Contrail Mitigation by means of 4D Aircraft Trajectory Optimisation

Abstract

Aviation has a significant impact on the global atmosphere. Next to particle and gas emissions, the increase of cloud cover might increasingly contribute to aviation induced climate change. Water contained in the exhaust of jet engines locally raises the relative humidity levels sufficiently for contrail formation to occur. These contrails can persist and evolve into anthropogenic clouding, often indiscernible from natural clouding, in case the ambient atmosphere is saturated. With the projected continued growth of air traffic, increasing propulsive efficiencies and possible introduction of fuels with higher hydrogen to carbon ratios, the importance of contrail mitigation is likely to increase. In this study, contrail mitigation by means of flexible free flight was studied, as this is likely the most efficient (short term) mitigation strategy. Contrail avoidance strategies by means of flexible free flight have been in use with air-forces around the globe for some decades. For commercial aviation however, the implementation of any such strategies, requires a thorough assessment of cost-effectiveness. The purpose of this study is to get a better understanding of the wider context of the problem, next to this a viable proposal is made for a short term contrail mitigation strategy. In this research, the formation of contrails and the costs and benefits associated with contrail mitigation is studied. A tool has been developed so that time and fuel burn optimal trajectories can be determined, while mitigating the formation of persistent contrail formation. Flexible free flight in four dimensions forms the basic means by which the optimisation tool can help evading regions prone to persistent contrail formation. The tool has been designed to rely on up-to-date meteorologic Numerical Weather Predictions (NWP), made available by meteorologic institutes. Test cases have been performed under realistic atmospheric conditions, using the Temperature, Pressure, Specific Humidity and Wind vector output from Numerical Weather Predictions, generated by the Canadian National Meteorologic Institute. With the, there from obtained results, an indication for the technical feasibility of this contrail mitigation strategy for commercial aviation has been created. This study has shown it is technically feasible for commercial aviation to evade contrail regions, thereby greatly reducing the effective anthropogenic cloud cover. The obtained results, clearly indicate persistent contrail formation can be avoided, at the cost of increased fuel consumption. The contrail reductions are more than sufficient to offset the additional CO2 emission, induced by the increases in fuel burn, leading to large scale reductions in the flights effective Radiative Forcing. The case dependency of contrail mitigation costs are susceptible to ambient atmospheric conditions. Indicating the great potential of using up-to-date atmospheric data over parametrized and more generic models often used in climate studies. The implementation of realistic, high resolution atmospheric data, significantly impacts the optimal trajectory in a number of ways. Deviations indicate that the realistic non-uniform temperature distribution can significantly affect the path constraints and hence the optimal trajectory. The inclusion of realistic wind data enhances the accurate representation of commercial aviation. For contrail mitigation, trajectory adjustments in the vertical plane are seemingly preferred over adjustments in the horizontal plane. This is due to the large horizontal extent and limited vertical thickness of contrail regions. A number of characteristic contrail evading manoeuvres are seen in the vertical flight trajectory. The sensitivity case analysed herein indicates, 90% of the induced contrails can be mitigated against an increased fuel consumption of less than 1.5%. The marginal costs of contrail mitigation are shown to be non-uniform. The more contrails are avoided, the higher the fuel burn penalty becomes. It might therefore be more cost effective for commercial aviation to implement a partial `contrail-reduction' plan rather than a `zero-tolerance on contrail' strategy. The great merit of implementing flexible free flight contrail mitigation in commercial aviation has been shown. This study presents an academic methodology potentially forming the basis for a commercial tool, to be used for scheduling of environmental optimal trajectories throughout the aviation sector. This would enable airliners to optimise their fleet movements while accounting for both economic and ecological impact factors.