through the nozzle 19 to provide additional propulsive thrust the high intermediate and low pressure turbines 16 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by concentric shafts 20 21 and 22 which rotate about an axis x x fig2 shows part of the duct 24 indicated in fig1 the duct is defined by radially outer and radially inner walls respectively 26 and 28 and contains an annular array of vanes 30 of which one is shown the vanes 30 are secured between the duct walls 26 28 in use a gas stream flows through the duct 24 in the direction shown by the arrow 32 under appropriate atmospheric conditions entrained water or ice crystals in the gas stream may form ice on the leading edge region 34 of the vane 30 a short distance upstream of the leading edge 34 a nozzle 36 is provided in the duct wall 26 air 38 is fed through this nozzle creating a jet of air 40 directed along the leading edge 34 of the vane 30 and substantially perpendicular to the gas stream direction 32 each vane 30 in the annular array is provided with a nozzle 36 the air feed to the nozzles 36 may be by any convenient means u2014 for example an annular manifold may surround the duct wall 26.