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 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 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.