A drain valve assembly for positively sealing an aircraft waste system drain line to prevent waste fluid leakage and the formation of so-called "blue ice" due to such leakage during high altitude flight. The valve assembly has an inlet for connection to the drain line, an outlet for connection to a waste drain hose of an airport waste disposal facility, and an outer valve cover and inner ball valve between the inlet and outlet which provide truly redundant seals for the drain port in flight, can be opened with relative ease when frozen, are relatively immune to improper sealing due to obstruction by waste, enable removal of a frozen waste blockage in the waste flow path without a sudden large waste dump, and permit inspection and easy replacement, if necessary, of the valve seals.