Gas turbine engine

A gas turbine engine for an aircraft, comprising:

• a core engine comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
• a fan, which is positioned upstream of the core engine, wherein the fan includes a plurality of fan blades;
• a gear box between the core engine and the fan, wherein the gear box is driven by the core shaft, and wherein the fan is driven at a lower rotational speed than the core shaft via the gear box;
• a static part of the gas turbine engine including a rotational mounting of the core shaft, wherein the core shaft includes a length between a drive side of the gear box and an axial position of the rotational mounting of the core shaft;
• wherein the core shaft includes an axial first region and an axial second region, wherein the axial first region is arranged between the drive side of the gear box and the axial position of the rotational mounting of the core shaft, and wherein the axial first region includes an axial first region length which is greater than 50% of the length between the drive side of the gear box and the axial position of the rotational mounting of the core shaft;
• and
• wherein, spanning the axial first region length, the axial first region of the core shaft includes a diameter greater than a diameter of the axial second region, resulting in an axial majority of the core shaft which is widened toward the drive side of the gear box.