In many industrial applications the liquid trapped inside long pipelines can cause a number of problems. Intrusion of the pressurized air on top of the water column inside the horizontal pipeline can result in a less or more mixed stratified flow. The dynamics of a moving air–water front during the emptying of a PVC pipeline with the diameter-to-length ratio 1 : 1100 were experimentally and theoretically studied. In the experiments, the water was driven out of the pipeline with an initial upstream air pressure of 2 barg and a 4.5 m high downstream-end siphon, where the water outflow was restricted by a valve that was closed 11%. The measured discharges and water-level variations are analysed together with Control Volume modelling results. During the ‘forced’ (not only gravity-driven) emptying process, both the downstream-end drainage and tail leakage behind the moving air–water front decreased over the full water-column length. The water-column mass loss due to the tail leakage is referred to as holdup. The Zukoski dimensionless number is used to parameterize the relative shortening of the water column associated with the unidirectional movement of the air–water front along the large-scale horizontal test section of the pipeline, where surface-tension effects and minor losses at joints and turns are negligible. Keywords: pipeline, air–water interactions, two-phase flow, unsteady flow, Reynolds number, Froude number, Zukoski number.