Contrail, or not contrail, that is the question: the "feasibility" of climate-optimal routing

The environmental impact of aviation has been the subject of significant research efforts for several decades. While reducing carbon emissions has reached a consensus across different stakeholders, leading to efforts to reduce route inefficiencies in air traffic management systems, other climate effects like contrails have spurred a different kind of discussion in the research community. Some research has rapidly moved into the pre-operational phase, aiming to reduce the climate impact of aviation by optimizing flight trajectories to avoid contrail formation. Notably, recent commercial projects from Google and Breakthrough Energy have been fast-tracking the operational perspective of contrail avoidance. In our past research, we have established a robust and fast methodology for trajectory optimization to minimize contrail formation based on TOP, a trajectory optimizer using the OpenAP aircraft performance model. In this paper, we address the practical challenges of implementing contrail-aware routing strategies in the aviation industry. We analyze the trade-offs between fuel consumption and contrail avoidance, the impact of weather forecast uncertainty on contrail mitigation strategies, the effects of contrail mitigation on airspace capacity and network operations, and the implications of contrail reduction strategies on the aviation industry and regulatory frameworks. We use a dataset of flight trajectories over Europe on a day with significant contrail potential to conduct a data-driven analysis of these challenges. Then, we demonstrate the potential difficulties in implementing contrail-optimal routing, especially concerning the uncertainties in weather forecasts, airspace capacity, and the responsibility for optimal routing. Overall, we argue that contrail optimal routing should be approached with caution, and it may not be as straightforward as promoted by some stakeholders.