An air cycle machine pack system (10; 10a-e) for an aircraft, the system having an improved cooling performance at low inlet pressure, the system comprising: an air cycle machine comprising at least one compressor, at least one turbine and an ACM shaft (78; 78a), the at least one compressor being mechanically coupled to the at least one turbine in series on the ACM shaft (78; 78a); a pack heat exchanger coupled to and in fluid communication with the at least one compressor of the ACM; and, a bypass air flow assembly coupled in a parallel operation path to the ACM, the bypass air flow assembly having a bypass air flow regulating element, wherein the bypass air flow assembly is coupled to transfer energy extracted by the bypass air flow assembly to the ACM to aid the air cycle machine pack system (10; 10a-e) in cooling performance at a low inlet pressure, resulting in an improved cooling performance at low inlet pressure of the air cycle machine pack system (10; 10a-e); characterized in that: a) the bypass air flow assembly comprises a parallel turbine coupled to a turbine shaft, and a mechanical drive element connecting the turbine shaft to the ACM shaft to drive the turbine of the ACM and the turbine of the bypass air flow assembly in the parallel operation path, and wherein preferably the mechanical drive element comprises a mechanical shaft or a geared shaft; or b) the bypass air flow assembly comprises a turbo-compressor assembly coupled to a regeneration heat exchanger and a check valve, all coupled to the ACM, and the flow regulating element comprises a parallel turbine valve coupled to the turbo-compressor assembly; or c) the air cycle machine (ACM) comprises a four wheel pack with a compressor, a first turbine and a second turbine mechanically coupled in series on the ACM shaft, and wherein the bypass air flow regulating element comprises a parallel turbine valve coupled between the first turbine and the second turbine. an air cycle machine comprising at least one compressor, at least one turbine and an ACM shaft (78; 78a), the at least one compressor being mechanically coupled to the at least one turbine in series on the ACM shaft (78; 78a); a pack heat exchanger coupled to and in fluid communication with the at least one compressor of the ACM; and, a bypass air flow assembly coupled in a parallel operation path to the ACM, the bypass air flow assembly having a bypass air flow regulating element, wherein the bypass air flow assembly is coupled to transfer energy extracted by the bypass air flow assembly to the ACM to aid the air cycle machine pack system (10; 10a-e) in cooling performance at a low inlet pressure, resulting in an improved cooling performance at low inlet pressure of the air cycle machine pack system (10; 10a-e); characterized in that: a) the bypass air flow assembly comprises a parallel turbine coupled to a turbine shaft, and a mechanical drive element connecting the turbine shaft to the ACM shaft to drive the turbine of the ACM and the turbine of the bypass air flow assembly in the parallel operation path, and wherein preferably the mechanical drive element comprises a mechanical shaft or a geared shaft; or b) the bypass air flow assembly comprises a turbo-compressor assembly coupled to a regeneration heat exchanger and a check valve, all coupled to the ACM, and the flow regulating element comprises a parallel turbine valve coupled to the turbo-compressor assembly; or c) the air cycle machine (ACM) comprises a four wheel pack with a compressor, a first turbine and a second turbine mechanically coupled in series on the ACM shaft, and wherein the bypass air flow regulating element comprises a parallel turbine valve coupled between the first turbine and the second turbine. a) the bypass air flow assembly comprises a parallel turbine coupled to a turbine shaft, and a mechanical drive element connecting the turbine shaft to the ACM shaft to drive the turbine of the ACM and the turbine of the bypass air flow assembly in the parallel operation path, and wherein preferably the mechanical drive element comprises a mechanical shaft or a geared shaft; or b) the bypass air flow assembly comprises a turbo-compressor assembly coupled to a regeneration heat exchanger and a check valve, all coupled to the ACM, and the flow regulating element comprises a parallel turbine valve coupled to the turbo-compressor assembly; or c) the air cycle machine (ACM) comprises a four wheel pack with a compressor, a first turbine and a second turbine mechanically coupled in series on the ACM shaft, and wherein the bypass air flow regulating element comprises a parallel turbine valve coupled between the first turbine and the second turbine.