The S-duct was invented as a solution for positioning the central engine on trijets. The S-duct is easier to service than alternative trijet designs. Most trijet designs opted for the S-duct layout. Only the
McDonnell Douglas DC-10 and
MD-11 trijets' designers chose not to use the S-duct and go with a "straight-through" layout. The straight-through layout leaves the engine high above the ground, making access difficult. The straight-through layout also increases total
aerodynamic drag by 2–4%. The S-shaped duct is a complicated and costly design. Since modern jet engines have more power and reliability than those of the 1970s and can safely power the aircraft with only two engines, the trijet design is no longer used for large commercial airliners but is used on the latest
Dassault Falcon 7X and
Falcon 8X business jets in order to provide more total thrust while enabling the continued use of smaller engines in the class with which Falcons have historically been designed. In international aviation regulations the triple engine layout is also considered inherently safer for the increased redundancy, which allows the use of certain airfields at high elevation only for aircraft with more than two engines. ==On stealth aircraft==