A method of providing bleed air to an environmental control system of an aircraft using a gas turbine engine , the method comprising: determining a bleed air demand for the environmental control system , the bleed air demand comprising at least one of an air pressure, an air temperature, and a flow rate demand for the environmental control system, or a combination thereof; selectively supplying low pressure bleed air and high pressure bleed air from a compressor of the gas turbine engine to a first turbine section of a turbo air cycle machine , with the first turbine section emitting a cooled air stream; whereby when low pressure bleed air is selectively supplied from the compressor of the gas turbine engine , a portion is also supplied to a compressor section of the turbo air cycle machine , with the compressor section emitting a compressed air stream; selectively supplying the cooled air stream to a second turbine section with the second turbine section emitting a further cooled air stream; combining at least one of the cooled air stream emitted from the first turbine section or the further cooled air stream emitted from the second turbine section with the compressed air stream emitted from the compressor section to form a conditioned air stream; transforming excess mechanical power into electrical power via a generator ; and delivering electrical power to at least one device or system; wherein the selectively supplying low pressure bleed air and high pressure bleed air and selectively supplying the cooled air stream are controlled such that the conditioned air stream satisfies the determined bleed air demand and wherein operation of at least one of the first turbine section or the second turbine section creates the excess mechanical power. determining a bleed air demand for the environmental control system , the bleed air demand comprising at least one of an air pressure, an air temperature, and a flow rate demand for the environmental control system, or a combination thereof; selectively supplying low pressure bleed air and high pressure bleed air from a compressor of the gas turbine engine to a first turbine section of a turbo air cycle machine , with the first turbine section emitting a cooled air stream; whereby when low pressure bleed air is selectively supplied from the compressor of the gas turbine engine , a portion is also supplied to a compressor section of the turbo air cycle machine , with the compressor section emitting a compressed air stream; selectively supplying the cooled air stream to a second turbine section with the second turbine section emitting a further cooled air stream; combining at least one of the cooled air stream emitted from the first turbine section or the further cooled air stream emitted from the second turbine section with the compressed air stream emitted from the compressor section to form a conditioned air stream; transforming excess mechanical power into electrical power via a generator ; and delivering electrical power to at least one device or system; wherein the selectively supplying low pressure bleed air and high pressure bleed air and selectively supplying the cooled air stream are controlled such that the conditioned air stream satisfies the determined bleed air demand and wherein operation of at least one of the first turbine section or the second turbine section creates the excess mechanical power. <EOS>