This work presents the methodology for the conceptual modeling and preliminary sizing of Solid Oxide Fuel Cell–Gas Turbine (SOFC–GT) hybrid powertrains within a conceptual aircraft design framework. SOFC–GT hybrid systems, long investigated in commercial power generation, offer higher combined power output, improved overall thermal efficiency, and greater fuel flexibility compared to similar FC– GT architectures. As high-temperature solid oxide fuel cells (HT–SOFCs) are not electrochemically bound by a theoretical efficiency limit like the Carnot cycle for gas turbines, their integration with conventional engines has the potential to significantly improve system efficiencies while retaining high power densities. The primary objective of this work is to evaluate their integration into regional turboprop aircraft and compare them against conventional aircraft with kerosene/SAF and LH2-based gas turbine powertrains for similar Top Level Aircraft Requirements (TLAR). The sizing methodologies for SOFC-GT aircraft are incorporated within the Class 1 sizing loop of the Aircraft Design Initiator (ADI), an in-house conceptual aircraft design tool developed by the Faculty of Aerospace Engineering at Delft University of Technology for rapid design evaluation. For this, the powertrain model previously developed by De Vries (for hybridelectric powertrains) and Borgia (for hydrogen-based hybrid-electric powertrains) was extended for the sizing of SOFC-GT aircraft. The modeling approach starts at the fuel cell level, using validated electrochemical and thermodynamic models for the SOFC. A baseline tubular SOFC geometry is selected and modified to improve stacklevel gravimetric power density through reductions in component thicknesses, dominant ohmic losses, and an increase in the effective cell area. The cell-level model is then integrated into a system-level representation of the SOFC-GT powertrain, which is then incorporated into the ADI’s Class 1 sizing loop to meet the propulsion and power requirements of the baseline ATR 72-600 aircraft. At the system level, the Class 1 sizing results of the SOFC-GT aircraft (at a design current density of 400 mA/cm², cruise H2 split of 50%, and 75% fuel utilization) show an overall thermal efficiency of 44.17% in cruise, representing a 48% improvement over the baseline ATR 72-600 and 44% over the LH2-based aircraft. These gains are accompanied by increases in MTOM (19.2% and 14.7%) and OEM (+45.2% and +23%) compared to kerosene and LH2 aircraft, respectively. Nevertheless, the higher efficiency reduces the required energy mass for the full-range design mission by 65% and 13.5%, while the larger wingspan (29.44 m) remains within ICAO Class C limits. Consequently, the lower H2 demand also decreases H2O and NOX emissions by 15.3% and 50.26% relative to conventional LH2 aircraft. Results from parametric analyses indicate that the hydrogen power split between the gas turbine and the SOFC stacks in cruise is the most influential design parameter for maximizing performance, with higher values improving overall thermal efficiency but also significantly increasing the aircraft operating empty mass (OEM) and energy requirements for the design mission. The trade-off between fuel cell and gas turbine contributions is sensitive to fuel utilisation, where low utilisation reduces efficiency by shifting power production toward the less efficient gas turbine. On the other hand, increasing anode fuel utilisation above 75% improves overall thermal efficiency but can reduce gravimetric power density at the stack level due to mass transport losses, the effect of which can be partially mitigated by increasing the excess air supply at the cathode. Additionally, the SOFC system pressure and operating temperature were also observed to be important design parameters that could improve the performance of the SOFC-GT powertrain. However, the cruise thermal efficiency exhibits an asymptotic trend at higher pressures and temperatures, beyond which the benefits diminish. At elevated temperatures, further gains are offset by an increase in system mass, while at higher pressures, the additional Balance of Plant (BoP) power requirements lead to reductions in both gas turbine and SOFC efficiencies.