Abstract: A water supply system for an aircraft includes a ventilatable water tank, a water supply line to supply water to water-using devices such as galleys and handwash basins, fill and drain lines, and a multi-function valve for carrying out filling and draining, and other system functions. The multi-function valve is interposed between the water tank, the water supply line, the fill line, and the drain line. The valve includes a filling port connected to the filling line, a tank port connected to the water tank, a supply port connected to the supply line, a drain port connected to the drain line, and preferably also a tank overflow port and an outside overflow port. The valve includes a shut-off element arranged within a housing, wherein the shut-off element has therein a water passage which includes first, second, and third branches, and preferably also an air passage.