<p>  </p> <p>Recently, aircraft using liquid hydrogen gas-turbine engines (also referred to as liquid hydrogen aircraft) have drawn more attention from the aviation industry as an option to decarbonize commercial aviation. Liquid hydrogen aircraft can have a lower environmental impact than kerosene aircraft and can be a vital part of the net-zero carbon emission plan. However, uncertainties and challenges still exist, including the design of liquid hydrogen aircraft, how their operation might differ from kerosene aircraft, and most importantly, the feasibility of achieving aviation decarbonization goals when these aircraft operate in fleets alongside existing fleets of aircraft. This paper focuses on the modeling and the design of liquid hydrogen aircraft and describes operational studies to evaluate the environmental impact of introducing liquid hydrogen aircraft into a fleet operating on a worldwide network. The studies use the Fleet-Level Environmental Evaluation Tool (FLEET) to simulate different scenarios and account for the estimated hydrogen cost and equivalent carbon emissions. Results show that the “most likely" scenario where a 175-seat class single-aisle liquid hydrogen aircraft enter service in 2035, could result in a total fleet-level carbon emission reduction of 7.11% compared to a baseline scenario with no liquid hydrogen aircraft in the airline fleet. The simulation considers different liquid hydrogen pricing scenarios and equivalent carbon emissions assumptions. A fleet with multiple seat classes of liquid hydrogen aircraft may result in smaller carbon emission reduction than might initially be expected due to the delayed replacements of Jet-A aircraft in the fleet caused by the high costs of liquid hydrogen aircraft. The FLEET simulations show a maximum possible reduction of 45.72% of the total fleet-level carbon emission in 2050 comparing to the baseline scenario with no-liquid hydrogen aircraft, obtained by introducing multiple models of liquid hydrogen aircraft using green hydrogen to replace the existing fleet instead of future kerosene aircraft. The introduction of liquid hydrogen aircraft also significantly impact the profitability of the total fleet, which has an unexpected impact on the fleet allocation and composition. Further studies on the liquid hydrogen fleet pricing and implementation, as well as refinements on the FLEET tool, are recommended in order to gain a more realistic understanding of how the liquid hydrogen fleet might impact the global aviation.</p>