Abstract: A gas turbine engine comprises a gearbox comprising a sun gear, an annulus gear, a plurality of planet gears and a carrier. The sun gear meshes with the planet gears and the planet gears mesh with the annulus gear. A planet gear comprises a first annular structure, a plurality of teeth extending radially from the first annular structure and a second annular structure abutting and secured to a cylindrical surface of the first annular structure. The first annular structure and the teeth consist of steel, the second annular structure comprises a metal matrix composite, the metal matrix comprising aluminium, aluminium alloy, steel, titanium or titanium alloy, the reinforcing material comprising titanium diboride, titanium carbide or titanium nitride. The second annular structure reduces the mass but maintains, or increases, the stiffness of the planet gear to carry centrifugal loads and to maintain roundness of the planet gears.

Abstract: A male or female element (12, 2) for a conic coupling, the element comprising: an end surface (16, 6) and two angled side surfaces (18, 8) disposed either side of the end surface (16, 6); wherein at least one of the side surfaces (18, 8) has a non-planar profile.

Abstract: A male or female element (12, 2) for a conic coupling, the element comprising: an end surface (16, 6) and two angled side surfaces (18, 8) disposed either side of the end surface (16, 6); wherein a coating is applied to at least one of the side surfaces, the coating having a varying coefficient of friction across the surface.