The ratio of the energy liberated during a flight to the revenue work done (ETRW) of an airplane can be employed as a key indicator to assess its environmental impact. It remains constant during the life cycle of the aircraft and is fixed by its designers. The goal of an environmentally optimum airplane is to minimize the ETRW. For an existing airplane, there are two major parameters that can greatly affect the ETRW, which are the ratio of actual payload to maximum possible payload “c” and the flight range R. The goal of this paper is to study the effect of c and R on ETRW and minimize it by using a genetic algorithm (GA). The study is performed on a Boeing 737-800 and a Boeing 747-400 aircraft. The optimization study is valuable in determining the payload and range of an existing aircraft for minimal environmental impact; it turns out that the maximum possible values of payload and range do not necessarily lead to a flight with minimal environmental impact. For new aircraft designs, the minimization of ETRW should account for advances in materials, alternative fuels, structures, aerodynamics and propulsion technologies which can be taken into consideration at design stage. It should be noted that other factors which also affect the emissions, namely the aircraft operations and air traffic management are not included in the ETRW.