A combination air seal for opposed flanges of a casing structure containing air at elevated pressure includes rectangular cross-section groove in one face of one of the opposed flanges and a narrow compressible gasket or seal strip in the groove at one side thereof to define an open transverse channel in the groove oriented transversely to the direction of air leakage through the flanges from the elevated pressure conditions in the structure. Air under elevated pressure leaks across the interface of the flanges to be taken up by the open transverse channel and conducted to a lower pressure region of the structure for further utilization. The gasket is adapted to be adequately but not excessively compressed when metal to metal engagement of the flanges is achieved. The compressed gasket prevents excess air in the channel from escaping from the channel to pass across the flange interface and escape from the casing structure.