It's the most common trijet configuration known, in this case the air intake is under the vertical fin which catches the air and then it descends to an inclined duct to the center body level until it finally meets the engine and scapes through the center-back of the plane. It's name comes from the letter "S" shape of the duct. The Boeing 727 and Lockheed L-1011 Tristar feature this system. * Direct flow