A gas turbine engine comprising: an inner cowl; an annular air-oil cooler; and a fan air diverter, the fan air diverter comprising: a nozzle flap disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap being pivotally 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 actuation 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; characterised by: a plurality of cooling fins of the annular air-oil cooler extending into a fan duct and nozzle and into a path of the fan airflow; wherein the nozzle flap is axially positioned in the fore of the annular air-oil cooler, the nozzle flap having a leading edge that is pivotally hinged to a surface of the inner cowl and a trailing edge that is movable between the open position and the closed position relative to an inlet of the annular air-oil cooler. an inner cowl; an annular air-oil cooler; and a fan air diverter, the fan air diverter comprising: a nozzle flap disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap being pivotally 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 actuation 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; characterised by: a plurality of cooling fins of the annular air-oil cooler extending into a fan duct and nozzle and into a path of the fan airflow; wherein the nozzle flap is axially positioned in the fore of the annular air-oil cooler, the nozzle flap having a leading edge that is pivotally hinged to a surface of the inner cowl and a trailing edge that is movable between the open position and the closed position relative to an inlet of the annular air-oil cooler. a nozzle flap disposed circumferentially about the inner cowl and coaxially adjacent to the annular air-oil cooler, the nozzle flap being pivotally 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 actuation 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; characterised by: a plurality of cooling fins of the annular air-oil cooler extending into a fan duct and nozzle and into a path of the fan airflow; wherein the nozzle flap is axially positioned in the fore of the annular air-oil cooler, the nozzle flap having a leading edge that is pivotally hinged to a surface of the inner cowl and a trailing edge that is movable between the open position and the closed position relative to an inlet of the annular air-oil cooler. a plurality of cooling fins of the annular air-oil cooler extending into a fan duct and nozzle and into a path of the fan airflow; wherein the nozzle flap is axially positioned in the fore of the annular air-oil cooler, the nozzle flap having a leading edge that is pivotally hinged to a surface of the inner cowl and a trailing edge that is movable between the open position and the closed position relative to an inlet of the annular air-oil cooler.