An aircraft air conditioning system including a process air line having a first end connected to a process air source to allow a flow of process air therethrough. A ram air channel of the air conditioning system allows ambient air flow therethrough. A heat exchanger unit is disposed in the process air line and arranged within the ram air channel to thermally couple process air flowing through the process air line to ambient air flowing through the ram air channel. A water extractor disposed in the process air line downstream of the heat exchanger unit extracts water from the process air. A water supply system supplies extracted water into the ram air channel. The water supply system has a control arrangement to control a fluid connection between the water extractor and the ram air channel in dependence on an operating state of the aircraft air conditioning system.