A gas turbine engine comprising: a nacelle extending circumferentially around an engine core and defining a fan bypass duct that is substantially annular between an inner wall and an outer wall, the nacelle comprising: a first side; a second side opposite the first side; a lower bifurcation structure extending between the inner wall and the outer wall and bifurcating the fan bypass duct, the lower bifurcation structure comprising: a first side surface connected to the inner wall of the first side and extending radially away from an engine central longitudinal axis; a second side surface connected to the inner wall of the second side opposite the first side surface connected to the inner wall and extending radially away from the engine central longitudinal axis; a first bifurcation duct wall extending radially away from the engine central longitudinal axis; a second bifurcation duct wall extending radially away from the engine central longitudinal axis; the lower bifurcation structure defines a bifurcation duct extending along a central axis (A BD) of the lower bifurcation structure; and a heat exchanger positioned in the bifurcation duct; characterized in that the first side surface and the first bifurcation duct wall combine to define a first lower bifurcation cavity; the second side surface and the second bifurcation duct wall combine to define a second lower bifurcation cavity; and the first side is separable from the second side such that the first side surface is separable from the first bifurcation duct wall and second side surface is separable from second bifurcation duct wall. a nacelle extending circumferentially around an engine core and defining a fan bypass duct that is substantially annular between an inner wall and an outer wall, the nacelle comprising: a first side; a second side opposite the first side; a first side; a second side opposite the first side; a lower bifurcation structure extending between the inner wall and the outer wall and bifurcating the fan bypass duct, the lower bifurcation structure comprising: a first side surface connected to the inner wall of the first side and extending radially away from an engine central longitudinal axis; a second side surface connected to the inner wall of the second side opposite the first side surface connected to the inner wall and extending radially away from the engine central longitudinal axis; a first bifurcation duct wall extending radially away from the engine central longitudinal axis; a second bifurcation duct wall extending radially away from the engine central longitudinal axis; a first side surface connected to the inner wall of the first side and extending radially away from an engine central longitudinal axis; a second side surface connected to the inner wall of the second side opposite the first side surface connected to the inner wall and extending radially away from the engine central longitudinal axis; a first bifurcation duct wall extending radially away from the engine central longitudinal axis; a second bifurcation duct wall extending radially away from the engine central longitudinal axis; the lower bifurcation structure defines a bifurcation duct extending along a central axis (A BD) of the lower bifurcation structure; and