A method for converting a turbofan engine including providing a turbofan engine and converting the turbofan engine. The turbofan engine includes a core engine (including at least one high pressure spool assembly and a combustion chamber), and an unmodified fan configured for providing at least a bypass flow bypassing the core engine, the fan being mechanically coupled to a low pressure turbine that is in turn driven by the core engine. The conversion includes modifying or replacing the unmodified fan to provide a modified fan, the modified fan configured for generating a reduced bypass flow with respect to said fan bypass flow during operation of the converted turbofan engine corresponding to at least one set of engine conditions, enabling said low pressure turbine to generate an excess shaft power above a baseline shaft power required for driving the modified fan during operation of the converted turbofan engine.