In this study, an unordinary methodology for energy efficiency calculation of a turbofan engine is introduced. Emission indexes of the engine are used to find out energy efficiency and novel parameters for environmental impact assessment. Energy efficiency is proven at take-off, climb-out and approach phases of the flight for the turbofan engine. Additionally, power and energy emission index variations of the turbofan engine for carbon monoxide and unburned hydro carbon constituents of the exhaust are presented in the paper. As a result of the study, energy efficiency of the turbofan engine is determined 35.68%, 41.12% and 36.05 at take-off, climb-out and approach flight phases respectively.