After COVID-19, a full recovery compared to the 2019 situation with a subsequent growth of global air traffic is expected for the next three to six years . Regarding carbon dioxide emissions, Coronavirus lockdown helped the environment to bounce back, but this will be a temporary situation. It is important to continue investigating additional mitigation measurements to achieve long-term environmental benefits, especially after the recovery. At that point, the question of how to reduce aviation's impact on the climate change will certainly arise again, and will re-gain its importance for the world-wide community. Since no fundamental breakthroughs in CO 2 reduction in aviation are expected in the near future, research should focus on several measures to sustainably reduce the environmental impact of aviation. The air traffic management can contribute to an overall reduction of emissions of greenhouse gases by optimizing traffic flows not only towards maximum airspace capacity and maximum efficiency, but also increasingly towards minimum environmental impact. A set of concept elements that were investigated in the frame of the European-Chinese project Greener Air Traffic Operations (GreAT) can already constitute simple and suitable means towards a greener air traffic management. One of these concept elements is the 'Lowest Impact of Deviation' principle: Whenever two flights need to deviate from their most fuel-efficient route, speed or altitude due to de-conflicting, this deviation should be done by the flight with the lowest fuel consumption, and consequently, with the lowest amount of emissions produced with this maneuver. This principle is currently neither reflected in air traffic control regulations, nor in common practices. In the frame of the work presented in this paper, this principle has been further investigated and analyzed with a fast-time simulation, which models a free route airspace environment under ideal conditions. The flights are generated according to a configurable traffic density. De-conflicting is done automatically either by following the standard right of way rules, which also often serve as a guiding principle for air traffic controllers; or by following the 'Lowest Impact of Deviation' principle. Based on EUROCONTROL’s Base of Aircraft Data (BADA), the simulation estimates the fuel consumption for each flight as well as for the whole simulation, and consequently also the CO 2 emissions, as a function of traffic density.This paper gives basic information about the principle itself, which is then further tailored down and applied to a free route airspace environment for en-route traffic. It briefly describes the used fast time simulation and illustrates the obtained results. This paper quantifies the theoretical benefit that can be achieved by applying the mentioned principle in the described way. When knowing the traffic density of real air traffic control sectors, the results can easily and directly be transferred to them.