Abstract: A gas turbine jet engine is disclosed with a fan delivering into a bypass duct to generate mainly propulsion thrust, and with a thrust reversing device having flaps which for reverse thrust operation pivot into the fan air stream and cooperate with an axially extended extreme section of the outer wall of the bypass duct to uncover delection ports arranged at breakthroughs in the outer wall. The extreme section of the outer wall and the thrust reverser flaps are linked one with the other for relative movement between them such that in a first phase of actuation relative to a fixed extreme nozzle section, the extreme section provides an additional fan nozzle area which communicates with the inlet flow areas formed by the flaps relative to the bypass duct.