An aircraft environmental control system comprising: a fresh air input and a RAM air input, heat exchanger means for receiving fresh air from the fresh air input and RAM air from the RAM air input and using the RAM air to cool the fresh air, and means for providing the cooled fresh air to an interior of the aircraft, the system further comprising: means for feeding back exhaust air that is conditioned air that has been emitted from the interior of the aircraft to combine with the RAM air to further cool the fresh air, whereby the heat exchanger means comprises a main heat exchanger and a primary heat exchanger; wherein the main heat exchanger comprising a main heat exchanger cooling fluid inlet, a main heat exchanger cooling fluid outlet and a main heat exchanger fresh air inlet, the cooling fluid working to cool fresh air in the main heat exchanger; wherein the primary heat exchanger comprises a primary heat exchanger cooling fluid inlet, a primary heat exchanger fresh air inlet and a conditioned air outlet; and characterized by the system further comprising a controller to control, in a first condition, flow of the exhaust air to the main heat exchanger cooling fluid inlet and flow of the RAM air to mix with fluid from the main heat exchanger cooling fluid outlet to be provided to the primary heat exchanger cooling fluid inlet, whereby, in the first condition, no RAM air is provided to the main heat exchanger cooling fluid inlet, and, in a second condition, flow of the RAM air to the main heat exchanger cooling fluid inlet. a fresh air input and a RAM air input, heat exchanger means for receiving fresh air from the fresh air input and RAM air from the RAM air input and using the RAM air to cool the fresh air, and means for providing the cooled fresh air to an interior of the aircraft, the system further comprising: means for feeding back exhaust air that is conditioned air that has been emitted from the interior of the aircraft to combine with the RAM air to further cool the fresh air, whereby the heat exchanger means comprises a main heat exchanger and a primary heat exchanger; wherein the main heat exchanger comprising a main heat exchanger cooling fluid inlet, a main heat exchanger cooling fluid outlet and a main heat exchanger fresh air inlet, the cooling fluid working to cool fresh air in the main heat exchanger; wherein the primary heat exchanger comprises a primary heat exchanger cooling fluid inlet, a primary heat exchanger fresh air inlet and a conditioned air outlet; and characterized by the system further comprising a controller to control, in a first condition, flow of the exhaust air to the main heat exchanger cooling fluid inlet and flow of the RAM air to mix with fluid from the main heat exchanger cooling fluid outlet to be provided to the primary heat exchanger cooling fluid inlet, whereby, in the first condition, no RAM air is provided to the main heat exchanger cooling fluid inlet, and, in a second condition, flow of the RAM air to the main heat exchanger cooling fluid inlet. means for feeding back exhaust air that is conditioned air that has been emitted from the interior of the aircraft to combine with the RAM air to further cool the fresh air, whereby the heat exchanger means comprises a main heat exchanger and a primary heat exchanger; wherein the main heat exchanger comprising a main heat exchanger cooling fluid inlet, a main heat exchanger cooling fluid outlet and a main heat exchanger fresh air inlet, the cooling fluid working to cool fresh air in the main heat exchanger; wherein the primary heat exchanger comprises a primary heat exchanger cooling fluid inlet, a primary heat exchanger fresh air inlet and a conditioned air outlet; and characterized by the system further comprising a controller to control, in a first condition, flow of the exhaust air to the main heat exchanger cooling fluid inlet and flow of the RAM air to mix with fluid from the main heat exchanger cooling fluid outlet to be provided to the primary heat exchanger cooling fluid inlet, whereby, in the first condition, no RAM air is provided to the main heat exchanger cooling fluid inlet, and, in a second condition, flow of the RAM air to the main heat exchanger cooling fluid inlet.