The invention provides a method of controlling gas flow in an axial flow compressor (C) in which the flow at least at one chosen station in the direction of flow through the compressor the flow is sensed at a series of circumferentially spaced positions. Flow variations above a predetermined limit are evaluated to give an actuating response if a disturbance above a predetermined acceptable level is detected. When such a disturbance is detected, higher pressure gas bled from further downstream is injected at said station to supplement the main gas flow there. An incipient rotating stall cell will appear as a variation occurring sequentially at the circumferentially spaced positions. By responding to such a condition with a pressure injection flow, it is found possible to suppress both rotating stall and surge conditions in the compressor before the disturbance develops fully. The same means can be arranged to counter steady state distortion.
