Aircraft environmental control system

1. 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 RAM air to cool the fresh air, and

means for providing the cooled fresh air to a cabin of the aircraft;

the system further comprising means for feeding back exhaust air emitted from the cabin of the aircraft to combine with the RAM air to further cool the fresh air, a main heat exchanger and a primary heat exchanger to condition fresh air, a first mixer to mix expanded exhaust air with RAM air prior to entry into the main heat exchanger and a second mixer to mix expanded exhaust air with RAM air after the main heat exchanger and prior to entry into the primary heat exchanger.

2. The system of claim 1 , wherein a ratio of fed-back exhaust air and RAM air to cool the fresh air is determined based on the temperature and/or pressure of the exhaust air.

3. The system of claim 1 , wherein a controller is provided to control the ratio of fed-back exhaust air and RAM air to cool the fresh air.

4. The system of claim 1 , wherein the exhaust air is expanded by a turbine and divided into two streams, a first stream to be mixed with RAM air entering into a main heat exchanger for cooling fresh air, and a second stream to be mixed with RAM air exiting the main heat exchanger.

5. The system of claim 4 , wherein the flow of exhaust air via the two streams is controlled by a controller based on the temperature of the exhaust air compared to the temperature of the RAM air.

6. The system of claim 1 , wherein the fresh air is compressed ambient air.

7. The system of claim 1 , wherein the fresh air is bleed air from an engine of the aircraft.