A type of Boundary Layer Control System, blown flaps pass engine-generated air or exhaust over the flaps to increase lift beyond that attainable with mechanical flaps. Types include the original (internally blown flap) which blows compressed air from the engine over the top of the flap, the externally blown flap, which blows engine exhaust over the upper and lower surfaces of the flap, and upper surface blowing which blows engine exhaust over the top of the wing and flap. While testing was done in Britain and Germany before the Second World War,[26] and flight trials started, the first production aircraft with blown flaps wasn't until the 1957 Lockheed T2V SeaStar.[27] Upper Surface Blowing was used on the Boeing YC-14 in 1976.