A gas turbine engine power plant assembly comprising a gas turbine engine power plant attached to and supported by a pylon, said power plant comprising a core gas turbine engine having a longitudinal centerline associated therewith which lies in a central plane of the engine, said core gas turbine engine driving a fan, which discharges airflow therefrom during operation of said engine, and being disposed within a nacelle, said nacelle comprising: a fan cowl defining a fan duct and surrounding said fan; and a core cowl disposed interiorly of said fan duct and surrounding said core engine, wherein downstream portions of said fan cowl and core cowl define therebetween a fan nozzle, said fan nozzle being divided into two portions by said central plane, with a first portion disposed on one side of said central plane and a second portion disposed on the opposite side of said central plane, said first portion having a greater cross-sectional flow area than said second portion, a portion of said pylon extends between said fan cowl and said core cowl and partially restricts the flow area of said fan duct on said one side of the central plane, and said fan cowl has a longitudinal centerline associated therewith and said core cowl has a longitudinal centerline associated therewith, characterised in that: the amount of flow area restricted by said pylon being substantially equal to the difference in area between said first and opposite fan nozzle portions; said fan cowl longitudinal centerline, at a downstream portion thereof, being offset toward said pylon from the longitudinal centerline of said core engine, or said core cowl longitudinal centerline, at a downstream portion thereof, being offset from the longitudinal centerline of said core engine in a direction away from said pylon; and the fan cowl and the core cowl are circular in cross section. a fan cowl defining a fan duct and surrounding said fan; and a core cowl disposed interiorly of said fan duct and surrounding said core engine, wherein downstream portions of said fan cowl and core cowl define therebetween a fan nozzle, said fan nozzle being divided into two portions by said central plane, with a first portion disposed on one side of said central plane and a second portion disposed on the opposite side of said central plane, said first portion having a greater cross-sectional flow area than said second portion, a portion of said pylon extends between said fan cowl and said core cowl and partially restricts the flow area of said fan duct on said one side of the central plane, and said fan cowl has a longitudinal centerline associated therewith and said core cowl has a longitudinal centerline associated therewith, the amount of flow area restricted by said pylon being substantially equal to the difference in area between said first and opposite fan nozzle portions; said fan cowl longitudinal centerline, at a downstream portion thereof, being offset toward said pylon from the longitudinal centerline of said core engine, or said core cowl longitudinal centerline, at a downstream portion thereof, being offset from the longitudinal centerline of said core engine in a direction away from said pylon; and the fan cowl and the core cowl are circular in cross section.