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; 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 and characterized by : 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 ; and wherein a fan blade mass ratio of: first flange radius 104 mass of each fan blade is equal to or less than 41.9 mm/kg (19.0 mm/lb) for each fan blade. 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 and characterized by : 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 ; and wherein a fan blade mass ratio of: first flange radius 104 mass of each fan blade is equal to or less than 41.9 mm/kg (19.0 mm/lb) for each fan blade. 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 ; and wherein a fan blade mass ratio of: first flange radius 104 mass of each fan blade is equal to or less than 41.9 mm/kg (19.0 mm/lb) for each fan blade. <EOS>