Abstract: A helicopter transmission system includes a gearbox comprising a gearcase (11) having a hollow stub axle (12) by which operational loads are transmitted from a main sustaining rotor to a fuselage structure. One or more input speed reducing gear trains (20, 21, 22, 23) are individually supported in hollow lobe portion(s) (14) supported from the axle (12), the or each lobe portion having an individual lubricating oil sump (18) and circulating means (31) adapted to circulate lubricating oil to the respective gear trains. Preferably, interconnecting means (33) are provided to connect the respective individual sumps, and valve means (32, 35) are adapted to maintain a flow of lubricating oil in the event of a loss of oil from one of the sumps.