A secondary power unit for an aircraft comprises a gas turbine engine having at least one drive shaft, a twin-load compressor including first and second load compressors both coupled with the drive shaft, a first ram-air intake in fluid communication with the twin-load compressor, and a flow regulating arrangement coupled with the first and second compressors outputs to individually regulate the generated flow of compressed air. The flow regulating arrangement is also adapted to be fluidly communicated with an Air Cycle Machine of an aircraft, and a control system is adapted to feed the flow of compressed air as bleed air to the Air Cycle Machine of an aircraft, both when the aircraft is on ground and in flight. The secondary power unit is capable of avoiding bleed air extraction from the main engines with the aim of reducing fuel consumption.