## 1 Introduction  
Aviation's contribution to anthropogenic radiative forcing is estimated at around 3.5%, and therefore significantly contributes to climate change [1]. In comparison with other transport sectors, it is the second largest contributor following road transportation [2]. Although the COVID-19 pandemic has led to a significant reduction in air transport in the short term, a quick recovery to previous levels is expected. Hence, a broad set of measures is required to achieve compliance with the ambitious goals that have been defined, for example, in the context of the Paris agreement [3]. In contrast to technological improvements, operational approaches require shorter implementation times and are able to realise mitigation potentials faster [4]. Thus, these measures are expected to play an important role in the near future. In addition to well-investigated climate effects from carbon dioxide (CO\({}_{2}\)), direct and indirect effects induced by non-CO\({}_{2}\) emissions, such as water vapour (H\({}_{2}\)O), nitrogen oxides (NO\({}_{x}\)), and contrail cirrus, contribute to about two thirds of the total net-radiative forcing in aviation [1]. These non-CO\({}_{2}\) effects do not only depend on the fuel consumption, but also on emission location, time, and meteorological boundary conditions [1, 5, 6]. Hence, a sole focus on fuel efficiency and resulting reduction in CO\({}_{2}\) emissions does not necessarily reduce the total climate impact. A comparison of these approaches either aiming for minimal fuel consumption or a minimal climate impact is subject to this study. The goal of intermediate stop operations (ISO, also referred to as staging or multi-step operations) is to increase fuel efficiency by shortening the stage length of a mission by performing one or more intermediate stops for refuelling. Shorter stage lengths allow to reduce the amount of fuel that is needed to carry the required fuel on the respective mission, which reduces the aircraft mass. As a result, kerosene can be saved compared to the non-stop flight. ISO has been subject to a variety of research studies aiming for fuel efficiency [7, 8, 9, 10, 11, 12, 13]: depending on the different boundary conditions of the studies,e.g., considered mission length, wind conditions, and aircraft type, fuel saving potentials between 5% and 20% are identified on a single mission basis, whereas the maximum global potential is estimated at around 5% to 10% for missions with distances of 2500 nautical miles (NM) and more. With a re-design of the aircraft, i.e., targeting shorter design ranges, up to 30% of fuel can be saved [13; 14]. Although ISO enhances fuel efficiency for a large share of long-haul flights, cost savings strongly depend on further mission parameters, such as the connected city pair, fuel prices, wind conditions, and crew composition as the resulting longer travel times negatively influence operating cost [9; 10]. This effect is enhanced for concepts with more than one refuelling stop, which only achieve marginal additional improvements in comparison to the one-stop set ups [11]. A reduction in fuel consumption through reduced aircraft weight leads to higher flight altitudes if maximum fuel efficiency is targeted [15; 16; 17]. Thus, engine emissions are generally released in higher altitudes, which can result in an increase in the climate impact caused by non-CO\({}_{2}\) effects, such as NOx and contrails [18; 19]. Therefore, a reduction in climate impact from fuel-proportional CO\({}_{2}\) emissions can be overcompensated by effects induced by other species. [PERSON] et al. [8] confirmed this for a global ISO scenario, where a fuel-optimal implementation was found to be associated with a rise in climate impact by 2.3%, given as the average change of the Earth's surface temperature over a period of 100 years. In this context, climate impact from these species can be reduced with lower cruise altitudes [18; 20; 21].