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A type of gas turbine engine that has a fan or a set of lengthened blades either on a low-pressure compressor or on a low-pressure turbine. The term also applies to a less common arrangement that has the fan at the rear of the engine, with the fan as an external extension of one or more turbines. In some axial-flow engines, the airflow through the compressor is in the forward direction, parallel to the axis of the rotor. This is known as a reverseflow turbofan-type engine. Air moved by the fan blades propels bypass airflows along the engine axis but between the inner and outer engine casings. This air is not combusted but does provide additional thrust caused by the propulsive effect imparted to it by the extended compressor blading. Fan air is responsible for production of 25 to 75% of the thrust produced by the engine. Also called a turbofan engine