an air conditioning system in an aircraft is provided . the system comprises at least one air cycle air - conditioning pack with a compressor comprising a bleed air outlet and with a compressed - air line connected to the bleed air connection , which compressed - air line is connectable to a starter turbine for a main engine . with the use of air from a compressor of an air conditioning pack it is not necessary to switch off the air conditioning pack ; bleed air removal from the auxiliary power units becomes obsolete ; and , since there is no unnecessary discharge of compressed air from a bleed air system , noise pollution on the ground is reduced .