the blade is cast as is well known in the art so as to have a hollow interior 3 an impingement tube indicated generally by the arrow 4 is inserted into the hollow aerofoil for the express purpose of providing a conduit for cooling air in accordance with the invention the impingement tube is shown split into two portions 4 a 4 b in a chordal plane c located at about the mid point of the blade 39 s span as shown in fig2 the blade is preferably cast with internal ribs 5 extending in the general direction of chords of the aerofoil in this case the ribs extend around the leading edge 2 of the aerofoil although only one rib 5 is shown in this sectional view there are usually several ribs radially spaced apart over the span of the blade and the provision of more than one such rib will be assumed during the remainder of this description the advantage of this is that the ribs provide additional cooling surface area and may seem to guide the cooling air exiting from small holes not shown in the impingement tube towards the more heat critical surfaces of the blade eventually the spent coolant passes from the blade into the surrounding freestream through film cooling holes slots or other apertures 11 as known in the art some of these may be provided in the trailing edge 6 of the blade and in fact fig2 shows the aerofoil as provided.