S-duct

An S-duct (or serpentine inlet) is a type of jet engine intake duct, used in several types of trijet aircraft. In this configuration, the intake is in the upper rear center of the aircraft, above or below the stabilizer, while the exhaust and engine is at the rear of the aircraft (Serpentine shape). The S-duct is located in the tail, or empennage, of the aircraft. The shape of the S-duct is distinctive and easily recognized, and was used in several aircraft, beginning in 1962 with the Hawker Siddeley Trident. Currently, the Dassault Falcon 7X and Dassault Falcon 900 business jets, and the Tupolev Tu-154M narrow-body airliner in limited production, are the only aircraft in production that use the S-duct design.

Benefits and drawbacks

The S-duct was invented as a solution for positioning the central engine on trijets. The S-duct was easier to service than alternative trijet designs. Most trijet designs opted for the S-duct layout. Only the DC-10 and MD-11 trijets choose 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 layout also increases total aircraft aerodynamic drag by 2–4%.