Additionally, airports considering this as a solution may double-check if, after the retrofit, the bus fleet would achieve the same efficiency, range and reliability compared to purpose-built zero- emission vehicles, particularly in demanding airside environments. Geneva Airport10 illustrates the potential of this approach: in 2024 it invested in two retrofitted buses at an average cost of CHF 350,000 each, with four more to be delivered in 2025. Together with other fleet measures, this brings the airport to 24 electric buses in a total of 27, showing how retrofitting can provide a cost-efficient bridge while infrastructure for next-generation technologies is developed. Fossil-fuel and biofuel-mix buses HVO buses represent a growing trend in airport ground transport decarbonization strategies. HVO is a renewable diesel alternative produced from vegetable oils or waste fats, offering a significant reduction in life-cycle carbon emissions – up to 90% compared to conventional diesel. These fuels are in most cases compatible with existing diesel engines and fuelling infrastructure, making them a practical and immediate solution for airports11 seeking to lower their operational emissions without extensive new infrastructure investments. From an infrastructure perspective, HVO’s compatibility with existing diesel fuelling systems means that airports can transition their fleets with minimal operational disruption or capital expenditure. This contrasts with the previously analysed scenarios. As such, HVO serves as a valuable bridge technology, supporting airports’ decarbonization goals in the near term and complementing the broader shift towards SAF and zero-emission ground transport. While this paper has focused on zero-emission technologies such as battery-electric and hydrogen fuel cell, HVO-powered buses have not been included in the core analysis due to their status as a low-carbon, rather than zero- emission, solution. Nevertheless, it is important to acknowledge that HVO buses are being adopted at a growing number of airports as a transitional technology. These deployments are enabling airports to achieve immediate emissions reduction while longer-term electrification and hydrogen infrastructure are developed. Biomethane-powered buses This alternative uses methane produced from biological sources, such as organic waste, sewage sludge or agricultural residues, which is used as a drop-in replacement for compressed or liquefied natural gas in standard gas engines or hybrid systems. It enables airports and transit operators to decarbonize their fleets without completely replacing existing vehicles or refuelling infrastructure. Biomethane buses have quick refuelling times, long operational ranges and significantly lower particulate and nitrogen oxide emissions compared with diesel. When the biomethane is sourced from waste streams, the overall greenhouse gas balance can approach carbon neutrality. The Munich Airport’s biomethane buses12 have a range of up to 800 km and refuel in about five minutes. They have reduced particulate emissions by roughly 90% and nitrogen oxides by over 60% compared with Euro VI diesel buses. Hydrogen internal combustion engine Hydrogen ICE is another retrofit option under development, showing promise particularly as a bridge between existing diesel engines and zero- emission technologies, since many components (such as ignition, cooling and transmission) are shared with conventional buses. Projects such as the TRIMIS HyFLEET:CUTE13 trials in Berlin have already demonstrated the potential of this approach. While hydrogen ICE has not been included in the present TCO analysis, it is an area worth tracking, and its evolution could be captured in future assessments to provide a more complete picture of available decarbonization pathways. As airports advance on their course towards decarbonization, the choice of bus technology is more than a technical decision – it is shaped by evolving priorities, operational realities and the ambition to create a cleaner future. Across all available technologies – diesel, retrofit diesel, battery electric and hydrogen fuel cell – the decision to adopt one pathway over another requires a careful analysis of each airport’s unique operational constraints, infrastructure readiness, regulatory environment and TCO. Figure 3 represents how these options compare in terms of climate impact, investment, ongoing costs and operational fit.2.2 Key features comparison Decarbonizing Aviation Ground Operations: Alternative Bus Technologies 11

Decarbonizing Aviation Ground Operations 2025