Abstract: Disclosed is a valve used in the water drainage system of an aircraft having a pressurized cabin. The valve includes a housing with a water inlet and outlet and in which a float is mounted by a spring. The float includes a stopper element adapted to engage the water outlet in the housing. The water inlet is in communication with the cabin pressure and the water being discharged and the outlet is in communication with atmospheric pressure outside the aircraft. The spring normally biases the float so that the stopper element is away from the water outlet when the plane is on the ground. As the plane gains altitude, the differential in pressure between atmospheric pressure and cabin pressure closes the valve, with the spring moving the stopper element into engagement with the water outlet. This occurs in response to the changing differential in pressure between the cabin pressure and atmospheric pressure.