An airplane comprising: a first medium; a second medium; a third medium; and an air conditioning system comprising: a first turbine, a second turbine, a first heat exchanger configured to reject heat from the first medium to the third medium, a second heat exchanger configured to reject heat from the second medium to the third medium, a third heat exchanger and a fourth heat exchanger, a compressor located upstream of the first turbine in a flow path of the first medium, and a mixing point at which the first medium mixes with the second medium, wherein the mixing point is downstream of the compressor and upstream of the first turbine, and the mixing point is located downstream of the first heat exchanger and the second heat exchanger; wherein the third heat exchanger is downstream of the mixing point, characterised in that the second turbine is configured to receive a flow of cabin discharge air from a cabin of the aircraft or from a cabin and a flight deck of the aircraft and to power the compressor the second turbine is downstream of the fourth heat exchanger in the flow of cabin discharge air. a first medium; a second medium; a third medium; and an air conditioning system comprising: a first turbine, a second turbine, a first heat exchanger configured to reject heat from the first medium to the third medium, a second heat exchanger configured to reject heat from the second medium to the third medium, a third heat exchanger and a fourth heat exchanger, a compressor located upstream of the first turbine in a flow path of the first medium, and a mixing point at which the first medium mixes with the second medium, wherein the mixing point is downstream of the compressor and upstream of the first turbine, and the mixing point is located downstream of the first heat exchanger and the second heat exchanger; wherein the third heat exchanger is downstream of the mixing point, characterised in that the second turbine is configured to receive a flow of cabin discharge air from a cabin of the aircraft or from a cabin and a flight deck of the aircraft and to power the compressor the second turbine is downstream of the fourth heat exchanger in the flow of cabin discharge air. a first turbine, a second turbine, a first heat exchanger configured to reject heat from the first medium to the third medium, a second heat exchanger configured to reject heat from the second medium to the third medium, a third heat exchanger and a fourth heat exchanger, a compressor located upstream of the first turbine in a flow path of the first medium, and a mixing point at which the first medium mixes with the second medium, wherein the mixing point is downstream of the compressor and upstream of the first turbine, and the mixing point is located downstream of the first heat exchanger and the second heat exchanger; wherein the third heat exchanger is downstream of the mixing point, characterised in that the second turbine is configured to receive a flow of cabin discharge air from a cabin of the aircraft or from a cabin and a flight deck of the aircraft and to power the compressor the second turbine is downstream of the fourth heat exchanger in the flow of cabin discharge air.