A gas turbine engine comprising an inner cowl; an annular air-oil cooler circumferentially disposed about the inner cowl, the annular air-oil cooler optionally including an integral annular oil tank and a plurality of cooling fins radially extending therefrom, the cooling fins being disposed in at least partial communication with a fan duct and nozzle; and a fan air diverter circumferentially disposed about the inner cowl and coaxially adjacent to the annular air-oil cooler or cooling fins, the fan air diverter being selectively movable relative to the annular air-oil cooler or cooling fins; characterised in that: the fan air diverter comprises: a nozzle flap (132, 134) disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap (132, 134) being coupled to the inner cowl and selectively movable relative to the annular air-oil cooler between an open position and a closed position; and an actuator system assembly operatively coupling the nozzle flap (132, 134) to the inner cowl, the actuation system assembly being configured to actuate the nozzle flap (132, 134) between the open position and the closed position; the actuator system assembly comprising at least one sync ring circumferentially disposed between a low pressure compressor case and the nozzle flap (132, 134), the sync ring being rotatable between a first angular position and a second angular position about an engine axis, wherein rotation of the at least one sync ring between the first angular position and the second angular position repositions the fan air diverter relative to the annular air-oil cooler or cooling fins; and a plurality of idler links radially coupling the sync ring to the nozzle flap (132, 134), the idler links being pivotally configured such that rotation of the sync ring to the first angular position moves the nozzle flap (132, 134) to the open position, and rotation of the sync ring to the second angular position moves the nozzle flap (132, 134) to the closed position. the fan air diverter comprises: a nozzle flap (132, 134) disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap (132, 134) being coupled to the inner cowl and selectively movable relative to the annular air-oil cooler between an open position and a closed position; and an actuator system assembly operatively coupling the nozzle flap (132, 134) to the inner cowl, the actuation system assembly being configured to actuate the nozzle flap (132, 134) between the open position and the closed position; a nozzle flap (132, 134) disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap (132, 134) being coupled to the inner cowl and selectively movable relative to the annular air-oil cooler between an open position and a closed position; and an actuator system assembly operatively coupling the nozzle flap (132, 134) to the inner cowl, the actuation system assembly being configured to actuate the nozzle flap (132, 134) between the open position and the closed position; the actuator system assembly comprising at least one sync ring circumferentially disposed between a low pressure compressor case and the nozzle flap (132, 134), the sync ring being rotatable between a first angular position and a second angular position about an engine axis, wherein rotation of the at least one sync ring between the first angular position and the second angular position repositions the fan air diverter relative to the annular air-oil cooler or cooling fins; and a plurality of idler links radially coupling the sync ring to the nozzle flap (132, 134), the idler links being pivotally configured such that rotation of the sync ring to the first angular position moves the nozzle flap (132, 134) to the open position, and rotation of the sync ring to the second angular position moves the nozzle flap (132, 134) to the closed position.