An aircraft pneumatic system comprises a main gas turbine engine (10) comprising a main compressor (14, 16), a recuperated auxiliary gas turbine engine (40) comprising a core compressor (42) having an inlet in fluid communication with a main compressor bleed (32) of the main gas turbine engine(10) and an environmental control system inlet in fluid communication with the main compressor bleed (32) of the main gas turbine engine (10). The environmental control system comprises a compressor (68) driveable by power provided by the auxiliary gas turbine engine (40).