Aircraft air conditioning system and method of operating an aircraft air conditioning system

An aircraft air conditioning system comprising:

• a process air line configured to supply compressed process air provided by a process air source to an air conditioning unit of the aircraft air conditioning system,
• a trim air line branching off from the process air line upstream of the air conditioning unit and being configured such that trim air will flow through the trim air line, the trim air having been branched off from the compressed process air flowing through the process air line,
• a compressor arranged in the trim air line and being configured to compress the trim air flowing through the trim air line,
• a turbine configured to drive the compressor,
• a cabin exhaust air line configured to supply cabin exhaust air discharged from an aircraft cabin to the turbine for driving the turbine, and
• a connecting line connecting the trim air line to the process air line and being configured to discharge excess trim air from the trim air line into the process air line.