An aircraft engine assembly having a turbo-compounded internal combustion engine having an engine shaft. A coolant cooler is fluidly connected to a coolant circuitry of the internal combustion engine and to the environment. A plenum is connected with the environment via the coolant cooler and via an air outlet. A fan is disposed adjacent the air outlet and is operable to drive an airflow from the environment into the plenum via the coolant cooler. The fan is spaced apart from the internal combustion engine in a direction perpendicular to the engine shaft. A method of defining a cooling air circulation is also discussed.