A gas turbine engine comprises a bypass duct and a heat exchanger assembly, the heat exchanger assembly comprising a heat exchanger and a heat exchanger duct having an inlet region, an inflection region and an outlet region. A direction of a centreline of the heat exchanger duct has a tangential component with respect to a principal rotational axis of the gas turbine engine at one or more of the inlet region, the inflection region and the outlet region. The heat exchanger is disposed within the inflection region and configured to transfer heat generated by the gas turbine engine into the flow of air as it passes through the inflection region.