Abstract: A drain structure for facilitating the draining of liquid condensate from inside an aircraft whenever air pressures inside and outside the aircraft are substantially equal. A pressure-sensitive apparatus is provided in conjunction with a drain opening through the aircraft's outer skin so that liquid can pass through the drain opening only while the apparatus is open. The apparatus includes a housing and a valve in the form of a slider movable within the housing to open and close the apparatus in response to the pressure differential between the inside and the outside of the aircraft. The housing defines a liquid passageway between the inside of the aircraft and the drain opening, and the slider closes the apparatus by blocking this liquid passageway whenever there is a substantial pressure differential between the inside and the outside of the aircraft.