Abstract: An apparatus and method for introducing a transmission line into a duct through which it is to be propelled by viscous drag of fluid propellant. A high local flow of fluid is created by inputting fluid, preferably compressed air, adjacent the input of the transmission line to the duct and bleeding a proportion of the fluid from the duct a short distance along from the input. This local flow provides the driving force to progress the transmission line along the first third of the duct against the hydrostatic potential. The extent of the venting via the bleed valve is controllable to control the driving force on the transmission line. This technique is especially useful for transmission lines including optical fibres.