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.