speed of the supplemental engine, wherein, during the operating step, a clutch interoperably coupled to the supplemental engine is interoperably decoupled from the main rotor and, responsive to a command to interoperably engage the clutch, if the clutch successfully engages such that the clutch is interoperably coupled to the main rotor, providing, by the supplemental engine, power to the main rotor.