Air-conditioning system (10) for aircraft, in particular for commercial aircraft, with a bleed air source (12), a bleed air line (22-30) comprising a main valve (16), and an air-conditioning unit (32), wherein a mass air flow conveyed from the bleed air source (12) through the bleed air line (22-30) to the air-conditioning unit (32) can be controlled by means of the main valve (16). According to the invention it is envisaged that a by-pass line (36-50, 58, 60) comprising a by-pass valve (34) and by-passing at least a part (22-26, 16) of the bleed air line, is present between the bleed air source (12) and the air-conditioning unit (32).