An environmental control system for an engine buffer air system of a turbocompressor of an aircraft engine for an aircraft comprising: the turbocompressor; a higher pressure tap configured to be associated with a higher compression location in a main compressor section associated with the aircraft engine, and a lower pressure tap configured to be associated with a lower pressure location in the main compressor section associated with the aircraft engine, said lower pressure location being at a lower pressure than said higher pressure location; the lower pressure tap communicating to a first passage leading to a downstream outlet to be delivered to the aircraft, and having a second passage configured to lead into a compressor section of the turbocompressor; the higher pressure tap configured to lead into a turbine section of the turbocompressor such that air in the higher pressure tap drives the turbine section to in turn drive the compressor section of the turbocompressor; a turbine outlet configured to receive airflow exhausted from the turbine section; a compressor outlet configured to receive airflow exhausted from the compressor section; a combined outlet configured to receive airflow from the turbine outlet and the compressor outlet configured to intermix airflow and pass the mixed airflow downstream to be delivered to the aircraft; a housing configured to support the compressor section and a buffer air outlet (112, 114) within the housing, the buffer air outlet (112, 114) being configured to receive airflow supplied to the turbocompressor and configured to communicate airflow to an engine buffer air system; a check valve configured to control airflow from the lower pressure tap through a bypass passage between the lower pressure tap and the combined outlet; a first control valve positioned on the higher pressure tap and operable to control operation of the turbocompressor, wherein when the first control valve is in an open position, airflow is drawn into the compressor section of the turbocompressor from the lower pressure tap, and when the first control valve is in a closed position, airflow is not drawn through the compressor section of the turbocompressor and passes through the bypass passage; and a second control valve positioned downstream of a location at which the bypass passage and the combined outlet intermix into a common conduit operable to control airflow through the common conduit to an aircraft system, characterised in that the engine buffer air system is configured to utilize the airflow communicated thereto in a low pressure turbine section or a high pressure turbine section of the aircraft engine. the turbocompressor; a higher pressure tap configured to be associated with a higher compression location in a main compressor section associated with the aircraft engine, and a lower pressure tap configured to be associated with a lower pressure location in the main compressor section associated with the aircraft engine, said lower pressure location being at a lower pressure than said higher pressure location; the lower pressure tap communicating to a first passage leading to a downstream outlet to be delivered to the aircraft, and having a second passage configured to lead into a compressor section of the turbocompressor; the higher pressure tap configured to lead into a turbine section of the turbocompressor such that air in the higher pressure tap drives the turbine section to in turn drive the compressor section of the turbocompressor; a turbine outlet configured to receive airflow exhausted from the turbine section; a compressor outlet configured to receive airflow exhausted from the compressor section; a combined outlet configured to receive airflow from the turbine outlet and the compressor outlet configured to intermix airflow and pass the mixed airflow downstream to be delivered to the aircraft; a housing configured to support the compressor section and a buffer air outlet (112, 114) within the housing, the buffer air outlet (112, 114) being configured to receive airflow supplied to the turbocompressor and configured to communicate airflow to an engine buffer air system; a check valve configured to control airflow from the lower pressure tap through a bypass passage between the lower pressure tap and the combined outlet; a first control valve positioned on the higher pressure tap and operable to control operation of the turbocompressor, wherein when the first control valve is in an open position, airflow is drawn into the compressor section of the turbocompressor from the lower pressure tap, and when the first control valve is in a closed position, airflow is not drawn through the compressor section of the turbocompressor and passes through the bypass passage; and a second control valve positioned downstream of a location at which the bypass passage and the combined outlet intermix into a common conduit operable to control airflow through the common conduit to an aircraft system, characterised in that the engine buffer air system is configured to utilize the airflow communicated thereto in a low pressure turbine section or a high pressure turbine section of the aircraft engine.