Trajectory Optimisation for Contrail Reduction of Hydrogen-Powered Aircraft

Abstract

Airlines worldwide have committed to the goal of achieving carbon neutrality in 2050. This goal is to be reached using a combination of sustainable aviation fuel (SAF), carbon offsetting, operational improvements, and the implementation of new propulsion technologies. Electric propulsion is expected to be applied for short-haul flights, while hydrogen-powered aircraft have the potential for medium-range routes too. Although these aircraft types are certain to cut the in-flight carbon emissions of the aviation sector if they are integrated into commercial fleets, the impact of hydrogen-powered aircraft on non-CO2 climate effects is still rather unknown. The non-CO2 effects of conventional aircraft have been researched extensively, focussing primarily on the climate impact of NOX and contrails. Both climate effects are location dependent, meaning that the climate impact is correlated with local values of atmospheric properties. As the location dependency of contrails is the most significant, the bulk of the research on this topic is focused on trajectory optimisation for contrail reduction. The introduction of hydrogen-powered aircraft in the aviation system could add an extra dimension to this research, as the lack of carbon emissions reduce the negative effects of route diversions.
This research considers three aircraft concepts: a current aircraft powered by Jet A-1, an aircraft with technological improvements powered by a 70% SAF blend, and an aircraft powered by hydrogen combustion. The effect of trajectory optimisation for each aircraft type highly depends on the contrail climate impact of the fuel-optimal flight. Simulations in pycontrails show that flying on hydrogen can cause a decrease in contrail ice crystal numbers of 89% compared the contrail of a conventional aircraft, and 81% of that of future aircraft powered by a 70% SAF blend. The resulting decrease in climate impact is 87% and 79% compared to the two other aircraft types, respectively.
Two approaches to using contrail analysis for trajectory optimisation are proposed in this research. The first is based on using pycontrails to find correlations between the contrail climate impact and atmospheric properties, which can be used for formulate new algorithmic climate change functions (aCCFs). This method proved not to be applicable as low correlations were found between atmospheric variables at the location of contrail formation and contrail energy forcing. The physical processes modelled in pycontrails do not seem to sufficiently well-captured by aCCFs that only take into account the conditions at the contrail onset. The second method is based on directly forming a contrail cost grid in pycontrails. Although this method is more complex to implement, it yields more accurate results.
The resulting contrail climate impact grids are subsequently used for trajectory optimisation, performed in OpenAP-TOP. The fuel-optimal route per concept aircraft is determined for six different trajectories, after which weighted combinations of fuel and climate costs are passed through the program to end up with a range of small and larger route diversions for climate cost savings. The results show a high impact of trajectory optimisation for the Jet A-1 aircraft concept and the future aircraft concept flown on the 70% SAF blend, decreasing the P-ATR20 climate impact by approximately 25 − 50 pK and 10− 30 pK, respectively, for the analysed case studies. Route diversions for hydrogen-powered aircraft show limited benefits (< 3 pK), which can be directly related to the relatively small contribution of contrails to the total climate impact of hydrogen-powered flights. For their fuel-optimal routes though, hydrogenpowered
aircraft reduce the total climate impact of a flight greatly: generally, a 80-90% reduction in
total climate impact is realised compared to Jet A-1.