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 (M1) at which the first medium mixes with the second medium, wherein the mixing point (M1) is downstream of the compressor and upstream of the first turbine , and the mixing point (M1) 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 (M1) at which the first medium mixes with the second medium, wherein the mixing point (M1) is downstream of the compressor and upstream of the first turbine , and the mixing point (M1) 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 (M1) at which the first medium mixes with the second medium, wherein the mixing point (M1) is downstream of the compressor and upstream of the first turbine , and the mixing point (M1) 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. <EOS>