A gas turbine engine for an aircraft, the gas turbine engine having a principal rotational axis and comprising: a fan located upstream of an engine core , the fan comprising a plurality of fan blades and having a fan diameter ; a nacelle surrounding the engine core and defining a bypass duct between the engine core and the nacelle ; a fan outlet guide vane (OGV) extending radially across the bypass duct between an outer surface of the engine core and an inner surface of the nacelle , the fan OGV having a radially inner edge and a radially outer edge , wherein an axial midpoint of the radially inner edge is defined as the fan OGV root centrepoint ; the engine core comprising: a compressor system with compressor blades comprising respective aerofoils, the compressor system comprising a first, lower pressure, compressor , and a second, higher pressure, compressor ; and an outer core casing surrounding the compressor system; the gas turbine engine being characterized by the outer core casing comprising a first flange connection arranged to allow separation of the outer core casing into different portions at an axial position of the first flange connection , the first flange connection having a first flange radius defined as the radial distance between the rotational axis and a flange connector of the first flange connection , wherein the first flange connection is the first flange connection that is downstream of an axial position defined by the axial midpoint between the mid-span axial location on the trailing edge of the most downstream aerofoil of the first compressor and the mid-span axial location on the leading edge of the most upstream aerofoil of the second compressor ; wherein a fan OGV root position ratio of: axial distance 108 between the first flange connection 60 and the fan OGV root centrepoint 58 a first flange radius 104 is equal to or less than 2.6. a fan located upstream of an engine core , the fan comprising a plurality of fan blades and having a fan diameter ; a nacelle surrounding the engine core and defining a bypass duct between the engine core and the nacelle ; a fan outlet guide vane (OGV) extending radially across the bypass duct between an outer surface of the engine core and an inner surface of the nacelle , the fan OGV having a radially inner edge and a radially outer edge , wherein an axial midpoint of the radially inner edge is defined as the fan OGV root centrepoint ; the engine core comprising: a compressor system with compressor blades comprising respective aerofoils, the compressor system comprising a first, lower pressure, compressor , and a second, higher pressure, compressor ; and an outer core casing surrounding the compressor system; a compressor system with compressor blades comprising respective aerofoils, the compressor system comprising a first, lower pressure, compressor , and a second, higher pressure, compressor ; and an outer core casing surrounding the compressor system; the gas turbine engine being characterized by the outer core casing comprising a first flange connection arranged to allow separation of the outer core casing into different portions at an axial position of the first flange connection , the first flange connection having a first flange radius defined as the radial distance between the rotational axis and a flange connector of the first flange connection , wherein the first flange connection is the first flange connection that is downstream of an axial position defined by the axial midpoint between the mid-span axial location on the trailing edge of the most downstream aerofoil of the first compressor and the mid-span axial location on the leading edge of the most upstream aerofoil of the second compressor ; wherein a fan OGV root position ratio of: axial distance 108 between the first flange connection 60 and the fan OGV root centrepoint 58 a first flange radius 104 is equal to or less than 2.6. <EOS>