## 1 Introduction
While SAFs are produced from low-carbon feedstocks, LCAF refers to a fuel from fossil sources, the production of which is able to displace at least 10% lower lifecycle GHG emissions against a certain benchmark value. ICAO CORSIA developed a set of sustainability criteria for aviation fuels to qualify as CEFs that cover several themes, including greenhouse gases (GHG), carbon stock (for fuels produced before 1 January 2024), water, soil, air, conservation, waste and chemicals, human and labour rights, land use rights and land use, water use rights, local and social development and food security (for fuels produced after 1 January 2024). The CORSIA criterion for eligible fuels on GHG emissions requires a lifecycle emission reduction of at least 10% as compared to the baseline lifecycle emissions values for aviation fuels of 89 gCO2 eq/MJ using a lifecycle assessment (LCA) approach. [7]. CORSIA developed default lifecycle emissions values that may be used to claim emission reductions from the use of CEFs [8]. CORSIA produced guidance for sustainability certification schemes (SCS) for application of CORSIA sustainability criteria [9] and developed a methodology for calculating actual lifecycle emission values [10]. This is the first global LCA methodology agreement on the calculation of GHG emissions of aviation fuels. According to the methodology, a default lifecycle emissions value is a sum of a default core LCA value and a default indirect land use change (ILUC) value. The SAF pathways offer potentially significant GHG emission reductions in attributional lifecycle GHG emissions relative to fossil jet fuel [11]. The concept of LCAF and sustainability criteria are still under evaluation in CORSIA as its implementation poses many challenges both from a theoretical and practical point of view. For the moment, sustainable aviation fuels are identified as one of the most important ways for reducing GHG emissions in the international aviation sector. A study [12] assessed the ways of controlling aviation carbon emissions, identifying improvements in aircraft aerodynamic efficiency, structural efficiency, air traffic management and the use of using alternative fuels as main determining factors. Several studies have been conducted to assess the technical and economic feasibility of the production and use of SAFs and to assess the technical potential and GHG emissions that can be ensured by different pathways [13; 14; 15]. They have addressed the technologies to produce aviation biofuels, their potential to be upscaled and techno-economic perspectives, as well as the impact of SAF on sustainability. A study [16] assessed the current use of biofuels in aviation and the production potential of aviation fuels in the EU and the constraints related to costs and feedstock availability and competition with demand. [PERSON] et al. [11] presented the lifecycle GHG emissions of sustainable aviation fuel pathways calculated following the CORSIA methodology. The GHG emission savings can be up to 94% when compared to petroleum-derived baseline jet fuel (and more than 100% when considering negative GHG emissions of ILUC contribution for some SAF pathways). This showed that many SAF pathways can play a major role in reducing the carbon footprint of the aviation sector. A study [15] concluded that the use of CORSIA-eligible fuels (CEFs) is beneficial to reducing the cost associated with CORSIA's offsetting requirement operational costs after considering the fuel cost, carbon pricing and GHG emission savings from reduction of different pathways. A study [17] estimated the EU's technical potential for the production of LCAF considering the data on crude oil used in EU refineries, aviation fuel use and relevant carbon intensity reduction technologies. This showed that, even if LCAF could offer GHG saving opportunities, its possible impact is rather limited, especially when compared to the demand for the decarbonisation of aviation.