A gas turbine engine assembly (210; 215) comprising: a fan section delivering air into a main compressor section, said main compressor section compressing air and delivering air into a combustion section, products of combustion passing from said combustion section over a turbine section to drive said fan section and main compressor section, wherein a gearbox is driven by said turbine section to drive said fan section; a pylon supporting the gas turbine engine (210; 215); an environmental control system including a higher pressure tap at a higher pressure location in said main compressor section, and a lower pressure tap at a lower pressure location, said lower pressure location being at a lower pressure than said higher pressure location; said lower pressure tap communicating to a first passage leading to a downstream outlet and a compressor section of a turbocompressor; said higher pressure tap leading into a turbine section of said turbocompressor such that air in said higher pressure tap drives said turbine section to in turn drive said compressor section of said turbocompressor, wherein the pylon includes a lowermost surface and the higher pressure tap does not extend above a plane including the lowermost surface; and a combined outlet of said compressor section and said turbine section of said turbocompressor intermixing and passing downstream to be delivered to an aircraft use; characterised in that: a control valve is disposed between the low pressure tap and the compressor section of the turbocompressor and is operable to control and modulate low pressure airflow from said low pressure tap into the compressor section of the turbocompressor. a fan section delivering air into a main compressor section, said main compressor section compressing air and delivering air into a combustion section, products of combustion passing from said combustion section over a turbine section to drive said fan section and main compressor section, wherein a gearbox is driven by said turbine section to drive said fan section; a pylon supporting the gas turbine engine (210; 215); an environmental control system including a higher pressure tap at a higher pressure location in said main compressor section, and a lower pressure tap at a lower pressure location, said lower pressure location being at a lower pressure than said higher pressure location; said lower pressure tap communicating to a first passage leading to a downstream outlet and a compressor section of a turbocompressor; said higher pressure tap leading into a turbine section of said turbocompressor such that air in said higher pressure tap drives said turbine section to in turn drive said compressor section of said turbocompressor, wherein the pylon includes a lowermost surface and the higher pressure tap does not extend above a plane including the lowermost surface; and a combined outlet of said compressor section and said turbine section of said turbocompressor intermixing and passing downstream to be delivered to an aircraft use; characterised in that: a control valve is disposed between the low pressure tap and the compressor section of the turbocompressor and is operable to control and modulate low pressure airflow from said low pressure tap into the compressor section of the turbocompressor.